Published Papers Discussing The Global Warming Hiatus

This is a list of more than 50 published papers that try to explain how the global warming hiatus can be consistent with greenhouse gas theory. This list is not complete. The papers are in reverse order by year.

Santer, B. D., Fyfe, J. C., Pallotta, G., Flato, G. M., Meehl, G. A., England, M. H., Hawkins, E., Mann, M. E., Painter, J. F., Bonfils, C., Cvijanovic, I., Mears, C., Wentz, F. J., Po-Chedley, S., Fu, Q., and Zou, C.-Z., 2017, Causes of differences in model and satellite tropospheric warming rates, Nature Geoscience. (Santer et al., 2017)

In the early twenty-first century, satellite-derived tropospheric warming trends were generally smaller than trends estimated from a large multi-model ensemble. Because observations and coupled model simulations do not have the same phasing of natural internal variability, such decadal differences in simulated and observed warming rates invariably occur. Here we analyse global-mean tropospheric temperatures from satellites and climate model simulations to examine whether warming rate differences over the satellite era can be explained by internal climate variability alone. We find that in the last two decades of the twentieth century, differences between modelled and observed tropospheric temperature trends are broadly consistent with internal variability. Over most of the early twenty-first century, however, model tropospheric warming is substantially larger than observed; warming rate differences are generally outside the range of trends arising from internal variability. The probability that multi-decadal internal variability fully explains the asymmetry between the late twentieth and early twenty-first century results is low (between zero and about 9%). It is also unlikely that this asymmetry is due to the combined effects of internal variability and a model error in climate sensitivity. We conclude that model overestimation of tropospheric warming in the early twenty-first century is partly due to systematic deficiencies in some of the post-2000 external forcings used in the model simulations.

Medhaug, I., Stolpe, B., Fischer, M., Erich M., and Knutti, R., 2017, Reconciling controversies about the ‘global warming hiatus’: Nature, v. 545, p. 41–47, (Medhaug et al., 2017)

Between about 1998 and 2012, a time that coincided with political negotiations for preventing climate change, the surface of Earth seemed hardly to warm. This phenomenon, often termed the ‘global warming hiatus’, caused doubt in the public mind about how well anthropogenic climate change and natural variability are understood. Here we show that apparently contradictory conclusions stem from different definitions of ‘hiatus’ and from different datasets. A combination of changes in forcing, uptake of heat by the oceans, natural variability and incomplete observational coverage reconciles models and data. Combined with stronger recent warming trends in newer datasets, we are now more confident than ever that human influence is dominant in long-term warming.

Rahmstorf, S., G. Foster, and N. Cahill (2017), Global temperature evolution: recent trends and some pitfalls, Environ. Res. Lett., 12(5), 054001, doi:10.1088/1748-9326/aa6825.(Rahmstorf et al., 2017)

Global surface temperatures continue to rise. In most surface temperature data sets, the years 2014, 2015 and again 2016 set new global heat records since the start of regular measurements. Never before have three record years occurred in a row. We show that this recent streak of record heat does not in itself provide statistical evidence for an acceleration of global warming, nor was it preceded by a ‘slowdown period’ with a significantly reduced rate of warming. Rather, the data are fully consistent with a steady global warming trend since the 1970s, superimposed with random, stationary, short-term variability. All recent variations in short-term trends are well within what was to be expected, based on the observed warming trend and the observed variability from the 1970s up to the year 2000. We discuss some pitfalls of statistical analysis of global temperatures which have led to incorrect claims of an unexpected or significant warming slowdown.

Ballantyne, A., Smith, W., Anderegg, W., Kauppi, P., Sarmiento, J., Tans, P., Shevliakova, E., Pan, Y., Poulter, B., and Anav, A., 2017, Accelerating net terrestrial carbon uptake during the warming hiatus due to reduced respiration: Nature Climate Change, v. 7, p. 148-152, 10.1038/nclimate3204.(Ballantyne et al., 2017)

The recent ‘warming hiatus’ presents an excellent opportunity to investigate climate sensitivity of carbon cycle processes. Here we combine satellite and atmospheric observations to show that the rate of net biome productivity (NBP) has significantly accelerated from −0.007 ± 0.065 PgC yr−2 over the warming period (1982 to 1998) to 0.119 ± 0.071 PgC yr−2 over the warming hiatus (1998–2012). This acceleration in NBP is not due to increased primary productivity, but rather reduced respiration that is correlated (r = 0.58; P = 0.0007) and sensitive (γ = 4.05 to 9.40 PgC yr−1 per °C) to land temperatures. Global land models do not fully capture this apparent reduced respiration over the warming hiatus; however, an empirical model including soil temperature and moisture observations better captures the reduced respiration.

Huang, J., Xie, Y., Guan, X., Li, D., and Ji, F., 2017, The dynamics of the warming hiatus over the Northern Hemisphere: Climate Dynamics, v. 48, no. 1-2, p. 429-446, 10.1007/s00382-016-3085-8.(Huang et al., 2017)

A warming hiatus is a period of relatively little change in global mean surface air temperatures (SAT). Many studies have attributed the current warming hiatus to internal climate variability (ICV). But there is less work on discussion of the dynamics about how these ICV modes influence cooling over land in the Northern Hemisphere (NH). Here we demonstrate the warming hiatus was more significant over the continental NH. We explored the dynamics of the warming hiatus from a global perspective and investigated the mechanisms of the reversing from accelerated warming to hiatus, and how ICV modes influence SAT change throughout the NH land. It was found that these ICV modes and Arctic amplification can excite a decadal modulated oscillation (DMO), which enhances or suppresses the long-term trend on decadal to multi-decadal timescales. When the DMO is in an upward (warming) phase, it contributes to an accelerated warming trend, as in last 20 years of twentieth-century. It appears that there is a downward swing in the DMO occurring at present, which has balanced or reduced the radiative forced warming and resulted in the recent global warming hiatus. The DMO modulates the SAT, in particular, the SAT of boreal cold months, through changes in the asymmetric meridional and zonal thermal forcing (MTF and ZTF). The MTF represents the meridional temperature gradients between the mid- and high-latitudes, and the ZTF represents the asymmetry in temperatures between the extratropical large-scale warm and cold zones in the zonal direction. Via the different performance of combined MTF and ZTF, we found that the DMO’s modulation effect on SAT was strongest when both weaker (stronger) MTF and stronger (weaker) ZTF occurred simultaneously. And the current hiatus is a result of a downward DMO combined with a weaker MTF and stronger ZTF, which stimulate both a weaker polar vortex and westerly winds, along with the amplified planetary waves, thereby facilitating southward invasion of cold Arctic-air and promoting the blocking formation. The results conclude that the DMO can not only be used to interpret the current warming hiatus, it also suggests that global warming will accelerate again when it swings upward.

Dong, L., and McPhaden, M. J., 2016, Interhemispheric SST Gradient Trends in the Indian Ocean prior to and during the Recent Global Warming Hiatus: Journal of Climate, v. 29, no. 24, p. 9077-9095, 10.1175/JCLI-D-16-0130.1.(Dong and McFaden, 2016)

Sea surface temperatures (SSTs) have been rising for decades in the Indian Ocean in response to greenhouse gas forcing. However, this study shows that during the recent hiatus in global warming, a striking interhemispheric gradient in Indian Ocean SST trends developed around 2000, with relatively weak or little warming to the north of 10°S and accelerated warming to the south of 10°S. Evidence is presented from a wide variety of data sources showing that this interhemispheric gradient in SST trends is forced primarily by an increase of Indonesian Throughflow (ITF) transport from the Pacific into the Indian Ocean induced by stronger Pacific trade winds. This increased transport led to a depression of the thermocline that facilitated SST warming, presumably through a reduction in the vertical turbulent transport of heat in the southern Indian Ocean. Surface wind changes in the Indian Ocean linked to the enhanced Walker circulation also may have contributed to thermocline depth variations and associated SST changes, with downwelling-favorable wind stress curls between 10° and 20°S and upwelling-favorable wind stress curls between the equator and 10°S. In addition, the anomalous southwesterly wind stresses off the coast of Somalia favored intensified coastal upwelling and offshore advection of upwelled water, which would have led to reduced warming of the northern Indian Ocean. Although highly uncertain, lateral heat advection associated with the ITF and surface heat fluxes may also have played a role in forming the interhemispheric SST gradient change.

Wang, L., Yuan, X., Xie, Z., Wu, P., and Li, Y., 2016, Increasing flash droughts over China during the recent global warming hiatus: Scientific Reports, v. 6, 10.1038/srep30571(Wang et al., 2016)

The recent global warming slowdown or hiatus after the big El Niño event in 1997/98 raises the questions of whether terrestrial hydrological cycle is being decelerated and how do the hydrological extremes respond to the hiatus. However, the rapidly developing drought events that are termed as “flash droughts” accompanied by extreme heat, low soil moisture and high evapotranspiration (ET), occurred frequently around the world, and caused devastating impacts on crop yields and water supply. Here, we investigate the long-term trend and variability of flash droughts over China. Flash droughts are most likely to occur over humid and semi-humid regions, such as southern and northeastern China. Flash drought averaged over China increased by 109% from 1979 to 2010, and the increase was mainly due to a long term warming of temperature (50%), followed by the contributions from decreasing soil moisture and increasing ET. There was a slight drop in temperature after 1997, but the increasing trend of flash droughts was tripled. Further results indicate that the decreasing temperature was compensated by the accelerated drying trends of soil moisture and enhanced ET, leading to an acceleration of flash droughts during the warming hiatus. The anthropogenic warming in the next few decades may exacerbate future flash drought conditions in China.

Yao, S.-L., Huang, G., Wu, R.-G., and Qu, X., 2016, The global warming hiatus—a natural product of interactions of a secular warming trend and a multi-decadal oscillation: Theoretical and Applied Climatology, v. 123, no. 1-2, p. 349-360, 10.1007/s00704-014-1358-x. (Yao et al., 2016)

The globally-averaged annual combined land and ocean surface temperature (GST) anomaly change features a slowdown in the rate of global warming in the mid-twentieth century and the beginning of the twenty-first century. Here, it is shown that the hiatus in the rate of global warming typically occurs when the internally generated cooling associated with the cool phase of the multi-decadal variability overcomes the secular warming from human-induced forcing. We provide compelling evidence that the global warming hiatus is a natural product of the interplays between a secular warming tendency due in a large part to the buildup of anthropogenic greenhouse gas concentrations, in particular CO2 concentration, and internally generated cooling by a cool phase of a quasi-60-year oscillatory variability that is closely associated with the Atlantic multi-decadal oscillation (AMO) and the Pacific decadal oscillation (PDO). We further illuminate that the AMO can be considered as a useful indicator and the PDO can be implicated as a harbinger of variations in global annual average surface temperature on multi-decadal timescales. Our results suggest that the recent observed hiatus in the rate of global warming will very likely extend for several more years due to the cooling phase of the quasi-60-year oscillatory variability superimposed on the secular warming trend.

Arora, A., Rao, S. A., Chattopadhyay, R., Goswami, T., George, G., and Sabeerali, C., 2016, Role of Indian Ocean SST variability on the recent global warming hiatus: Global and Planetary Change, v. 143, p. 21-30, 10.1016/j.gloplacha.2016.05.009. (Arora et al., 2016)

Previous studies have shown a slowdown in the warming rate of the annual mean global surface temperature in the recent decade and it is referred to as the hiatus in global warming. Some recent studies have suggested that the hiatus in global warming is possibly due to strong cooling in the tropical Pacific. This study investigates the possible role of the Indian Ocean warming on the tropical Pacific cooling. Despite the continued rise in sea surface temperature (SST) over the tropical Indian Ocean, SST over the tropical Pacific has shown a cooling trend in the recent decade (2002 − 2012). It is well known fact that the Indian Ocean and the Pacific Ocean are strongly coupled to each other and the Indian Ocean basin wide warming is triggered by El Niño on interannual time scale. However, in the recent decade, this relationship is weakening. The recent Indian Ocean warming is triggering a Matsuno-Gill type response in the atmosphere by generating anomalous cyclonic circulations on either side of equator over the tropical Indian Ocean and anomalous easterlies along the tropical Pacific Ocean. These anomalous easterlies result in Ekman divergence in the equatorial Pacific and produce upwelling Kelvin waves, cools the tropical Pacific and therefore indirectly contributes to the hiatus in global warming.

