Global Warming Hiatus Research Articles

Spatio-temporal variations of Arctic amplification and their linkage with the Arctic oscillation

The Arctic near-surface air temperatures are increasing more than twice as fast as the global average–a feature known as Arctic amplification (AA). A modified AA index is constructed in this paper to emphasize the contrast of warming rate between polar and mid-latitude regions, as well as the spatial and temporal characteristics of AA and their influence on atmospheric circulation over the Northern Hemisphere. Results show that AA has a pronounced annual cycle. The positive or negative phase activities are the strongest in autumn and winter, the weakest in summer. After experiencing a remarkable decadal shift from negative to positive phase in the early global warming hiatus period, the AA has entered into a state of being enlarged continuously, and the decadal regime shift of AA in about 2002 is affected mainly by decadal shift in autumn. In terms of spatial distribution, AA has maximum warming near the surface in almost all seasons except in summer. Poleward of 20°N, AA in autumn has a significant influence on the atmospheric circulation in the following winter. The reason may be that the autumn AA increases the amplitude of planetary waves, slows the wave speeds and weakens upper-level zonal winds through the thermal wind relation, thus influencing surface air temperature in the following winter. The AA correlates to negative phase of the Arctic oscillation (AO) and leads AO by 0–3 months within the period 1979–2002. However, weaker relationship between them is indistinctive after the decadal shift of AA.

Trajectories toward the 1.5°C Paris target: Modulation by the Interdecadal Pacific Oscillation

AbstractGlobal temperature is rapidly approaching the 1.5°C Paris target. In the absence of external cooling influences, such as volcanic eruptions, temperature projections are centered on a breaching of the 1.5°C target, relative to 1850–1900, before 2029. The phase of the Interdecadal Pacific Oscillation (IPO) will regulate the rate at which mean temperature approaches the 1.5°C level. A transition to the positive phase of the IPO would lead to a projected exceedance of the target centered around 2026. If the Pacific Ocean remains in its negative decadal phase, the target will be reached around 5 years later, in 2031. Given the temporary slowdown in global warming between 2000 and 2014, and recent initialized decadal predictions suggestive of a turnaround in the IPO, a sustained period of rapid temperature rise might be underway. In that case, the world will reach the 1.5°C level of warming several years sooner than if the negative IPO phase persists.

On semi‐empirical decomposition of multidecadal climate variability into forced and internally generated components

ABSTRACTThis paper combines CMIP5 historical simulations and observations of surface temperature to investigate relative contributions of forced and internal climate variability to long‐term climate trends. A suite of estimated forced signals based on surrogate multi‐model ensembles mimicking the statistical characteristics of individual models is used to show that, in contrast to earlier claims, scaled versions of the multi‐model ensemble mean cannot adequately characterize the full spectrum of CMIP5 forced responses, due to misrepresenting the model uncertainty. The same suite of multiple forced signals is also used to derive unbiased estimates of the model simulated internal variability in historical simulations and, after appropriate scaling to match the observed climate sensitivity, to estimate the internal component of climate variability in the observed temperatures. On average, climate models simulate the non‐uniform warming of Northern Hemisphere mean surface temperature well, but are overly sensitive to forcing in the North Atlantic and North Pacific, where the simulations have to be scaled back to match observed trends. In contrast, the simulated internal variability is much weaker than observed. There is no evidence of coupling between the model simulated forced signals and internal variability, suggesting that their underlying dominant physical mechanisms are different. Analysis of regional contributions to the recent global warming hiatus points to the presence of a hemispheric mode of internal climate variability, rather than to internal processes local to the Pacific Ocean. Large discrepancies between present estimates of the simulated and observed multidecadal internal climate variability suggest that our ability to attribute and predict climate change using current generation of climate models is limited.

Reconciling controversies about the 'global warming hiatus'.

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.

Climate science: The 'pause' unpacked.