Liu, W., Xie, S.-P., and Lu, J., 2016, Tracking ocean heat uptake during the surface warming hiatus: Nature communications, v. 7, 10.1038/ncomms10926.(Liu et al., 2016)

Ocean heat uptake is observed to penetrate deep into the Atlantic and Southern Oceans during the recent hiatus of global warming. Here we show that the deep heat penetration in these two basins is not unique to the hiatus but is characteristic of anthropogenic warming and merely reflects the depth of the mean meridional overturning circulation in the basin. We find, however, that heat redistribution in the upper 350 m between the Pacific and Indian Oceans is closely tied to the surface warming hiatus. The Indian Ocean shows an anomalous warming below 50 m during hiatus events due to an enhanced heat transport by the Indonesian throughflow in response to the intensified trade winds in the equatorial Pacific. Thus, the Pacific and Indian Oceans are the key regions to track ocean heat uptake during the surface warming hiatus.

Lewandowsky, S., Risbey, J. S., and Oreskes, N., 2016, The “pause” in global warming: Turning a routine fluctuation into a problem for science: Bulletin of the American Meteorological Society, v. 97, no. 5, p. 723-733, 10.1175/BAMS-D-14-00106.1. (Lewandowsky et al., 2016)

There has been much recent published research about a putative “pause” or “hiatus” in global warming. We show that there are frequent fluctuations in the rate of warming around a longer-term warming trend, and that there is no evidence that identifies the recent period as unique or particularly unusual. In confirmation, we show that the notion of a pause in warming is considered to be misleading in a blind expert test. Nonetheless, the most recent fluctuation about the longer-term trend has been regarded by many as an explanatory challenge that climate science must resolve. This departs from long-standing practice, insofar as scientists have long recognized that the climate fluctuates, that linear increases in CO2 do not produce linear trends in global warming, and that 15-yr (or shorter) periods are not diagnostic of long-term trends. We suggest that the repetition of the “warming has paused” message by contrarians was adopted by the scientific community in its problem-solving and answer-seeking role and has led to undue focus on, and mislabeling of, a recent fluctuation. We present an alternative framing that could have avoided inadvertently reinforcing a misleading claim.

Yan, X. H., Boyer, T., Trenberth, K., Karl, T. R., Xie, S. P., Nieves, V., Tung, K. K., and Roemmich, D., 2016, The global warming hiatus: Slowdown or redistribution?: Earth’s Future, v. 4, no. 11, p. 472-482, 10.1002/2016EF000417.(Yan et al., 2016)

Global mean surface temperatures (GMST) exhibited a smaller rate of warming during 1998–2013, compared to the warming in the latter half of the 20th Century. Although, not a “true” hiatus in the strict definition of the word, this has been termed the “global warming hiatus” by IPCC (2013). There have been other periods that have also been defined as the “hiatus” depending on the analysis. There are a number of uncertainties and knowledge gaps regarding the “hiatus.” This report reviews these issues and also posits insights from a collective set of diverse information that helps us understand what we do and do not know. One salient insight is that the GMST phenomenon is a surface characteristic that does not represent a slowdown in warming of the climate system but rather is an energy redistribution within the oceans. Improved understanding of the ocean distribution and redistribution of heat will help better monitor Earth’s energy budget and its consequences. A review of recent scientific publications on the “hiatus” shows the difficulty and complexities in pinpointing the oceanic sink of the “missing heat” from the atmosphere and the upper layer of the oceans, which defines the “hiatus.” Advances in “hiatus” research and outlooks (recommendations) are given in this report.

Xie, S.-P., Kosaka, Y., and Okumura, Y. M., 2016, Distinct energy budgets for anthropogenic and natural changes during global warming hiatus: Nature Geoscience, v. 9, no. 1, p. 29-33, 10.1038/ngeo2581.(Xie et al., 2016)

The Earth’s energy budget for the past four decades can now be closed1, and it supports anthropogenic greenhouse forcing as the cause for climate warming. However, closure depends on invoking an unrealistically large increase in aerosol cooling2 during the so-called global warming hiatus since the late 1990s (refs 3,4) that was due partly to tropical Pacific Ocean cooling5, 6, 7. The difficulty with this closure lies in the assumption that the same climate feedback applies to both anthropogenic warming and natural cooling. Here we analyse climate model simulations with and without anthropogenic increases in greenhouse gas concentrations, and show that top-of-the-atmosphere radiation and global mean surface temperature are much less tightly coupled for natural decadal variability than for the greenhouse-gas-induced response, implying distinct climate feedback between anthropogenic warming and natural variability. In addition, we identify a phase difference between top-of-the-atmosphere radiation and global mean surface temperature such that ocean heat uptake tends to slow down during the surface warming hiatus. This result deviates from existing energy theory but we find that it is broadly consistent with observations. Our study highlights the importance of developing metrics that distinguish anthropogenic change from natural variations to attribute climate variability and to estimate climate sensitivity from observations.

Fyfe, J. C., Meehl, G. A., England, M. H., Mann, M. E., Santer, B. D., Flato, G. M., Hawkins, E., Gillett, N. P., Xie, S-P., Kosaka, Y., and Swart, N. C., 2016, Making sense of the early-2000s warming slowdown: Nature Climate Change, v. 6, p. 224-228, doi:10.1038/nclimate2938. (Fyfe et al., 2016)

It has been claimed that the early-2000s global warming slowdown or hiatus, characterized by a reduced rate of global surface warming, has been overstated, lacks sound scientific basis, or is unsupported by observations. The evidence presented here contradicts these claims.

Karl, T. R., Arguez, A., Huang, B., Lawrimore, J. H., McMahon, J. R., Menne, M. J., Peterson, T. C., Vose, R. S., and Zhang, H.-M., 2015, Possible artifacts of data biases in the recent global surface warming hiatus: Science, v. 348, no. 6242, p. 1469-1472 10.1126/science.aaa5632. (Karl et al., 2015)

Much study has been devoted to the possible causes of an apparent decrease in the upward trend of global surface temperatures since 1998, a phenomenon that has been dubbed the global warming “hiatus.” Here we present an updated global surface temperature analysis that reveals that global trends are higher than reported by the IPCC, especially in recent decades, and that the central estimate for the rate of warming during the first 15 years of the 21st century is at least as great as the last half of the 20th century. These results do not support the notion of a “slowdown” in the increase of global surface temperature.

Trenberth, K. E., 2015, Has there been a hiatus?: Science (New York, NY), v. 349, p. 691-692, doi:10.1126/science.aac9225 (Trenberth, 2015)

Natural fluctuations are big enough to overwhelm the steady background warming at any point in time.

Steinman, B. A., Mann, M. E., and Miller, S. K., 2015, Atlantic and Pacific multidecadal oscillations and Northern Hemisphere temperatures: Science, v. 347, no. 6225, p. 988-991, 10.1126/science.1257856. (Steinman et al., 2015)

The recent slowdown in global warming has brought into question the reliability of climate model projections of future temperature change and has led to a vigorous debate over whether this slowdown is the result of naturally occurring, internal variability or forcing external to Earth’s climate system. To address these issues, we applied a semi-empirical approach that combines climate observations and model simulations to estimate Atlantic- and Pacific-based internal multidecadal variability (termed “AMO” and “PMO,” respectively). Using this method, the AMO and PMO are found to explain a large proportion of internal variability in Northern Hemisphere mean temperatures. Competition between a modest positive peak in the AMO and a substantially negative-trending PMO are seen to produce a slowdown or “false pause” in warming of the past decade.

Nieves, V., Willis, J. K., and Patzert, W. C., 2015, Recent hiatus caused by decadal shift in Indo-Pacific heating: Science, v. 349, no. 6247, p. 532-535, 10.1126/science.aaa4521. (Nieves et al., 2015)

Recent modeling studies have proposed different scenarios to explain the slowdown in surface temperature warming in the most recent decade. Some of these studies seem to support the idea of internal variability and/or rearrangement of heat between the surface and the ocean interior. Others suggest that radiative forcing might also play a role. Our examination of observational data over the past two decades shows some significant differences when compared to model results from reanalyses and provides the most definitive explanation of how the heat was redistributed. We find that cooling in the top 100-meter layer of the Pacific Ocean was mainly compensated for by warming in the 100- to 300-meter layer of the Indian and Pacific Oceans in the past decade since 2003.

Dai, A., Fyfe, J. C., Xie, S.-P., and Dai, X., 2015, Decadal modulation of global surface temperature by internal climate variability: Nature Climate Change, v. 5, no. 6, p. 555-559, 10.1038/nclimate2605. (Dai et al., 2015)

Despite a steady increase in atmospheric greenhouse gases (GHGs), global-mean surface temperature (T) has shown no discernible warming since about 2000, in sharp contrast to model simulations, which on average project strong warming1, 2, 3. The recent slowdown in observed surface warming has been attributed to decadal cooling in the tropical Pacific1, 4, 5, intensifying trade winds5, changes in El Niño activity6, 7, increasing volcanic activity8, 9, 10 and decreasing solar irradiance7. Earlier periods of arrested warming have been observed but received much less attention than the recent period, and their causes are poorly understood. Here we analyse observed and model-simulated global T fields to quantify the contributions of internal climate variability (ICV) to decadal changes in global-mean T since 1920. We show that the Interdecadal Pacific Oscillation (IPO) has been associated with large T anomalies over both ocean and land. Combined with another leading mode of ICV, the IPO explains most of the difference between observed and model-simulated rates of decadal change in global-mean T since 1920, and particularly over the so-called ‘hiatus’ period since about 2000. We conclude that ICV, mainly through the IPO, was largely responsible for the recent slowdown, as well as for earlier slowdowns and accelerations in global-mean T since 1920, with preferred spatial patterns different from those associated with GHG-induced warming or aerosol-induced cooling. Recent history suggests that the IPO could reverse course and lead to accelerated global warming in the coming decades.

Karl, T. R., Arguez, A., Huang, B., Lawrimore, J. H., McMahon, J. R., Menne, M. J., Peterson, T. C., Vose, R. S., and Zhang, H.-M., 2015, Possible artifacts of data biases in the recent global surface warming hiatus: Science, v. 348, no. 6242, p. 1469-1472 10.1126/science.aaa5632. (Karl et al., 2015)

Much study has been devoted to the possible causes of an apparent decrease in the upward trend of global surface temperatures since 1998, a phenomenon that has been dubbed the global warming “hiatus.” Here we present an updated global surface temperature analysis that reveals that global trends are higher than reported by the IPCC, especially in recent decades, and that the central estimate for the rate of warming during the first 15 years of the 21st century is at least as great as the last half of the 20th century. These results do not support the notion of a “slowdown” in the increase of global surface temperature.

Rajaratnam, B., Romano, J., Tsiang, M., and Diffenbaugh, N. S., 2015, Debunking the climate hiatus: Climatic Change, v. 133, no. 2, p. 129-140, 10.​1007/​s10584-015-1495-y. (Rajaratnam et al., 2015)

The reported “hiatus” in the warming of the global climate system during this century has been the subject of intense scientific and public debate, with implications ranging from scientific understanding of the global climate sensitivity to the rate in which greenhouse gas emissions would need to be curbed in order to meet the United Nations global warming target. A number of scientific hypotheses have been put forward to explain the hiatus, including both physical climate processes and data artifacts. However, despite the intense focus on the hiatus in both the scientific and public arenas, rigorous statistical assessment of the uniqueness of the recent temperature time-series within the context of the long-term record has been limited. We apply a rigorous, comprehensive statistical analysis of global temperature data that goes beyond simple linear models to account for temporal dependence and selection effects. We use this framework to test whether the recent period has demonstrated i) a hiatus in the trend in global temperatures, ii) a temperature trend that is statistically distinct from trends prior to the hiatus period, iii) a “stalling” of the global mean temperature, and iv) a change in the distribution of the year-to-year temperature increases. We find compelling evidence that recent claims of a “hiatus” in global warming lack sound scientific basis. Our analysis reveals that there is no hiatus in the increase in the global mean temperature, no statistically significant difference in trends, no stalling of the global mean temperature, and no change in year-to-year temperature increases.

Marotzke, J., and Forster, P. M., 2015, Forcing, feedback and internal variability in global temperature trends: Nature, v. 517, no. 7536, p. 565-570, 10.1038/nature14117. target=”_blank”>(Marotzke, and Forster, 2015)Most present-generation climate models simulate an increase in global-mean surface temperature (GMST) since 1998, whereas observations suggest a warming hiatus. It is unclear to what extent this mismatch is caused by incorrect model forcing, by incorrect model response to forcing or by random factors. Here we analyse simulations and observations of GMST from 1900 to 2012, and show that the distribution of simulated 15-year trends shows no systematic bias against the observations. Using a multiple regression approach that is physically motivated by surface energy balance, we isolate the impact of radiative forcing, climate feedback and ocean heat uptake on GMST—with the regression residual interpreted as internal variability—and assess all possible 15- and 62-year trends. The differences between simulated and observed trends are dominated by random internal variability over the shorter timescale and by variations in the radiative forcings used to drive models over the longer timescale. For either trend length, spread in simulated climate feedback leaves no traceable imprint on GMST trends or, consequently, on the difference between simulations and observations. The claim that climate models systematically overestimate the response to radiative forcing from increasing greenhouse gas concentrations therefore seems to be unfounded.

England, M. H., Kajtar, J. B., and Maher, N., 2015, Robust warming projections despite the recent hiatus: Nature Climate Change, v. 5, no. 5, p. 394-396, doi:10.1038/nclimate2575. (England et al., 2015)

The hiatus in warming has led to questions about the reliability of long-term projections, yet here we show they are statistically unchanged when considering only ensemble members that capture the recent hiatus. This demonstrates the robust nature of twenty-first century warming projections.

Douville, H., Voldoire, A., and Geoffroy, O., 2015, The recent global‐warming hiatus: What is the role of Pacific variability?: Geophysical Research Letters, 10.1002/2014GL062775. (Douville et al., 2015)

The observed global mean surface air temperature (GMST) has not risen over the last 15 years, spurring outbreaks of skepticism regarding the nature of global warming and challenging the upper-range transient response of the current-generation global climate models. Recent numerical studies have however tempered the relevance of the observed pause in global warming by highlighting the key role of tropical Pacific internal variability. Here we first show that many climate models overestimate the influence of the El Niño Southern Oscillation on GMST, thereby shedding doubt on their ability to capture the tropical Pacific contribution to the hiatus. Moreover, we highlight that model results can be quite sensitive to the experimental design. We argue that overriding the surface wind stress is more suitable than nudging the sea surface temperature for controlling the tropical Pacific ocean heat uptake and, thereby, the multi-decadal variability of GMST. Using the former technique, our model captures several aspects of the recent climate evolution, including the weaker slowdown of global warming over land and the transition towards a negative phase of the Pacific Decadal Oscillation. Yet, the observed global warming is still overestimated, not only over the recent 1998–2012 hiatus period but also over former decades, thereby suggesting that the model might be too sensitive to the prescribed radiative forcings.

Wei, M., Qiao, F., and Deng, J., 2015, A Quantitative Definition of Global Warming Hiatus and 50-Year Prediction of Global Mean Surface Temperature: Journal of the Atmospheric Sciences, no. 2015, doi: 10.1175/JAS-D-14-0296.1 (Wei et al., 2015)

Recent global warming hiatus has received much attention; however, a robust and quantitative definition for the hiatus is still lacking. Recent studies (Scafetta, 2010; Wu et al., 2011; Tung and Zhou, 2013) showed that multi-decadal variability (MDV), is responsible for the multi-decadal accelerated warming and hiatuses in historical global mean surface temperature (GMST) records, though MDV itself has not received sufficient attention thus far. Here, we introduce four key episodes in GMST evolution, according to different phases of the MDV extracted by the ensemble empirical mode decomposition method from the ensemble HadCRUT4 monthly GMST time series. The “warming/cooling hiatus” and “typical warming/cooling” periods are defined as the 95% confidence intervals for the locations of local MDV maxima/minima and of their derivatives, respectively. Since 1850, the warming hiatuses, cooling hiatuses and typical warming have already occurred three times, and the typical cooling, twice. At present, the MDV is in its third warming hiatus period, which started in 2012 and would last until 2017, followed by a 30-year cooling episode, while the trend will sustain the current steady growth in the next 50 years. Their superposition presents ladder-like rising since 1850. It is currently ascending a new height and will stay there until the next warming phase of the MDV carries it higher.

Su, H., Wu, X., Yan, X.-H., and Kidwell, A., 2015, Estimation of subsurface temperature anomaly in the Indian Ocean during recent global surface warming hiatus from satellite measurements: A support vector machine approach: Remote Sensing of Environment, v. 160, p. 63-71, doi:10.1016/j.rse.2015.01.001. (Su et al., 2015)

Estimating the thermal information in the subsurface and deeper ocean from satellite measurements over large basin-wide scale is important but also challenging. This paper proposes a support vector machine (SVM) method to estimate subsurface temperature anomaly (STA) in the Indian Ocean from a suite of satellite remote sensing measurements including sea surface temperature anomaly (SSTA), sea surface height anomaly (SSHA), and sea surface salinity anomaly (SSSA). The SVM estimation of STA features the inclusion of in-situ Argo STA data for training and testing. SVM, one of the most popular machine learning methods, can well estimate the STA in the upper 1000 m of the Indian Ocean from satellite measurements of sea surface parameters (SSTA, SSHA and SSSA as input attributes for SVM). The results, based on the common SVM application of Support Vector Regression (SVR), were validated for accuracy and reliability using the Argo STA data. Both MSE and r2 for performance measures are improved after including SSSA for SVR (MSE decreased by 12% and r2 increased by 11% on average). The results showed that SSSA, in addition to SSTA and SSHA, is a useful parameter that can help detect and describe the deeper ocean thermal structure, as well as improve the STA estimation accuracy. Moreover, our method can provide a useful technique for studying subsurface and deeper ocean thermal variability which has played an important role in recent global surface warming hiatus since 1998, from satellite measurements in large basin-wide scale.

Roberts, C., Palmer, M., McNeall, D., and Collins, M., 2015, Quantifying the likelihood of a continued hiatus in global warming: Nature Climate Change, v. 5, p. 337–342, doi:10.1038/nclimate2531. (Roberts et al., 2015)

Since the end of the twentieth century, global mean surface temperature has not risen as rapidly as predicted by global climate models (GCMs). This discrepancy has become known as the global warming ‘hiatus’ and a variety of mechanisms1, have been proposed to explain the observed slowdown in warming. Focusing on internally generated variability, we use pre-industrial control simulations from an observationally constrained ensemble of GCMs and a statistical approach to evaluate the expected frequency and characteristics of variability-driven hiatus periods and their likelihood of future continuation. Given an expected forced warming trend of ~0.2 K per decade, our constrained ensemble of GCMs implies that the probability of a variability-driven 10-year hiatus is ~10%, but less than 1% for a 20-year hiatus. Although the absolute probability of a 20-year hiatus is small, the probability that an existing 15-year hiatus will continue another five years is much higher (up to 25%). Therefore, given the recognized contribution of internal climate variability to the reduced rate of global warming during the past 15 years, we should not be surprised if the current hiatus continues until the end of the decade. Following the termination of a variability-driven hiatus, we also show that there is an increased likelihood of accelerated global warming associated with release of heat from the sub-surface ocean and a reversal of the phase of decadal variability in the Pacific Ocean.

Johansson, D. J., O’Neill, B. C., Tebaldi, C., and Häggström, O., 2015, Equilibrium climate sensitivity in light of observations over the warming hiatus: Nature Climate Change, v. 5, p. 449–453 doi:10.1038/nclimate2573. (Johansson et al., 2015)

A key uncertainty in projecting future climate change is the magnitude of equilibrium climate sensitivity (ECS), that is, the eventual increase in global annual average surface temperature in response to a doubling of atmospheric CO₂ concentration. The lower bound of the likely range for ECS given in the IPCC Fifth Assessment Report (AR5; refs 1, 2) was revised downwards to 1.5 °C, from 2 °C in its previous report3, mainly as an effect of considering observations over the warming hiatus—the period of slowdown of global average temperature increase since the early 2000s. Here we analyse how estimates of ECS change as observations accumulate over time and estimate the contribution of potential causes to the hiatus. We find that including observations over the hiatus reduces the most likely value for ECS from 2.8 °C to 2.5 °C, but that the lower bound of the 90% range remains stable around 2 °C. We also find that the hiatus is primarily attributable to El Niño/Southern Oscillation-related variability and reduced solar forcing.

Delworth, T. L., Zeng, F., Rosati, A., Vecchi, G. A., and Wittenberg, A. T., 2015, A link between the hiatus in global warming and North American drought: Journal of Climate, v. 28, no. 9, p. 3834-3845, doi:10.1175/JCLI-D-14-00616.1. (Delworth et al., 2015)

Portions of western North America have experienced prolonged drought over the last decade. This drought has occurred at the same time as the global warming hiatus—a decadal period with little increase in global mean surface temperature. Climate models and observational analyses are used to clarify the dual role of recent tropical Pacific changes in driving both the global warming hiatus and North American drought. When observed tropical Pacific wind stress anomalies are inserted into coupled models, the simulations produce persistent negative sea surface temperature anomalies in the eastern tropical Pacific, a hiatus in global warming, and drought over North America driven by SST-induced atmospheric circulation anomalies. In the simulations herein the tropical wind anomalies account for 92% of the simulated North American drought during the recent decade, with 8% from anthropogenic radiative forcing changes. This suggests that anthropogenic radiative forcing is not the dominant driver of the current drought, unless the wind changes themselves are driven by anthropogenic radiative forcing. The anomalous tropical winds could also originate from coupled interactions in the tropical Pacific or from forcing outside the tropical Pacific. The model experiments suggest that if the tropical winds were to return to climatological conditions, then the recent tendency toward North American drought would diminish. Alternatively, if the anomalous tropical winds were to persist, then the impact on North American drought would continue; however, the impact of the enhanced Pacific easterlies on global temperature diminishes after a decade or two due to a surface reemergence of warmer water that was initially subducted into the ocean interior.

Maher, N., Gupta, A. S., and England, M. H., 2014, Drivers of decadal hiatus periods in the 20th and 21st centuries: Geophysical Research Letters, v. 41, no. 16, p. 5978-5986, 10.1002/2014GL060527. (Maher et al., 2014)

The latest generation of climate model simulations are used to investigate the occurrence of hiatus periods in global surface air temperature in the past and under two future warming scenarios. Hiatus periods are identified in three categories: (i) those due to volcanic eruptions, (ii) those associated with negative phases of the Interdecadal Pacific Oscillation (IPO), and (iii) those affected by anthropogenically released aerosols in the mid-twentieth century. The likelihood of future hiatus periods is found to be sensitive to the rate of change of anthropogenic forcing. Under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption. We further demonstrate that most nonvolcanic hiatuses across Coupled Model Intercomparison Project 5 (CMIP5) models are associated with enhanced cooling in the equatorial Pacific linked to the transition to a negative IPO phase.

Drijfhout, S., Blaker, A., Josey, S., Nurser, A., Sinha, B., and Balmaseda, M., 2014, Surface warming hiatus caused by increased heat uptake across multiple ocean basins: Geophysical Research Letters, v. 41, no. 22, p. 7868-7874, doi:10.1002/2014GL061456. (Drijfhout et al., 2014)

The first decade of the 21st century was characterized by a hiatus in global surface warming. Using ocean model hindcasts and reanalyses we show that heat uptake between the 1990s and 2000s increased by 0.7 ± 0.3W m⁻². Approximately 30% of the increase is associated with colder sea surface temperatures in the eastern Pacific. Other basins contribute via reduced heat loss to the atmosphere, in particular, the Southern and subtropical Indian Oceans (30%) and the subpolar North Atlantic (40%). A different mechanism is important at longer timescales (1960s–present) over which the Southern Annular Mode trended upward. In this period, increased ocean heat uptake has largely arisen from reduced heat loss associated with reduced winds over the Agulhas Return Current and southward displacement of Southern Ocean westerlies.

Huber, M., and Knutti, R., 2014, Natural variability, radiative forcing and climate response in the recent hiatus reconciled: Nature Geoscience, v. 7, p. 651–656, doi:10.1038/ngeo2228. (Huber, and Knutti, 2014)

Global mean surface warming over the past 15 years or so has been less than in earlier decades and than simulated by most climate models. Natural variability, a reduced radiative forcing, a smaller warming response to atmospheric carbon dioxide concentrations and coverage bias in the observations have been identified as potential causes. However, the explanations of the so-called ‘warming hiatus’ remain fragmented and the implications for long-term temperature projections are unclear. Here we estimate the contribution of internal variability associated with the El Niño/Southern Oscillation (ENSO) using segments of unforced climate model control simulations that match the observed climate variability. We find that ENSO variability analogous to that between 1997 or 1998 and 2012 leads to a cooling trend of about −0.06 °C. In addition, updated solar and stratospheric aerosol forcings from observations explain a cooling trend of similar magnitude (−0.07 °C). Accounting for these adjusted trends we show that a climate model of reduced complexity with a transient climate response of about 1.8°C is consistent with the temperature record of the past 15 years, as is the ensemble mean of the models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We conclude that there is little evidence for a systematic overestimation of the temperature response to increasing atmospheric CO₂ concentrations in the CMIP5 ensemble.