Short-term climate trends are sensitive to definitions, data and testing. This sensitivity underlies an alleged pause in global warming, and highlights the need for meaningful definitions to sustain claims that it was real. See Analysis p.41 After a spike in global-mean temperature associated with the 1998 El Nino, the climate system experienced several years of reduced warming, and perhaps even slight cooling. This period, variously termed the 'hiatus', 'pause' or 'slowdown', should have come as no surprise given our understanding of El Nino and natural climate variability. However, soon after the recognition of the reduced warming, it appeared that models and observations were diverging, raising the question of whether the models were missing important processes. Although global warming has since recommenced, the hiatus sparked an enormous research effort. Iselin Medhaug et al. synthesize the literature and reassess the model and observational evidence. Their assessment reconciles the apparent contradictions between models and data and obviates the need to revise our understanding of the underlying physics of climate systems. The hiatus was an episode of natural variability after all.

The long-term variability of Changma in the East Asian summer monsoon system: A review and revisit

Changma, which is a vital part of East Asian summer monsoon (EASM) system, plays a critical role in modulating water and energy cycles in Korea. Better understanding of its long-term variability and change is therefore a matter of scientific and societal importance. It has been indicated that characteristics of Changma have undergone significant interdecadal changes in association with the mid-1970s global-scale climate shift and the mid-1990s EASM shift. This paper reviews and revisits the characteristics on the long-term changes of Changma focusing on the underlying mechanisms for the changes. The four important features are manifested mainly during the last few decades: 1) mean and extreme rainfalls during Changma period from June to September have been increased with the amplification of diurnal cycle of rainfall, 2) the dry spell between the first and second rainy periods has become shorter, 3) the rainfall amount as well as the number of rainy days during August have significantly increased, probably due to the increase in typhoon landfalls, and 4) the relationship between the Changma rainfall and Western Pacific Subtropical High on interannual time scale has been enhanced. The typhoon contribution to the increase in heavy rainfall is attributable to enhanced interaction between typhoons and midlatitude baroclinic environment. It is noted that the change in the relationship between Changma and the tropical sea surface temperature (SST) over the Indian, Pacific, and Atlantic Oceans is a key factor in the long-term changes of Changma and EASM. Possible sources for the recent mid-1990s change include 1) the tropical dipole-like SST pattern between the central Pacific and Indo-Pacific region (the global warming hiatus pattern), 2) the recent intensification of tropical SST gradients among the Indian Ocean, the western Pacific, and the eastern Pacific, and 3) the tropical Atlantic SST warming.

The extreme El Niño of 2015–2016 and the end of global warming hiatus

AbstractSlower rates of increase in global mean surface temperature (GMST) after 2000, dubbed “global warming hiatus,” recently gave way to a rapid temperature rise. This rise coincided with persistent warm conditions in the equatorial Pacific between March 2014 and May 2016, which peaked as the 2015 extreme El Niño. Here we show that the El Niño–Southern Oscillation (ENSO) tightly controls interannual variations in atmospheric heating rate in the tropics (r > 0.9), allowing us to construct a simple, physically based model of GMST variations that incorporates greenhouse gas emissions, ENSO forcing, and stratospheric sulfate aerosols produced by volcanoes. The model closely reproduces GMST changes since 1880, including the global warming hiatus and the subsequent temperature rise. Our results confirm that weak El Niño activity, rather than volcanic eruptions, was the cause of the hiatus, while the rapid temperature rise is due to atmospheric heat release during 2014–2016 El Niño conditions concurrent with the continuing global warming trend.

The post‐2002 global surface warming slowdown caused by the subtropical Southern Ocean heating acceleration

AbstractThe warming rate of global mean surface temperature slowed down during 1998–2012. Previous studies pointed out role of increasing ocean heat uptake during this global warming slowdown, but its mechanism remains under discussion. Our numerical simulations, in which wind stress anomaly in the equatorial Pacific is imposed from reanalysis data, suggest that subsurface warming in the equatorial Pacific took place during initial phase of the global warming slowdown (1998–2002), as previously reported. It is newly clarified that the Ekman transport from tropics to subtropics is enhanced during the later phase of the slowdown (after 2002) and enhanced subtropical Ekman downwelling causes accelerated heat storage below depth of 700 m in the subtropical Southern Ocean, leading to the post‐2002 global warming slowdown. Observational data of ocean temperature also support this scenario. This study provides clear evidence that deeper parts of the Southern Ocean play a critical role in the post‐2002 warming slowdown.