Kamae, Y., Shiogama, H., Watanabe, M., and Kimoto, M., 2014, Attributing the increase in Northern Hemisphere hot summers since the late 20th century: Geophysical Research Letters, v. 41, no. 14, p. 5192-5199, doi:10.1002/2014GL061062. (Kamae et al., 2014)

Anomalously high summertime temperatures have occurred with increasing frequency since the late 20th century. It is not clear why hot summers are becoming more frequent despite the recent slowdown in the rise in global surface air temperature. To examine factors affecting the historical variation in the frequency of hot summers over the Northern Hemisphere (NH), we conducted three sets of ensemble simulations with an atmospheric general circulation model. The model accurately reproduced interannual variation and long-term increase in the occurrence of hot summers. Decadal variabilities in the Pacific and Atlantic Oceans accounted for 43 ± 27% of the recent increase over the NH middle latitudes. In addition, direct influence of anthropogenic forcing also contributes to increasing the frequency since the late 20th century. The results suggest that the heat extremes can become more frequent in the coming decade even with the persistent slowdown in the global-mean surface warming.

Boisséson, E., Balmaseda, M., Abdalla, S., Källén, E., and Janssen, P., 2014, How robust is the recent strengthening of the Tropical Pacific trade winds?: Geophysical Research Letters, v. 41, no. 12, p. 4398-4405, doi:10.1002/2014GL060257. (Boisséson et al., 2014)

The persistent strengthening of the trade winds over the Pacific Ocean over the past 20 years has recently been proposed as a driver for the increase of ocean heat uptake linked to the hiatus in surface global warming. Crucial aspects in this argument are the reliability of the wind signal, usually derived from atmospheric reanalyses, and the ability of models to represent it. This study addresses these two aspects by comparing various observations with reanalyses and model integrations from the European Centre for Medium-Range Weather Forecasts system. We show that the strengthening of trades over the Pacific is a robust feature in several observational data sets as well as in the reanalyses based on full and limited sets of observations. The wind trend is also reproduced in an atmospheric model integration forced by sea surface temperature analysis, a result that opens the doors to further investigation on the nature of the changes.

Dong, L., and Zhou, T., 2014, The formation of the recent cooling in the eastern tropical Pacific Ocean and the associated climate impacts: A competition of global warming, IPO, and AMO: Journal of Geophysical Research: Atmospheres, v. 119, no. 19, p. 11,272-211,287, doi:10.1002/2013JD021395. (Dong, and Zhou, 2014)

The cooling trend in the eastern tropical Pacific sea surface temperature (SST) during 1979–2008 is examined by using a wide variety of data sets for the ocean and atmosphere. The results show that the cooling trend is statistically significant at the 10% level out of the equator rather than along the equator. Diagnostic analysis indicates that the SST cooling in the eastern tropical Pacific is resulted from a competition of global warming mode, Interdecadal Pacific Oscillation (IPO) mode, and Atlantic Multidecadal Oscillation (AMO) mode. The cooling trend is preliminarily dominated by the phase transition of IPO from positive to negative phases in the year around 1998/1999, which overwhelms the effect of global warming in past three decades. Quantitative estimates based on the average of four different SST data sets indicate that the global warming mode offsets more than half of the cooling effect of IPO mode. The phase transition of AMO during 1990s causes a weak warming trend in the eastern tropical Pacific and partly weakens the cooling, making the trend along the equator less significant. Climate impacts associated with global warming mode, IPO mode, and AMO mode are further examined. The surface air temperature cooling over the eastern tropical Pacific, the easterly wind anomaly along the Pacific equator, the enhanced zonal gradient in sea level pressure over tropical Pacific, and the increased precipitation over the Asian monsoon region during 1979–2008 are dominated by the phase transition of IPO.

Fyfe, J. C., and Gillett, N. P., 2014, Recent observed and simulated warming: Nature Climate Change, v. 4, no. 3, p. 150-151, doi: 10.1038/nclimate2111. (Fyfe, and Gillett, 2014)

Fyfe et al. showed that global warming over the past 20 years is significantly less than that calculated from 117 simulations of the climate by 37 models participating in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). This might be due to some combination of errors.

Watanabe et al., 2014, Contribution of natural decadal variability to global warming acceleration and hiatus: Nature Climate Change, v. 4, p. 893–897, doi: 10.1038/nclimate2355. (Watanabe et al., 2014)

Reasons for the apparent pause in the rise of global-mean surface air temperature (SAT) after the turn of the century has been a mystery, undermining confidence in climate projections1, 2, 3. Recent climate model simulations indicate this warming hiatus originated from eastern equatorial Pacific cooling4 associated with strengthening of trade winds5. Using a climate model that overrides tropical wind stress anomalies with observations for 1958–2012, we show that decadal-mean anomalies of global SAT referenced to the period 1961–1990 are changed by 0.11, 0.13 and −0.11 °C in the 1980s, 1990s and 2000s, respectively, without variation in human-induced radiative forcing. They account for about 47%, 38% and 27% of the respective temperature change. The dominant wind stress variability consistent with this warming/cooling represents the deceleration/acceleration of the Pacific trade winds, which can be robustly reproduced by atmospheric model simulations forced by observed sea surface temperature excluding anthropogenic warming components. Results indicate that inherent decadal climate variability contributes considerably to the observed global-mean SAT time series, but that its influence on decadal-mean SAT has gradually decreased relative to the rising anthropogenic warming signal.

Meehl, G. A., Teng, H., and Arblaster, J. M., 2014, Climate model simulations of the observed early-2000s hiatus of global warming: Nature Climate Change, v. 4, p. 898–902, doi:10.1038/nclimate2357. (Meehl et al., 2014)

The slowdown in the rate of global warming in the early 2000s is not evident in the multi-model ensemble average of traditional climate change projection simulations1. However, a number of individual ensemble members from that set of models successfully simulate the early-2000s hiatus when naturally-occurring climate variability involving the Interdecadal Pacific Oscillation (IPO) coincided, by chance, with the observed negative phase of the IPO that contributed to the early-2000s hiatus. If the recent methodology of initialized decadal climate prediction could have been applied in the mid-1990s using the Coupled Model Intercomparison Project Phase 5 multi-models, both the negative phase of the IPO in the early 2000s as well as the hiatus could have been simulated, with the multi-model average performing better than most of the individual models. The loss of predictive skill for six initial years before the mid-1990s points to the need for consistent hindcast skill to establish reliability of an operational decadal climate prediction system.

Watanabe, M., Shiogama, H., Tatebe, H., Hayashi, M., Ishii, M., and Kimoto, M., 2014, Contribution of natural decadal variability to global warming acceleration and hiatus: Nature Climate Change, v. 4, p. 893–897, doi:10.1038/nclimate2355. (Watanabe et al., 2014)

Reasons for the apparent pause in the rise of global-mean surface air temperature (SAT) after the turn of the century has been a mystery, undermining confidence in climate projections. Recent climate model simulations indicate this warming hiatus originated from eastern equatorial Pacific cooling associated with strengthening of trade winds. Using a climate model that overrides tropical wind stress anomalies with observations for 1958–2012, we show that decadal-mean anomalies of global SAT referenced to the period 1961–1990 are changed by 0.11, 0.13 and −0.11 °C in the 1980s, 1990s and 2000s, respectively, without variation in human-induced radiative forcing. They account for about 47%, 38% and 27% of the respective temperature change. The dominant wind stress variability consistent with this warming/cooling represents the deceleration/acceleration of the Pacific trade winds, which can be robustly reproduced by atmospheric model simulations forced by observed sea surface temperature excluding anthropogenic warming components. Results indicate that inherent decadal climate variability contributes considerably to the observed global-mean SAT time series, but that its influence on decadal-mean SAT has gradually decreased relative to the rising anthropogenic warming signal.

McGregor, S., Timmermann, A., Stuecker, M. F., England, M. H., Merrifield, M., Jin, F.-F., and Chikamoto, Y., 2014, Recent Walker circulation strengthening and Pacific cooling amplified by Atlantic warming: Nature Climate Change, v. 4, no. 10, p. 888-892, doi:10.1038/nclimate2330. (McGregor et al., 2014)

An unprecedented strengthening of Pacific trade winds since the late 1990s has caused widespread climate perturbations, including rapid sea-level rise in the western tropical Pacific, strengthening of Indo-Pacific ocean currents, and an increased uptake of heat in the equatorial Pacific thermocline1. The corresponding intensification of the atmospheric Walker circulation is also associated with sea surface cooling in the eastern Pacific, which has been identified as one of the contributors to the current pause in global surface warming. In spite of recent progress in determining the climatic impacts of the Pacific trade wind acceleration, the cause of this pronounced trend in atmospheric circulation remains unknown. Here we analyze a series of climate model experiments along with observational data to show that the recent warming trend in Atlantic sea surface temperature and the corresponding trans-basin displacements of the main atmospheric pressure centers were key drivers of the observed Walker circulation intensification, eastern Pacific cooling, North American rainfall trends and western Pacific sea-level rise. Our study suggests that global surface warming has been partly offset by the Pacific climate response to enhanced Atlantic warming since the early 1990s.

Sillmann, J., Donat, M. G., Fyfe, J. C., and Zwiers, F. W., 2014, Observed and simulated temperature extremes during the recent warming hiatus: Environmental Research Letters, v. 9, no. 6, p. 064023, doi:10.1088/1748-9326/9/6/064023. (Sillmann et al., 2014)

The discrepancy between recent observed and simulated trends in global mean surface temperature has provoked a debate about possible causes and implications for future climate change projections. However, little has been said in this discussion about observed and simulated trends in global temperature extremes. Here we assess trend patterns in temperature extremes and evaluate the consistency between observed and simulated temperature extremes over the past four decades (1971–2010) in comparison to the recent 15 years (1996–2010). We consider the coldest night and warmest day in a year in the observational dataset HadEX2 and in the current generation of global climate models (CMIP5). In general, the observed trends fall within the simulated range of trends, with better consistency for the longer period. Spatial trend patterns differ for the warm and cold extremes, with the warm extremes showing continuous positive trends across the globe and the cold extremes exhibiting a coherent cooling pattern across the Northern Hemisphere mid-latitudes that has emerged in the recent 15 years and is not reproduced by the models. This regional inconsistency between models and observations might be a key to understanding the recent hiatus in global mean temperature warming.

Chen, X., and Tung, K.-K., 2014, Varying planetary heat sink led to global-warming slowdown and acceleration: Science, v. 345, no. 6199, p. 897-903, doi:10.1126/science.1254937. (Chen, and Tung, 2014)

A vacillating global heat sink at intermediate ocean depths is associated with different climate regimes of surface warming under anthropogenic forcing: The latter part of the 20th century saw rapid global warming as more heat stayed near the surface. In the 21st century, surface warming slowed as more heat moved into deeper oceans. In situ and reanalyzed data are used to trace the pathways of ocean heat uptake. In addition to the shallow La Niña–like patterns in the Pacific that were the previous focus, we found that the slowdown is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans, initiated by a recurrent salinity anomaly in the subpolar North Atlantic. Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years.

Santer, B. D., Bonfils, C., Painter, J. F., Zelinka, M. D., Mears, C., Solomon, S., Schmidt, G. A., Fyfe, J. C., Cole, J. N., and Nazarenko, L., 2014, Volcanic contribution to decadal changes in tropospheric temperature: Nature Geoscience, v. 7, no. 3, p. 185-189, doi:10.1038/ngeo2098. (Santer et al., 2014)

Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previously. Possible explanations for the slow-down include internal climate variability, external cooling influences and observational errors. Several recent modelling studies have examined the contribution of early twenty-first-century volcanic eruptions to the muted surface warming. Here we present a detailed analysis of the impact of recent volcanic forcing on tropospheric temperature, based on observations as well as climate model simulations. We identify statistically significant correlations between observations of stratospheric aerosol optical depth and satellite-based estimates of both tropospheric temperature and short-wave fluxes at the top of the atmosphere. We show that climate model simulations without the effects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998. In two simulations with more realistic volcanic influences following the 1991 Pinatubo eruption, differences between simulated and observed tropospheric temperature trends over the period 1998 to 2012 are up to 15% smaller, with large uncertainties in the magnitude of the effect. To reduce these uncertainties, better observations of eruption-specific properties of volcanic aerosols are needed, as well as improved representation of these eruption-specific properties in climate model simulations.

Song, Y., Yu, Y., and Lin, P., 2014, The hiatus and accelerated warming decades in CMIP5 simulations: Advances in Atmospheric Sciences, v. 31, no. 6, p. 1316-1330, doi:10.1007/s00376-014-3265-6. (Song et al., 2014)

Observed hiatus or accelerated warming phenomena are compared with numerical simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archives, and the associated physical mechanisms are explored based on the CMIP5 models. Decadal trends in total ocean heat content (OHC) are strongly constrained by net top-of-atmosphere (TOA) radiation. During hiatus decades, most CMIP5 models exhibit a significant decrease in the SST and upper OHC and a significant increase of heat penetrating into the subsurface or deep ocean, opposite to the accelerated warming decades. The shallow meridional overturning of the Pacific subtropical cell experiences a significant strengthening (slowdown) for the hiatus (accelerated warming) decades associated with the strengthened (weakened) trade winds over the tropical Pacific. Both surface heating and ocean dynamics contribute to the decadal changes in SST over the Indian Ocean, and the Indonesian Throughflow has a close relationship with the changes of subsurface temperature in the Indian Ocean. The Atlantic Meridional Overturing Circulation (Antarctic Bottom Water) tends to weaken (strengthen) during hiatus decades, opposite to the accelerated warming decades. In short, the results highlight the important roles of air-sea interactions and ocean circulations for modulation of surface and subsurface temperature.

Trenberth, K. E., Fasullo, J. T., Branstator, G., and Phillips, A. S., 2014, Seasonal aspects of the recent pause in surface warming: Nature Climate Change, v. 4, no. 10, p. 911-916, doi:10.1038/nclimate2341. (Trenberth et al., 2014)

Factors involved in the recent pause in the rise of global mean temperatures are examined seasonally. For 1999 to 2012, the hiatus in surface warming is mainly evident in the central and eastern Pacific. It is manifested as strong anomalous easterly trade winds, distinctive sea-level pressure patterns, and large rainfall anomalies in the Pacific, which resemble the Pacific Decadal Oscillation (PDO). These features are accompanied by upper tropospheric teleconnection wave patterns that extend throughout the Pacific, to polar regions, and into the Atlantic. The extratropical features are particularly strong during winter. By using an idealized heating to force a comprehensive atmospheric model, the large negative anomalous latent heating associated with the observed deficit in central tropical Pacific rainfall is shown to be mainly responsible for the global quasi-stationary waves in the upper troposphere. The wave patterns in turn created persistent regional climate anomalies, increasing the odds of cold winters in Europe. Hence, tropical Pacific forcing of the atmosphere such as that associated with a negative phase of the PDO produces many of the pronounced atmospheric circulation anomalies observed globally during the hiatus.

Ridley, D. A., Solomon, S., Barnes, J. E., Burlakov, V. D., Deshler5, T., Dolgii, S. I., Herber, A. B., Nagai, T., III, R. R. N., Nevzorov, A. V., Ritter, C., Sakai, T., Santer, B. D., Sato, M., Schmidt, A., O.Uchino, and Vernier, J. P., 2014, Total volcanic stratospheric aerosol optical depths and implications for global climate change: Geophysical Research Letters, v. 41, no. 22, p. 7763–7769, doi:10.1002/2014GL061541. (Ridley et al., 2014)

Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth (AOD) above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, AERONET and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at mid to high latitudes, and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be -0.19 ± 0.09 W m⁻². This translates into an estimated global cooling of 0.05 to 0.12°C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.

Franzke, C. L., 2014, Warming trends: Nonlinear climate change: Nature Climate Change, v. 4, no. 6, p. 423-424, doi:10.1038/nclimate2245. (Franzke, 2014)

Most studies assume that temperature trends are linear. Now, research demonstrates that warming trends are nonlinear, that warming accelerated over most of the twentieth century and is much stronger since 1980 than calculated by linear methods.

Clement, A., and DiNezio, P., 2014, The Tropical Pacific Ocean—Back in the Driver’s Seat?: Science, v. 343, no. 6174, p. 976-978, doi:10.1126/science.1248115. (Clement, and DiNezio, 2014)

First paragraph: Average temperatures at Earth’s surface are now higher than they were in the mid-19th century, but the rate of warming has not been steady. A pause in surface warming in the mid-20th century coincided with increases in the atmospheric concentrations of sulfate aerosols, which are generally understood to cool the planet. Surface warming resumed in the 1970s, when strong pollution controls were implemented in developed countries. Thus, a balance of warming by greenhouse gases and cooling by aerosols may explain the variable rates of surface warming in the past century. A pause in global warming since 2000—a global warming “hiatus”—has opened up new questions about natural and human activity-driven (anthropogenic) effects on global mean trends in surface temperature. Recent studies point to the importance of the tropical Pacific in driving these changes.

Meehl, G. A., and Teng, H., 2014, CMIP5 multi‐model hindcasts for the mid‐1970s shift and early 2000s hiatus and predictions for 2016–2035: Geophysical Research Letters, v. 41, no. 5, p. 1711-1716, doi:10.1002/2014GL059256. (Meehl, and Teng, 2014)

Compared to uninitialized climate change projections, a multi-model ensemble from the CMIP5 10 year decadal prediction experiments produces more warming during the mid-1970s climate shift and less warming in the early 2000s hiatus in both the tropical Indo-Pacific region and globally averaged surface air temperature (TAS) in closer agreement with observations. Assuming bias in TAS has stabilized in the 10 year predictions, after bias adjustment, TAS anomalies for the 2016–2035 period in the 30 year predictions initialized in 2006 are about 16% less than the uninitialized projections. One contributing factor for the improved climate simulation is the bias adjustment, which corrects the models’ systematic errors and higher-than-observed decadal warming trend. Another important factor is the initialization with observations which constrains the ocean such that the starting points of the initialized simulations are close to the observed initial states.

England, M. H., McGregor, S., Spence, P., Meehl, G. A., Timmermann, A., Cai, W., Gupta, A. S., McPhaden, M. J., Purich, A., and Santoso, A., 2014, Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus: Nature Climate Change, v. 4, no. 3, p. 222-227, doi:10.1038/nclimate2106. (England et al., 2014)

Despite ongoing increases in atmospheric greenhouse gases, the Earth’s global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing. Here we show that a pronounced strengthening in Pacific trade winds over the past two decades—unprecedented in observations/reanalysis data and not captured by climate models—is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake. The extra uptake has come about through increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline. At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering sea surface temperature there, which drives further cooling in other regions. The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2 °C, which can account for much of the hiatus in surface warming observed since 2001. This hiatus could persist for much of the present decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate.

Kosaka, Y., 2014, Atmospheric science: Increasing wind sinks heat: Nature Climate Change, v. 4, no. 3, p. 172-173, doi:10.1038/nclimate2138. (Kosaka, 2014)

Surface global warming has stalled since around 2000 despite increasing atmospheric CO₂. A study finds that recent strengthening of Pacific trade winds has enhanced heat transport from the surface to ocean depths, explaining most of the slowed surface warming.

Kintisch, E., 2014, Is Atlantic holding Earth’s missing heat?: Science, v. 345, no. 6199, p. 860-861, doi:10.1126/science.345.6199.860. (Kintisch, 2014)

First paragraph: rmchair detectives might call it the case of Earth’s missing heat: Why have average global surface air temperatures remained essentially steady since 2000, even as greenhouse gases have continued to accumulate in the atmosphere? The suspects include changes in atmospheric water vapor, a strong greenhouse gas, or the noxious sunshade of haze emanating from factories. Others believe the culprit is the mighty Pacific Ocean, which has been sending vast slugs of cold bottom water to the surface. But two fresh investigations finger a new suspect: the Atlantic Ocean. One study, in this issue of Science, presents sea temperature data implying that most of the missing heat has been stored deep in the Atlantic. The other, published online in Nature Climate Change, suggests a warming Atlantic is abetting the Pacific by driving wind patterns that help that ocean cool the atmosphere. But some climate specialists remain skeptical. In a third recent paper, also published online in Nature Climate Change, other researchers argue that the Pacific remains the kingpin. One reason some scientists remain convinced the Pacific is behind the hiatus is a measured speedup in trade winds that drive a massive upwelling of cold water in the eastern Pacific. But there, too, the Atlantic may be responsible, modeling experiments suggest. A consensus about what has put global warming on pause may be years away, but one scientist says the recent papers confirm that Earth’s warming has continued during the hiatus, at least in the ocean depths, if not in the air.

Held, I. M., 2013, Climate science: The cause of the pause: Nature, v. 501, no. 7467, p. 318-319, doi:10.1038/501318a. (Held, 2013)

A global climate model that factors in the observed temperature of the surface ocean in the eastern equatorial Pacific offers an explanation for the recent hiatus in global warming.

Trenberth, K. E., and Fasullo, J. T., 2013, An apparent hiatus in global warming?: Earth’s Future, v. 1, no. 1, p. 19-32, doi:10.1002/2013EF000165. (Trenberth, and Fasullo, 2013)

Global warming first became evident beyond the bounds of natural variability in the 1970s, but increases in global mean surface temperatures have stalled in the 2000s. Increases in atmospheric greenhouse gases, notably carbon dioxide, create an energy imbalance at the top-of-atmosphere (TOA) even as the planet warms to adjust to this imbalance, which is estimated to be 0.5–1 W m⁻² over the 2000s. Annual global fluctuations in TOA energy of up to 0.2 W m⁻² occur from natural variations in clouds, aerosols, and changes in the Sun. At times of major volcanic eruptions the effects can be much larger. Yet global mean surface temperatures fluctuate much more than these can account for. An energy imbalance is manifested not just as surface atmospheric or ground warming but also as melting sea and land ice, and heating of the oceans. More than 90% of the heat goes into the oceans and, with melting land ice, causes sea level to rise. For the past decade, more than 30% of the heat has apparently penetrated below 700 m depth that is traceable to changes in surface winds mainly over the Pacific in association with a switch to a negative phase of the Pacific Decadal Oscillation (PDO) in 1999. Surface warming was much more in evidence during the 1976–1998 positive phase of the PDO, suggesting that natural decadal variability modulates the rate of change of global surface temperatures while sea-level rise is more relentless. Global warming has not stopped; it is merely manifested in different ways.

Balmaseda, M. A., Trenberth, K. E., and Källén, E., 2013, Distinctive climate signals in reanalysis of global ocean heat content: Geophysical Research Letters, v. 40, no. 9, p. 1754-1759, doi:10.1002/grl.50382. (Balmaseda et al., 2013)

The elusive nature of the post-2004 upper ocean warming has exposed uncertainties in the ocean’s role in the Earth’s energy budget and transient climate sensitivity. Here we present the time evolution of the global ocean heat content for 1958 through 2009 from a new observation-based reanalysis of the ocean. Volcanic eruptions and El Niño events are identified as sharp cooling events punctuating a long-term ocean warming trend, while heating continues during the recent upper-ocean-warming hiatus, but the heat is absorbed in the deeper ocean. In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend. The warming below 700 m remains even when the Argo observing system is withdrawn although the trends are reduced. Sensitivity experiments illustrate that surface wind variability is largely responsible for the changing ocean heat vertical distribution.

Watanabe, M., Kamae, Y., Yoshimori, M., Oka, A., Sato, M., Ishii, M., Mochizuki, T., and Kimoto, M., 2013, Strengthening of ocean heat uptake efficiency associated with the recent climate hiatus: Geophysical Research Letters, v. 40, no. 12, p. 3175-3179, doi:10.1002/grl.50541. (Watanabe et al., 2013)

The rate of increase of global-mean surface air temperature (SATg) has apparently slowed during the last decade. We investigated the extent to which state-of-the-art general circulation models (GCMs) can capture this hiatus period by using multimodel ensembles of historical climate simulations. While the SATg linear trend for the last decade is not captured by their ensemble means regardless of differences in model generation and external forcing, it is barely represented by an 11-member ensemble of a GCM, suggesting an internal origin of the hiatus associated with active heat uptake by the oceans. Besides, we found opposite changes in ocean heat uptake efficiency (κ), weakening in models and strengthening in nature, which explain why the models tend to overestimate the SATg trend. The weakening of κ commonly found in GCMs seems to be an inevitable response of the climate system to global warming, suggesting the recovery from hiatus in coming decades.