Global Influence of Tropical Pacific Variability with Implications for Global Warming Slowdown

The impact of internal tropical Pacific variability on global mean surface temperature (GMST) is quantified using a multimodel ensemble. A tropical Pacific index (TPI) is defined to track tropical Pacific sea surface temperature (SST) variability. The simulated GMST is highly correlated with TPI on the interannual time scale but this correlation weakens on the decadal time scale. The time-scale dependency is such that the GMST regression equation derived from the observations, which are dominated by interannual variability, would underestimate the magnitude of decadal GMST response to tropical Pacific variability. The surface air temperature response to tropical Pacific variability is strong in the tropics but weakens in the extratropics. The regression coefficient of GMST against TPI shows considerable intermodel variations, primarily because of differences in high latitudes. The results have important implications for the planned intercomparison of pacemaker experiments that force Pacific variability to follow the observed evolution. The model dependency of the GMST regression suggests that in pacemaker experiments—model performance in simulating the recent “slowdown” in global warming—will vary substantially among models. It also highlights the need to develop observational constraints and to quantify the TPI effect on the decadal variability of GMST. Compared to GMST, the correlation between global mean tropospheric temperature and TPI is high on both interannual and decadal time scales because of a common structure in the tropical tropospheric temperature response that is upward amplified and meridionally broad.

What Caused the Global Surface Warming Hiatus of 1998–2013?

Research into the mechanisms for the global warming slowdown or “hiatus” of 1998–2013 is reviewed here. Observational and modeling studies identify tropical Pacific sea surface temperature variability as a major pacemaker of global mean surface temperature (GMST) change, as corroborated by the GMST increase following a major El Nino event. Specifically, the decadal cooling of the tropical Pacific contributes to the recent global warming hiatus. This tropical Pacific pacemaker effect appears larger for decadal than interannual variability, but the decadal effect remains to be quantified from observations. Our critical review of the literature reveals that the internal and radiatively forced GMST changes are distinct in pattern, energetics, mechanism, and predictability. In contrast to greenhouse gas-induced warming that is spatially uniform in sign and driven by energy perturbations, internal variability in GMST is an order of magnitude smaller than spatial variations, for which ocean-atmosphere interaction is of first-order importance while planetary energetics is not. In fact, decadal variability in GMST is poorly correlated with net radiation at the top of the atmosphere, highlighting the need to distinguish internal and forced GMST change in planetary energy budget. While the planetary energy budget can now be closed observationally over multi-decadal periods, the recent hiatus highlights the need and challenges to measure and quantify decadal changes in both global ocean heat uptake (e.g., for the effect of radiative forcing on the hiatus) and heat redistribution in the ocean. Hiatus research has led to a wide recognition of the importance of internal variability for GMST trends over a decade and longer. The strengthened connection between the climate variability and change communities is an important legacy of hiatus research.

Recent Enhanced Seasonal Temperature Contrast in Japan from Large Ensemble High-Resolution Climate Simulations

Since the late 1990s, land surface temperatures over Japan have increased during the summer and autumn, while global mean temperatures have not risen in this duration (i.e., the global warming hiatus). In contrast, winter and spring temperatures in Japan have decreased. To assess the impact of both global warming and global-scale decadal variability on this enhanced seasonal temperature contrast, we analyzed the outputs of 100 ensemble simulations of historical and counterfactual non-warming climate simulations conducted using a high-resolution atmospheric general circulation model (AGCM). Our simulations showed that atmospheric fields impacted by the La Nina-like conditions associated with Interdecadal Pacific Oscillation (IPO) have predominantly contributed to the seasonal temperature contrast over Japan. Compared with the impact of negative IPO, the influence of global warming on seasonal temperature contrasts in Japan was small. In addition, atmospheric variability has also had a large impact on temperatures in Japan over a decadal timescale. The results of this study suggest a future increase in heatwave risk during the summer and autumn when La Nina-like decadal phenomena and atmospheric perturbations coincide over a background of global warming.

Why Has the Relationship between Indian and Pacific Ocean Decadal Variability Changed in Recent Decades?