Kosaka, Y., and Xie, S.-P., 2013, Recent global-warming hiatus tied to equatorial Pacific surface cooling: Nature, doi:10.1038/nature12534. (Kosaka, and Xie, 2013)

Despite the continued increase in atmospheric greenhouse gas concentrations, the annual-mean global temperature has not risen in the twenty-first century, challenging the prevailing view that anthropogenic forcing causes climate warming. Various mechanisms have been proposed for this hiatus in global warming, but their relative importance has not been quantified, hampering observational estimates of climate sensitivity. Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model. Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970–2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase.

Meehl, G. A., Hu, A., Arblaster, J., Fasullo, J., and Trenberth, K. E., 2013, Externally forced and internally generated decadal climate variability associated with the Interdecadal Pacific Oscillation: Journal of Climate, v. 26, no. 18, p. 7298-7310, doi:10.1175/JCLI-D-12-00548.1. (Meehl et al., 2013)

Globally averaged surface air temperatures in some decades show rapid increases (accelerated warming decades), and in other decades there is no warming trend (hiatus decades). A previous study showed that the net energy imbalance at the top of the atmosphere of about 1 W m⁻² is associated with greater increases of deep ocean heat content below 750 m during the hiatus decades, while there is little globally averaged surface temperature increase or warming in the upper ocean layers. Here the authors examine processes involved with accelerated warming decades and address theFW m relative roles of external forcing from increasing greenhouse gases and internally generated decadal climate variability associated with interdecadal Pacific oscillation (IPO). Model results from the Community Climate System Model, version 4 (CCSM4), show that accelerated warming decades are characterized by rapid warming of globally averaged surface air temperature, greater increases of heat content in the upper ocean layers, and less heat content increase in the deep ocean, opposite to the hiatus decades. In addition to contributions from processes potentially linked to Antarctic Bottom Water (AABW) formation and the Atlantic meridional overturning circulation (AMOC), the positive phase of the IPO, adding to the response to external forcing, is usually associated with accelerated warming decades. Conversely, hiatus decades typically occur with the negative phase of the IPO, when warming from the external forcing is overwhelmed by internally generated cooling in the tropical Pacific. Internally generated hiatus periods of up to 15 years with zero global warming trend are present in the future climate simulations. This suggests that there is a chance that the current observed hiatus could extend for several more years.

de Elía, R., Biner, S., and Frigon, A., 2013, Interannual variability and expected regional climate change over North America: Climate Dynamics, v. 41, no. 5-6, p. 1245-1267, doi:10.1007/s00382-013-1717-9. (de Elía et al., 2013)

This study aims to analyze the interannual variability simulated by several regional climate models (RCMs), and its potential for disguising the effect of seasonal temperature increases due to greenhouse gases. In order to accomplish this, we used an ensemble of regional climate change projections over North America belonging to the North American Regional Climate Change Program, with an additional pair of 140-year continuous runs from the Canadian RCM. We find that RCM-simulated interannual variability shows important departures from observed one in some cases, and also from the driving models’ variability, while the expected climate change signal coincides with estimations presented in previous studies. The continuous runs from the Canadian RCM were used to illustrate the effect of interannual variability in trend estimation for horizons of a decade or more. As expected, it can contribute to the existence of transitory cooling trends over a few decades, embedded within the expected long-term warming trends. A new index related to signal-to-noise ratio was developed to evaluate the expected number of years it takes for the warming trend to emerge from interannual variability. Our results suggest that detection of the climate change signal is expected to occur earlier in summer than in winter almost everywhere, despite the fact that winter temperature generally has a much stronger climate change signal. In particular, we find that the province of Quebec and northwestern Mexico may possibly feel climate change in winter earlier than elsewhere in North America. Finally, we show that the spatial and temporal scales of interest are fundamental for our capacity of discriminating climate change from interannual variability.

Guemas, V., Doblas-Reyes, F. J., Andreu-Burillo, I., and Asif, M., 2013, Retrospective prediction of the global warming slowdown in the past decade: Nature Climate Change, v. 3, p. 649–653, doi:10.1038/nclimate1863. (Guemas et al., 2013)

Despite a sustained production of anthropogenic greenhouse gases, the Earth’s mean near-surface temperature paused its rise during the 2000–2010 period. To explain such a pause, an increase in ocean heat uptake below the superficial ocean layer has been proposed to overcompensate for the Earth’s heat storage. Contributions have also been suggested from the deep prolonged solar minimum, the stratospheric water vapour, the stratospheric and tropospheric aerosols. However, a robust attribution of this warming slowdown has not been achievable up to now. Here we show successful retrospective predictions of this warming slowdown up to 5 years ahead, the analysis of which allows us to attribute the onset of this slowdown to an increase in ocean heat uptake. Sensitivity experiments accounting only for the external radiative forcings do not reproduce the slowdown. The top-of-atmosphere net energy input remained in the [0.5–1] W m⁻² interval during the past decade, which is successfully captured by our predictions. Most of this excess energy was absorbed in the top 700 m of the ocean at the onset of the warming pause, 65% of it in the tropical Pacific and Atlantic oceans. Our results hence point at the key role of the ocean heat uptake in the recent warming slowdown. The ability to predict retrospectively this slowdown not only strengthens our confidence in the robustness of our climate models, but also enhances the socio-economic relevance of operational decadal climate predictions.

Smith, D., 2013, Oceanography: Has global warming stalled?: Nature Climate Change, v. 3, no. 7, p. 618-619, doi:10.1038/nclimate1938. (Smith, 2013)

Following a period of rapid warming from the 1970s, global temperatures seem to have stalled. New analysis of the uptake of heat by the upper ocean sheds light on the cause and suggests that the slowdown could have been predicted.

Fyfe, J. C., Gillett, N. P., and Zwiers, F. W., 2013, Overestimated global warming over the past 20 years: Nature Climate Change, v. 3, no. 9, p. 767-769, doi:10.1038/nclimate1972. (Fyfe et al., 2013)

Recent observed global warming is significantly less than that simulated by climate models. This difference might be explained by some combination of errors in external forcing, model response and internal climate variability.

Benestad, R., 2012, Reconciliation of global temperatures: Environmental Research Letters, v. 7, p. 011002 doi:10.1088/1748-9326/7/1/011002. (Benestad, 2012)

Basically promotes Foster and Rahmstorf (2011). First paragraph: In recent years there has been a public debate about whether the rate of global warming has waned, prompting the paper ‘Is the climate warming or cooling?’ in Geophysical Research Letters by Easterling and Wehner (2009). This question has also attracted attention in wider scientific circles, and in a recent paper in Science, Solomon et al (2010) suggested that a decrease in stratospheric water vapour concentrations has slowed the global surface temperature rate between 2000 and 2009. Yet another study by Kaufmann et al (2011) argued that the ‘hiatus’ in the global warming coincided with near constant combined anthropogenic and natural forcings. The reason: a declining solar insolation, a shift to La Niña conditions and a rapid growth in short-lived sulfur emissions have masked the effect from rising greenhouse gas concentrations (GHGs).

Douglass, D., and Knox, R., 2012, Ocean heat content and Earthʼs radiation imbalance. II. Relation to climate shifts: Physics Letters A, v. 376, no. 14, p. 1226-1229, doi:10.1016/j.physleta.2012.02.027. (Douglass, and Knox, 2012)

In an earlier study of ocean heat content (OHC) we showed that Earthʼs empirically implied radiation imbalance has undergone abrupt changes. Other studies have identified additional such climate shifts since 1950. The shifts can be correlated with features in recently updated OHC data. The implied radiation imbalance may possibly alternate in sign at dates close to the climate shifts. The most recent shifts occurred during 2001–2002 and 2008–2009. The implied radiation imbalance between these dates, in the direction of ocean heat loss, was −0.03±0.06 W m⁻², with a possible systematic error of [−0.00,+0.09] W m⁻².

Loeb, N. G., Lyman, J. M., Johnson, G. C., Allan, R. P., Doelling, D. R., Wong, T., Soden, B. J., and Stephens, G. L., 2012, Observed changes in top-of-the-atmosphere radiation and upper-ocean heating consistent within uncertainty: Nature Geoscience, v. 5, no. 2, p. 110-113, doi:10.1038/ngeo1375. (Loeb et al., 2012)

Global climate change results from a small yet persistent imbalance between the amount of sunlight absorbed by Earth and the thermal radiation emitted back to space. An apparent inconsistency has been diagnosed between interannual variations in the net radiation imbalance inferred from satellite measurements and upper-ocean heating rate from in situ measurements, and this inconsistency has been interpreted as ‘missing energy’ in the system. Here we present a revised analysis of net radiation at the top of the atmosphere from satellite data, and we estimate ocean heat content, based on three independent sources. We find that the difference between the heat balance at the top of the atmosphere and upper-ocean heat content change is not statistically significant when accounting for observational uncertainties in ocean measurements, given transitions in instrumentation and sampling. Furthermore, variability in Earth’s energy imbalance relating to El Niño-Southern Oscillation is found to be consistent within observational uncertainties among the satellite measurements, a reanalysis model simulation and one of the ocean heat content records. We combine satellite data with ocean measurements to depths of 1,800 m, and show that between January 2001 and December 2010, Earth has been steadily accumulating energy at a rate of 0.50±0.43 Wm⁻² (uncertainties at the 90% confidence level). We conclude that energy storage is continuing to increase in the sub-surface ocean.

Loeb, N. G., Lyman, J. M., Johnson, G. C., Allan, R. P., Doelling, D. R., Wong, T., Soden, B. J., and Stephens, G. L., 2012, Observed changes in top-of-the-atmosphere radiation and upper-ocean heating consistent within uncertainty: Nature Geoscience, v. 5, no. 2, p. 110-113, doi:10.1038/ngeo1375. (Loeb et al., 2012)

Global climate change results from a small yet persistent imbalance between the amount of sunlight absorbed by Earth and the thermal radiation emitted back to space1. An apparent inconsistency has been diagnosed between interannual variations in the net radiation imbalance inferred from satellite measurements and upper-ocean heating rate from in situ measurements, and this inconsistency has been interpreted as ‘missing energy’ in the system2. Here we present a revised analysis of net radiation at the top of the atmosphere from satellite data, and we estimate ocean heat content, based on three independent sources. We find that the difference between the heat balance at the top of the atmosphere and upper-ocean heat content change is not statistically significant when accounting for observational uncertainties in ocean measurements3, given transitions in instrumentation and sampling. Furthermore, variability in Earth’s energy imbalance relating to El Niño-Southern Oscillation is found to be consistent within observational uncertainties among the satellite measurements, a reanalysis model simulation and one of the ocean heat content records. We combine satellite data with ocean measurements to depths of 1,800 m, and show that between January 2001 and December 2010, Earth has been steadily accumulating energy at a rate of 0.50±0.43 Wm−2 (uncertainties at the 90% confidence level). We conclude that energy storage is continuing to increase in the sub-surface ocean.

Meehl, G. A., and Teng, H., 2012, Case studies for initialized decadal hindcasts and predictions for the Pacific region: Geophysical Research Letters, v. 39, no. 22, doi:10.1029/2012GL053423. (Meehl, and Teng, 2012)

Case studies involving notable past decadal climate variability are analyzed for the mid-1970s climate shift, when the tropical Pacific warmed over a decade and globally averaged temperature rapidly increased, and the early 2000s hiatus when the tropical Pacific cooled over a decade and global temperatures warmed little. Ten year hindcasts following the CMIP5 decadal climate prediction experiment design are analyzed for those two periods using two different initialization techniques in a global coupled climate model, the CCSM4. There is additional skill in the initialized hindcasts for surface temperature patterns over the Pacific region for those two case studies over and above that in free-running historical simulations with the same model. A 30 year hindcast also shows added skill over the Pacific compared to the historical simulations. A 30 year prediction from the initialized model simulations shows less global warming for the 2016–2035 period than the free-running model projection for that same time period.

Scafetta, N., 2012, Testing an astronomically based decadal-scale empirical harmonic climate model versus the general circulation climate models: Journal of Atmospheric and Solar-Terrestrial Physics, v. 80, p. 124-137, doi:10.1016/j.jastp.2011.12.005. (Scafetta, 2012)

We compare the performance of a recently proposed empirical climate model based on astronomical harmonics against all CMIP3 available general circulation climate models (GCM) used by the IPCC (2007) to interpret the 20th century global surface temperature. The proposed astronomical empirical climate model assumes that the climate is resonating with, or synchronized to a set of natural harmonics that, in previous works (Scafetta, 2010b, 2011b), have been associated to the solar system planetary motion, which is mostly determined by Jupiter and Saturn. We show that the GCMs fail to reproduce the major decadal and multidecadal oscillations found in the global surface temperature record from 1850 to 2011. On the contrary, the proposed harmonic model (which herein uses cycles with 9.1, 10–10.5, 20–21, 60–62 year periods) is found to well reconstruct the observed climate oscillations from 1850 to 2011, and it is shown to be able to forecast the climate oscillations from 1950 to 2011 using the data covering the period 1850–1950, and vice versa. The 9.1-year cycle is shown to be likely related to a decadal Soli/Lunar tidal oscillation, while the 10–10.5, 20–21 and 60–62 year cycles are synchronous to solar and heliospheric planetary oscillations. We show that the IPCC GCM’s claim that all warming observed from 1970 to 2000 has been anthropogenically induced is erroneous because of the GCM failure in reconstructing the quasi 20-year and 60-year climatic cycles. Finally, we show how the presence of these large natural cycles can be used to correct the IPCC projected anthropogenic warming trend for the 21st century. By combining this corrected trend with the natural cycles, we show that the temperature may not significantly increase during the next 30 years mostly because of the negative phase of the 60-year cycle. If multisecular natural cycles (which according to some authors have significantly contributed to the observed 1700–2010 warming and may contribute to an additional natural cooling by 2100) are ignored, the same IPCC projected anthropogenic emissions would imply a global warming by about 0.3–1.2 °C by 2100, contrary to the IPCC 1.0–3.6 °C projected warming. The results of this paper reinforce previous claims that the relevant physical mechanisms that explain the detected climatic cycles are still missing in the current GCMs and that climate variations at the multidecadal scales are astronomically induced and, in first approximation, can be forecast.

Solomon, S., Daniel, J., Neely, R., Vernier, J. P., Dutton, E., and Thomason, L., 2011, The Persistently Variable “Background” Stratospheric Aerosol Layer and Global Climate Change: Science, v. 333, no. 6044, p. 866-870. (Solomon et al., 2011)

Recent measurements demonstrate that the “background” stratospheric aerosol layer is persistently variable rather than constant, even in the absence of major volcanic eruptions. Several independent data sets show that stratospheric aerosols have increased in abundance since 2000. Near-global satellite aerosol data imply a negative radiative forcing due to stratospheric aerosol changes over this period of about –0.1 watt per square meter, reducing the recent global warming that would otherwise have occurred. Observations from earlier periods are limited but suggest an additional negative radiative forcing of about –0.1 watt per square meter from 1960 to 1990. Climate model projections neglecting these changes would continue to overestimate the radiative forcing and global warming in coming decades if these aerosols remain present at current values or increase.

Meehl, G. A., Arblaster, J. M., Fasullo, J. T., Hu, A., and Trenberth, K. E., 2011, Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods: Nature Climate Change, v. 1, no. 7, p. 360-364, doi:10.1038/NCLIMATE1229. (Meehl et al., 2011)

There have been decades, such as 2000–2009, when the observed globally averaged surface-temperature time series shows little increase or even a slightly negative trend1 (a hiatus period). However, the observed energy imbalance at the top-of-atmosphere for this recent decade indicates that a net energy flux into the climate system of about 1 W m⁻² (refs 2, 3) should be producing warming somewhere in the system4, 5. Here we analyze twenty-first-century climate-model simulations that maintain a consistent radiative imbalance at the top-of-atmosphere of about 1 W m⁻² as observed for the past decade. Eight decades with a slightly negative global mean surface-temperature trend show that the ocean above 300 m takes up significantly less heat whereas the ocean below 300 m takes up significantly more, compared with non-hiatus decades. The model provides a plausible depiction of processes in the climate system causing the hiatus periods, and indicates that a hiatus period is a relatively common climate phenomenon and may be linked to La Niña-like conditions.

Hansen, J., Sato, M., Kharecha, P., and von Schuckmann, K., 2011, Earth’s energy imbalance and implications: Atmospheric Chemnistry and Physics, v. 11, p. 13421-13449, doi:10.5194/acp-11-13421-2011. (Hansen et al., 2011)

Improving observations of ocean heat content show that Earth is absorbing more energy from the Sun than it is radiating to space as heat, even during the recent solar minimum. The inferred planetary energy imbalance, 0.58±0.15Wm−2 during the 6-yr period 2005–2010, confirms the dominant role of the human-made greenhouse effect in driving global climate change. Observed surface temperature change and ocean heat gain together constrain the net climate forcing and ocean mixing rates. We conclude that most climate models mix heat too efficiently into the deep ocean and as a result underestimate the negative forcing by human-made aerosols. Aerosol climate forcing today is inferred to be −1.6±0.3Wm−2, implying substantial aerosol indirect climate forcing via cloud changes. Continued failure to quantify the specific origins of this large forcing is untenable, as knowledge of changing aerosol effects is needed to understand future climate change. We conclude that recent slowdown of ocean heat uptake was caused by a delayed rebound effect from Mount Pinatubo aerosols and a deep prolonged solar minimum. Observed sea level rise during the Argo float era is readily accounted for by ice melt and ocean thermal expansion, but the ascendency of ice melt leads us to anticipate acceleration of the rate of sea level rise this decade.

Katsman, C., and van Oldenborgh, G. J., 2011, Tracing the upper ocean’s ‘missing heat’: Geophys. Res. Lett, v. 38, p. L14610, doi:10.1029/2011GL048417. (Katsman, and van Oldenborgh, 2011)

Over the period 2003–2010, the upper ocean has not gained any heat, despite the general expectation that the ocean will absorb most of the Earth’s current radiative imbalance. Answering to what extent this heat was transferred to other components of the climate system and by what process(-es) gets to the essence of understanding climate change. Direct heat flux observations are too inaccurate to assess such exchanges. In this study we therefore trace these heat budget variations by analyzing an ensemble of climate model simulations. The analysis reveals that an 8-yr period without upper ocean warming is not exceptional. It is explained by increased radiation to space (45%), largely as a result of El Niño variability on decadal timescales, and by increased ocean warming at larger depths (35%), partly due to a decrease in the strength of the Atlantic meridional overturning circulation. Recently-observed changes in these two large-scale modes of climate variability point to an upcoming resumption of the upward trend in upper ocean heat content.

Foster, G., and Rahmstorf, S., 2011, Global temperature evolution 1979–2010: Environmental Research Letters, v. 6, p. 044022, doi:10.1088/1748-9326/6/4/044022. (Foster, and Rahmstorf, 2011)

We analyze five prominent time series of global temperature (over land and ocean) for their common time interval since 1979: three surface temperature records (from NASA/GISS, NOAA/NCDC and HadCRU) and two lower-troposphere (LT) temperature records based on satellite microwave sensors (from RSS and UAH). All five series show consistent global warming trends ranging from 0.014 to 0.018 K yr-1. When the data are adjusted to remove the estimated impact of known factors on short-term temperature variations (El Niño/southern oscillation, volcanic aerosols and solar variability), the global warming signal becomes even more evident as noise is reduced. Lower-troposphere temperature responds more strongly to El Niño/southern oscillation and to volcanic forcing than surface temperature data. The adjusted data show warming at very similar rates to the unadjusted data, with smaller probable errors, and the warming rate is steady over the whole time interval. In all adjusted series, the two hottest years are 2009 and 2010.

Church, J. A., White, N. J., Konikow, L. F., Domingues, C. M., Cogley, J. G., Rignot, E., Gregory, J. M., van den Broeke, M. R., Monaghan, A. J., and Velicogna, I., 2011, Revisiting the Earth’s sea-level and energy budgets from 1961 to 2008: Geophysical Research Letters, v. 38, no. 18, p. L18601. (Church et al., 2011)

We review the sea-level and energy budgets together from 1961, using recent and updated estimates of all terms. From 1972 to 2008, the observed sea-level rise (1.8 ± 0.2 mm yr−1 from tide gauges alone and 2.1 ± 0.2 mm yr−1 from a combination of tide gauges and altimeter observations) agrees well with the sum of contributions (1.8 ± 0.4 mm yr−1) in magnitude and with both having similar increases in the rate of rise during the period. The largest contributions come from ocean thermal expansion (0.8 mm yr−1) and the melting of glaciers and ice caps (0.7 mm yr−1), with Greenland and Antarctica contributing about 0.4 mm yr−1. The cryospheric contributions increase through the period (particularly in the 1990s) but the thermosteric contribution increases less rapidly. We include an improved estimate of aquifer depletion (0.3 mm yr−1), partially offsetting the retention of water in dams and giving a total terrestrial storage contribution of −0.1 mm yr−1. Ocean warming (90% of the total of the Earth’s energy increase) continues through to the end of the record, in agreement with continued greenhouse gas forcing. The aerosol forcing, inferred as a residual in the atmospheric energy balance, is estimated as −0.8 ± 0.4 W m⁻² for the 1980s and early 1990s. It increases in the late 1990s, as is required for consistency with little surface warming over the last decade. This increase is likely at least partially related to substantial increases in aerosol emissions from developing nations and moderate volcanic activity.

Santer, B., Mears, C., Doutriaux, C., Caldwell, P., Gleckler, P., Wigley, T., Solomon, S., Gillett, N., Ivanova, D., and Karl, T., 2011, Separating signal and noise in atmospheric temperature changes: The importance of timescale: J. Geophys. Res, v. 116, p. D22105, doi:10.1029/2011JD016263. (Santer et al., 2011)

We compare global-scale changes in satellite estimates of the temperature of the lower troposphere (TLT) with model simulations of forced and unforced TLT changes. While previous work has focused on a single period of record, we select analysis timescales ranging from 10 to 32 years, and then compare all possible observed TLT trends on each timescale with corresponding multi-model distributions of forced and unforced trends. We use observed estimates of the signal component of TLT changes and model estimates of climate noise to calculate timescale-dependent signal-to-noise ratios (S/N). These ratios are small (less than 1) on the 10-year timescale, increasing to more than 3.9 for 32-year trends. This large change in S/N is primarily due to a decrease in the amplitude of internally generated variability with increasing trend length. Because of the pronounced effect of interannual noise on decadal trends, a multi-model ensemble of anthropogenically-forced simulations displays many 10-year periods with little warming. A single decade of observational TLT data is therefore inadequate for identifying a slowly evolving anthropogenic warming signal. Our results show that temperature records of at least 17 years in length are required for identifying human effects on global-mean tropospheric temperature.

Kaufmann, R. K., Kauppi, H., Mann, M. L., and Stock, J. H., 2011, Reconciling anthropogenic climate change with observed temperature 1998–2008: Proceedings of the National Academy of Sciences, v. 108, no. 29, p. 11790-11793, doi:10.1073/pnas.1102467108. (Kaufmann et al., 2011)

Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations. As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well-known warming and cooling effects.

Mochizuki, T., Ishii, M., Kimoto, M., Chikamoto, Y., Watanabe, M., Nozawa, T., Sakamoto, T. T., Shiogama, H., Awaji, T., and Sugiura, N., 2010, Pacific decadal oscillation hindcasts relevant to near-term climate prediction: Proceedings of the National Academy of Sciences, v. 107, no. 5, p. 1833-1837, doi:10.1073/pnas.0906531107. (Mochizuki et al., 2010)

Decadal-scale climate variations over the Pacific Ocean and its surroundings are strongly related to the so-called Pacific decadal oscillation (PDO) which is coherent with wintertime climate over North America and Asian monsoon, and have important impacts on marine ecosystems and fisheries. In a near-term climate prediction covering the period up to 2030, we require knowledge of the future state of internal variations in the climate system such as the PDO as well as the global warming signal. We perform sets of ensemble hindcast and forecast experiments using a coupled atmosphere-ocean climate model to examine the predictability of internal variations on decadal timescales, in addition to the response to external forcing due to changes in concentrations of greenhouse gases and aerosols, volcanic activity, and solar cycle variations. Our results highlight that an initialization of the upper-ocean state using historical observations is effective for successful hindcasts of the PDO and has a great impact on future predictions. Ensemble hindcasts for the 20th century demonstrate a predictive skill in the upper-ocean temperature over almost a decade, particularly around the Kuroshio-Oyashio extension (KOE) and subtropical oceanic frontal regions where the PDO signals are observed strongest. A negative tendency of the predicted PDO phase in the coming decade will enhance the rising trend in surface air-temperature (SAT) over east Asia and over the KOE region, and suppress it along the west coasts of North and South America and over the equatorial Pacific. This suppression will contribute to a slowing down of the global-mean SAT rise.