Abstract Both the Indian and Pacific Oceans exhibit prominent decadal time scale variations in sea surface temperature (SST), linked dynamically via atmospheric and oceanic processes. However, the relationship between SST in these two basins underwent a dramatic transformation beginning around 1985. Prior to that, SST variations associated with the Indian Ocean basin mode (IOB) and the interdecadal Pacific oscillation (IPO) were positively correlated, whereas afterward they were much less clearly synchronized. Evidence is presented from both observations and coupled state-of-the-art climate models that enhanced external forcing, particularly from increased anthropogenic greenhouse gases, was the principal cause of this changed relationship. Using coupled climate model experiments, it is shown that without external forcing, the evolution of the IOB would be strongly forced by variations in the IPO. However, with strong external forcing, the dynamical linkage between the IOB and the IPO weakens so that the negative phase IPO after 2000 is unable to force a negative phase IOB-induced cooling of the Indian Ocean. This changed relationship in the IOB and IPO led to unique SST patterns in the Indo-Pacific region after 2000, which favored exceptionally strong easterly trade winds over the tropical Pacific Ocean and a pronounced global warming hiatus in the first decade of the twenty-first century.

Revisiting the Relationship between Observed Warming and Surface Pressure in the Tibetan Plateau

Abstract The Tibetan Plateau (TP) has an average elevation of over 4000 m and with its surrounding mountains is regarded as Earth’s “third pole.” As a result of its size and height, climate change in the TP has its own unique characteristics that include a proposed positive correlation between the surface temperature and pressure. This study examines the trends and relationships between the surface pressure and temperature in the TP through the examination of monthly mean data from 71 stations during 1961–2013. On annual, seasonal, and monthly time scales, the TP exhibits a statistically significant warming trend that attains a rate of 0.30°C decade−1 for annual means over the period 1961–2013. The most pronounced warming occurs in winter, in agreement with previous studies, with evidence of acceleration in the rate after the mid-1980s and the global warming slowdown period. For the entire period of 1961–2013, the surface pressure in the TP has a positive trend of 0.08 hPa decade−1 on an annual basis, again with the largest trends occurring in winter. However, unlike what occurred with the surface temperature, the trend in surface pressure, in most cases, reversed sign after the mid-1980s. The trend in the geopotential height at 500 hPa from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis is consistent with the observed surface pressure trends. Over the period 1961–2013, there is a seasonal shift in the nature of the relationship between the surface temperature and pressure with a negative correlation during summer and autumn, and a positive correlation during winter. This suggests that the nature of the relationship between these two climate elements reflects the changing nature of the seasonal snow cover (land surface property) and cloud in the region.

Squabble over supposed global-warming pause illuminates new political challenges

Breitbart’s James Delingpole sees “the perfect excuse to drain the climate swamp.” Breitbart’s James Delingpole sees “the perfect excuse to drain the climate swamp.”

Modulation of air‐sea fluxes by extratropical planetary waves and its impact during the recent surface warming slowdown

AbstractIt is widely accepted that natural decadal variability played a major role in the slowdown in global warming observed in the 21st century, with sea surface cooling in the tropical Pacific recognized as a major contributor. However, the warming pause was most pronounced during boreal winter, with Northern Hemisphere flow anomalies also playing a role. Here we quantify the contribution of extratropical heat exchanges by comparing geopotential and temperature anomalies simulated by ensembles of seasonal forecasts with similar ocean temperature but different heat fluxes north of 40°N, as a result of planetary wave variability. We show that an important part of heat flux anomalies is associated with decadal variations in the phase of a specific planetary wave pattern. In model simulations covering the last three decades, this variability pattern accounts for a decrease of 0.35°C/decade in the post‐1998 wintertime temperature trend over northern continents.

Changes in aridity in response to the global warming hiatus

The global warming slowdown or warming hiatus, began around the year 2000 and has persisted for nearly 15 years. Most studies have focused on the interpretation of the hiatus in temperature. In this study, changes in a global aridity index (AI) were analyzed by using a newly developed dynamical adjustment method that can successfully identify and separate dynamically induced and radiatively forced aridity changes in the raw data. The AI and Palmer Drought Severity Index produced a wetting zone over the mid-to-high latitudes of the Northern Hemisphere in recent decades. The dynamical adjustment analysis suggested that this wetting zone occurred in response to the global warming hiatus. The dynamically induced AI (DAI) played a major role in the AI changes during the hiatus period, and its relationships with the North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), and Atlantic Multi-decadal Oscillation (AMO) also indicated that different phases of the NAO, PDO, and AMO contributed to different performances of the DAI over the Northern Hemisphere. Although the aridity wetting over the mid-to-high latitudes may relieve long-term drying in certain regions, the hiatus is temporary, and so is the relief. Accelerated global warming will return when the NAO, PDO, and AMO revert to their opposite phases in the future, and the wetting zone is likely to disappear.

Spatiotemporal Temperature Variability over the Tibetan Plateau: Altitudinal Dependence Associated with the Global Warming Hiatus

Abstract The recent slowdown in global warming has initiated a reanalysis of temperature data in some mountainous regions for understanding the consequences and impact that a hiatus has on the climate system. Spatiotemporal temperature variability is analyzed over the Tibetan Plateau because of its sensitivity to climate change with a station network updated to 2014, and its linkages to remote sensing–based variability of MODIS daytime and nighttime temperature are investigated. Results indicate the following: 1) Almost all stations have experienced a notable warming in the time interval 1961–2014, with most obvious warming in winter, which depends on the selected time intervals. 2) There is no clear shift from a predominant warming to a near stagnation during the most recent period (2001–present). 3) Uniform altitudinal dependence of temperature change trends could not be confirmed for all regions, time intervals, and seasons, but sometimes an altitude threshold around 3 km is apparent. 4) Most of the meteorological stations are associated with MODIS temperature warming pixels, and thus regional cooling is missing when considering only the locations of meteorological stations. In summarizing, previous studies based on station observations do not provide a complete picture for the temperature change over the Tibetan Plateau. Remote sensing–based analyses have the potential to find early signals of regional climate changes and assess the impact of global climate changes in complex regional, seasonal, and altitudinal environments.

An econometric analysis of global warming hiatus

ABSTRACTThere is an avid debate concerning the presence of a hiatus in global warming. Recognizing that this phenomenon has considerable economic implications, this study aims to make a threefold contribution to the climate econometrics literature. Firstly, it formulates a testable hypothesis; secondly, it proposes a systematic empirical procedure; thirdly, it performs the Bai–Perron test to detect the starting point of the global warming hiatus. The findings supported the existence of the global warming hiatus.

A reassessment of temperature variations and trends from global reanalyses and monthly surface climatological datasets

The ERA‐Interim and JRA‐55 reanalyses of synoptic data and several conventional analyses of monthly climatological data provide similar estimates of global‐mean surface warming since 1979. They broadly agree on the character of interannual variability and the extremity of the 2015/2016 warm spell to which a strong El Niño and low Arctic sea‐ice cover contribute. Nevertheless global and regional averages differ on various time‐scales due to differences in data coverage and sea‐surface temperature analyses; averages from those conventional datasets that infill where they lack direct observations agree better with the averages from the reanalyses. The latest warm event is less extreme when viewed in terms of atmospheric energy, which gives more weight to variability in the Tropics, where the thermal signal has greater vertical penetration and latent energy is a larger factor.Surface warming from 1998 to 2012 is larger than indicated by earlier versions of the conventional datasets used to characterize what the Fifth Assessment Report of the Intergovernmental Panel on Climate Change termed a hiatus in global warming. None of the datasets exhibit net warming over the Antarctic since 1979.Centennial trends from the conventional datasets, HadCRUT4 on the one hand and GISTEMP and NOAAGlobalTemp on the other, differ mainly because sea‐surface temperatures differ. Infilling of values where direct observations are lacking is more questionable for the data‐sparse earlier decades. Change since the eighteenth century is inevitably more uncertain than change over and after a modern baseline period. The latter is arguably best estimated separately for taking stock of actions to limit climate change, exploiting reanalyses and using satellite data to refine the conventional approach. Nevertheless, early in 2016 the global temperature appears to have first touched or briefly breached a level 1.5 °C above that early in the Industrial Revolution, having touched the 1.0 °C level in 1998 during a previous El Niño.

Interhemispheric SST Gradient Trends in the Indian Ocean prior to and during the Recent Global Warming Hiatusa

Abstract 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.