Trenberth, K., and Fasullo, J., 2010, Tracking Earth’s Energy: Science, v. 328, no. 5976, p. 316-317, doi:10.1126/science.1187272. (Trenberth, and Fasullo, 2010)

Where has the energy from global warming gone? Notes: CO₂ concentrations have further increased since 2003, and even more heat should have accumulated at a faster rate since then. Where has this energy gone? Ocean temperature measurements from 2004 to 2008 suggest a substantial slowing of the increase in global ocean heat content. Closure of the energy budget over the past 5 years is thus elusive (7). State-of-the art observations are unable to fully account for recent energy variability.

Trenberth, K., 2010, Global change: The ocean is warming, isn’t it?: Nature, v. 465, no. 7296, p. 304, doi:10.1038/465304a. (Trenberth, 2010)

A reappraisal of the messy data on upper-ocean heat content for 1993–2008 provides clear evidence for warming. But differences among various analyses and inconsistencies with other indicators merit attention. Notes: Lyman et al. are able to demonstrate a robust warming of the global upper ocean from 1993 to 2008, depicted by the red line in Figure 1, which averages 0.64 ± 0.29 watts per square meter (95% confidence interval) for the Earth as a whole. This is reasonably consistent with expectations from other indications of global warming6. Nonetheless, the results reveal that all curves flatten out after 2003 (as seen in the black line in Fig. 1; see also ref. 7, for example), suggesting that ocean warming has stalled. However, independent analysis8 of the full-depth Argo floats for 2003 to 2008 suggests that the 6-year heat-content increase is 0.77 ± 0.11 W m⁻² for the global ocean or 0.54 W m⁻² for the entire Earth, indicating that substantial warming may be taking place below the upper 700 m (Fig. 1, blue line).

Solomon, S., Rosenlof, K. H., Portmann, R. W., Daniel, J. S., Davis, S. M., Sanford, T. J., and Plattner, G. K., 2010, Contributions of stratospheric water vapor to decadal changes in the rate of global warming: Science, v. 327, no. 5970, p. 1219-1223, doi:10.1126/science.1182488. (Solomon et al., 2010)

Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000–2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.

Loeb, N. G., Wielicki, B. A., Doelling, D. R., Smith, G. L., Keyes, D. F., Kato, S., Manalo-Smith, N., and Wong, T., 2009, Toward optimal closure of the Earth’s top-of-atmosphere radiation budget: Journal of Climate, v. 22, no. 3, p. 748-766, doi:10.1175/2008JCLI2637.1. (Loeb et al., 2009)

Despite recent improvements in satellite instrument calibration and the algorithms used to determine reflected solar (SW) and emitted thermal (LW) top-of-atmosphere (TOA) radiative fluxes, a sizeable imbalance persists in the average global net radiation at the TOA from satellite observations. This imbalance is problematic in applications that use earth radiation budget (ERB) data for climate model evaluation, estimate the earth’s annual global mean energy budget, and in studies that infer meridional heat transports. This study provides a detailed error analysis of TOA fluxes based on the latest generation of Clouds and the Earth’s Radiant Energy System (CERES) gridded monthly mean data products [the monthly TOA/surface averages geostationary (SRBAVG-GEO)] and uses an objective constrainment algorithm to adjust SW and LW TOA fluxes within their range of uncertainty to remove the inconsistency between average global net TOA flux and heat storage in the earth–atmosphere system. The 5-yr global mean CERES net flux from the standard CERES product is 6.5W m⁻², much larger than the best estimate of 0.85W m⁻² based on observed ocean heat content data and model simulations. The major sources of uncertainty in the CERES estimate are from instrument calibration (4.2 W m⁻²) and the assumed value for total solar irradiance (1W m⁻²). After adjustment, the global mean CERES SW TOA flux is 99.5W m⁻², corresponding to an albedo of 0.293, and the global mean LW TOA flux is 239.6 W m⁻². These values differ markedly from previously published adjusted global means based on the ERB Experiment in which the global mean SW TOA flux is 107 W m⁻² and the LW TOA flux is 234 W m⁻².

Murphy, D. M., Solomon, S., Portmann, R. W., Rosenlof, K. H., Forster, P. M., and Wong, T., 2009, An observationally based energy balance for the Earth since 1950: Journal of Geophysical Research, v. 114, no. D17, doi:10.1029/2009jd012105. (Murphy et al., 2009)

We examine the Earth’s energy balance since 1950, identifying results that can be obtained without using global climate models. Important terms that can be constrained using only measurements and radiative transfer models are ocean heat content, radiative forcing by long-lived trace gases, and radiative forcing from volcanic eruptions. We explicitly consider the emission of energy by a warming Earth by using correlations between surface temperature and satellite radiant flux data and show that this term is already quite significant. About 20% of the integrated positive forcing by greenhouse gases and solar radiation since 1950 has been radiated to space. Only about 10% of the positive forcing (about 1/3 of the net forcing) has gone into heating the Earth, almost all into the oceans. About 20% of the positive forcing has been balanced by volcanic aerosols, and the remaining 50% is mainly attributable to tropospheric aerosols. After accounting for the measured terms, the residual forcing between 1970 and 2000 due to direct and indirect forcing by aerosols as well as semidirect forcing from greenhouse gases and any unknown mechanism can be estimated as −1.1 ± 0.4 W m⁻² (1σ). This is consistent with the Intergovernmental Panel on Climate Change’s best estimates but rules out very large negative forcings from aerosol indirect effects. Further, the data imply an increase from the 1950s to the 1980s followed by constant or slightly declining aerosol forcing into the 1990s, consistent with estimates of trends in global sulfate emissions. An apparent increase in residual forcing in the late 1990s is discussed.

Douglass, D. H., and Knox, R. S., 2009, Ocean heat content and Earth’s radiation imbalance: Physics Letters A, v. 373, no. 36, p. 3296-3300, doi:10.1016/j.physleta.2009.07.023. (Douglass, and Knox, 2009)

Earth’s radiation imbalance is determined from ocean heat content data and compared with results of direct measurements. Distinct time intervals of alternating positive and negative values are found: 1960–mid-1970s (−0.15), mid-1970s–2000 (+0.15), 2001–present (−0.2 W m⁻²), and are consistent with prior reports. These climate shifts limit climate predictability.

Lean, J. L., and Rind, D. H., 2009, How will Earth’s surface temperature change in future decades: Geophysical Research Letters, v. 36, p. L15708, doi:10.1029/2009GL038932. (Lean, and Rind, 2009)

Reliable forecasts of climate change in the immediate future are difficult, especially on regional scales, where natural climate variations may amplify or mitigate anthropogenic warming in ways that numerical models capture poorly. By decomposing recent observed surface temperatures into components associated with ENSO, volcanic and solar activity, and anthropogenic influences, we anticipate global and regional changes in the next two decades. From 2009 to 2014, projected rises in anthropogenic influences and solar irradiance will increase global surface temperature 0.15 ± 0.03°C, at a rate 50% greater than predicted by IPCC. But as a result of declining solar activity in the subsequent five years, average temperature in 2019 is only 0.03 ± 0.01°C warmer than in 2014. This lack of overall warming is analogous to the period from 2002 to 2008 when decreasing solar irradiance also countered much of the anthropogenic warming. We further illustrate how a major volcanic eruption and a super ENSO would modify our global and regional temperature projections.

Easterling, D. R., and Wehner, M. F., 2009, Is the climate warming or cooling?: Geophysical Research Letters, v. 36, p. L08706, doi:10.1029/2009GL037810. (Easterling, and Wehner, 2009)

Numerous websites, blogs and articles in the media have claimed that the climate is no longer warming, and is now cooling. Here we show that periods of no trend or even cooling of the globally averaged surface air temperature are found in the last 34 years of the observed record, and in climate model simulations of the 20th and 21st century forced with increasing greenhouse gases. We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer‐term warming.

Kerr, R. A., 2009, What happened to global warming? Scientists say just wait a bit: Science, v. 326, no. 2 October 2009, p. 28-29, doi:10.1126/science.326_28a. (Kerr, 2009)

Excerpts: Climate researchers are beginning to answer back in their preferred venue, the peer-reviewed literature. The pause in warming is real enough, but it’s just temporary, they argue from their analyses. A natural swing in climate to the cool side has been holding greenhouse warming back, and such swings don’t last forever. “In the end, global warming will prevail,” says climate scientist Gavin Schmidt of NASA’s Goddard Institute for Space Studies (GISS) in New York City. Climate researcher Jeff Knight and eight colleagues at the Met Office Hadley Centre in Exeter, U.K., first establish that—at least in one leading temperature record—greenhouse warming has been stopped in its tracks for the past 10 years. In the HadCRUT3 temperature record, the world warmed by 0.07°C±0.07°C from 1999 through 2008, not the 0.20°C expected by the Intergovernmental Panel on Climate Change. Corrected for the natural temperature effects of El Niño and its sister climate event La Niña, the decade’s trend is a perfectly flat 0.00°C. In 10 modeling runs of 21st century climate totaling 700 years worth of simulation, long-term warming proceeded about as expected: 2.0°C by the end of the century. But along the way in the 700 years of simulation, about 17 separate 10-year intervals had temperature trends resembling that of the past decade—that is, more or less flat. Pauses as long as 15 years are rare in the simulations, and “we expect that [real-world] warming will resume in the next few years,”

Knight, J., Kennedy, J., Folland, C., Harris, G., Jones, G., Palmer, M., Parker, D., Scaife, A., and Stott, P., 2009, Do global temperature trends over the last decade falsify climate predictions?, in Peterson, T. C., and Baringer, M. O., eds., State of the Climate in 2008, Volume 90, Bull. Am. Meteorol. Soc., 90 (8), p. S22-S23. (Knight et al., 2009)

Excerpts: The least squares trend for January 1999 to December 2008 calculated from the HadCRUT3 dataset (Brohan et al. 2006) is +0.07±0.07°C decade^-1— much less than the 0.18°C decade^-1 recorded between 1979 and 2005 and the 0.2°C decade^-1 expected in the next decade (IPCC; Solomon et al. 2007). The trend in the ENSO-related component for 1999–2008 is +0.08±0.07°C decade^-1, fully accounting for the overall observed trend. The trend after removing ENSO (the “ENSO-adjusted” trend) is 0.00°±0.05°C decade^-1. Ensembles with different modifications to the physical parameters of the model (within known uncertainties) (Collins et al. 2006) are performed for several of the IPCC SRES emissions scenarios (Solomon et al. 2007). Ten of these simulations have a steady long-term rate of warming between 0.15° and 0.25ºC decade^-1, close to the expected rate of 0.2ºC decade^-1. Near-zero and even negative trends are common for intervals of a decade or less in the simulations, due to the model’s internal climate variability. The simulations rule out (at the 95% level) zero trends for intervals of 15 year or more Given the likelihood that internal variability contributed to the slowing of global temperature rise in the last decade, we expect that warming will resume in the next few years, consistent with predictions from near-term climate forecasts (Smith et al. 2007; Haines et al. 2009).

Trenberth, K. E., 2009, An imperative for climate change planning: tracking Earth’s global energy: Current Opinion in Environmental Sustainability, v. 1, no. 1, p. 19-27, doi:10.1016/j.cosust.2009.06.001. (Trenberth, 2009)

Planned adaptation to climate change requires information about what is happening and why. While a long-term trend is for global warming, short-term periods of cooling can occur and have physical causes associated with natural variability. However, such natural variability means that energy is rearranged or changed within the climate system, and should be traceable. An assessment is given of our ability to track changes in reservoirs and flows of energy within the climate system. Arguments are given that developing the ability to do this is important, as it affects interpretations of global and especially regional climate change, and prospects for the future.

Kerr, R. A., 2007, Humans and nature duel over the next decade’s climate: Science, v. 317, p. 746-747. (Kerr, 2007)

Notes: Natural climate variability driven by the ocean appears to have held greenhouse warming at bay the past few years, but the warming, according to the forecast, should come roaring back before the end of the decade.