California is currently in the midst of a record-setting drought. The drought began in 2012 and now includes the lowest calendar-year and 12-mo precipitation, the highest annual temperature, and the most extreme drought indicators on record. The extremely warm and dry conditions have led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Analyzing historical climate observations from California, we find that precipitation deficits in California were more than twice as likely to yield drought years if they occurred when conditions were warm. We find that although there has not been a substantial change in the probability of either negative or moderately negative precipitation anomalies in recent decades, the occurrence of drought years has been greater in the past two decades than in the preceding century. In addition, the probability that precipitation deficits co-occur with warm conditions and the probability that precipitation deficits produce drought have both increased. Climate model experiments with and without anthropogenic forcings reveal that human activities have increased the probability that dry precipitation years are also warm. Further, a large ensemble of climate model realizations reveals that additional global warming over the next few decades is very likely to create ∼100% probability that any annual-scale dry period is also extremely warm. We therefore conclude that anthropogenic warming is increasing the probability of co-occurring warm-dry conditions like those that have created the acute human and ecosystem impacts associated with the "exceptional" 2012-2014 drought in California.drought | climate extremes | climate change detection | event attribution | CMIP5

Through modeling and international exchange, the Abdus Salam International Centre for Theoretical Physicsis fostering advanced climate research in countries where scientific resources are often scarce. P opulations in economically developing nations (EDNs) depend extensively on climate for their welfare (e.g., agriculture, water resources, power generation, industry) and likewise are vulnerable to variability in the climate system, whether due to anthropogenic forcing or natural processes. Furthermore, changes in atmospheric composition (e.g., greenhouse gases and aerosols) and land cover are likely to significantly alter regional climates (Nakicenovic et al. 2001), thereby affecting local socioeconomic development and livelihoods of EDN populations. Therefore, the evaluation of climate change and variability at seasonal-to-multidecadal time scales is of great benefit to these regions.Climate models, both global and regional, are the primary tools that aid in our understanding of the many processes that govern the climate system. In the past, a lack of computational resources has hindered the use of climate models by EDN scientists. However, in the last decade the computing power of the common desktop personal computer (PC) has dramatically increased •

Surface weather conditions are closely governed by the large-scale 1 circulation of the atmosphere. Recent increases in the occurrence of some extreme 2 weather phenomena 1,2 have led to multiple mechanistic hypotheses linking changes 3 in atmospheric circulation to increasing extreme event probability 3-5 . However, 4 observed evidence of long-term change in atmospheric circulation remains 5 inconclusive 6-8 . Here we identify statistically significant trends in the occurrence of 6 mid-atmospheric circulation patterns, which partially explain observed trends in 7 surface temperature extremes over seven mid-latitude regions of the Northern 8Hemisphere. Utilizing self-organizing map (SOM) cluster analysis 9-12 , we detect 9 robust pattern trends in a subset of these regions during both the satellite 10 observation era Although most land regions show robust warming over the past century 13 , the 21 pattern of change has not been spatially uniform 14 . This heterogeneity results from 22 regional differences in the response of the climate system to increasing radiative forcing, 23and from the background noise of climate variability. Together, these factors 24 substantially increase the challenge of climate change detection, attribution, and 25 projection at regional and local scales 14,16 . 26The spatial pattern of changes in extreme weather events has generated arguments 27 that global warming has caused dynamic and/or thermodynamic changes that have 28 differentially altered extreme event probabilities 1,17 . Thermodynamic arguments are well 29 3 understood and observed. For example, the accumulation of heat in the atmosphere has 30 resulted in upward trends in hot extremes, downward trends in the majority of cold 31 extremes, and more intense hydroclimatic events 1,2 . Dynamic arguments have greater 32 uncertainties [15][16][17][18][19] . Changes in the large-scale atmospheric circulation -for instance, an 33 increase in the occurrence or persistence of high-amplitude wave patterns -could alter 34 the likelihood of extreme events 20 . Recent extremes in the Northern Hemisphere mid-35 latitudes 1,2,17 have motivated hypotheses of a dynamic linkage between "Arctic 36 Amplification", altered atmospheric circulation patterns, and changes in the probability of 37 mid-latitude extremes e.g., [3][4][5]17 . Despite divergent views on the causal direction of this 38 linkage 17 , altered atmospheric dynamics are consistently invoked. Although trends in 39 mean-seasonal mid-atmospheric geopotential height anomalies have been identified (Fig. 40 2.36 ref. 21; Fig. 1), evidence of changes in the occurrence of sub-seasonal atmospheric 41 patterns remains equivocal, as does their contribution to extreme event probabilities [6][7][8] . 42Previous efforts to detect trends in atmospheric circulation may have been 43 hampered by narrowly-defined, spatially-sensitive, and/or non-standardized metrics 3,[6][7][8]17 . 44We therefore employ a large-scale spatial characterization approach -Self-Organizing 45Map ("SOM") cluster an...

We find that extreme temperature and precipitation events are likely to respond substantially to anthropogenically enhanced greenhouse forcing and that fine-scale climate system modifiers are likely to play a critical role in the net response. At present, such events impact a wide variety of natural and human systems, and future changes in their frequency and͞or magnitude could have dramatic ecological, economic, and sociological consequences. Our results indicate that fine-scale snow albedo effects influence the response of both hot and cold events and that peak increases in extreme hot events are amplified by surface moisture feedbacks. Likewise, we find that extreme precipitation is enhanced on the lee side of rain shadows and over coastal areas dominated by convective precipitation. We project substantial, spatially heterogeneous increases in both hot and wet events over the contiguous United States by the end of the next century, suggesting that consideration of fine-scale processes is critical for accurate assessment of local-and regional-scale vulnerability to climate change.extreme climate ͉ RegCM3 ͉ regional climate model ͉ United States ͉ CO2

We use a statistical metric of multi-dimensional climate change to quantify the emergence of global climate change hotspots in the CMIP5 climate model ensemble. Our hotspot metric extends previous work through the inclusion of extreme seasonal temperature and precipitation, which exert critical influence on climate change impacts. The results identify areas of the Amazon, the Sahel and tropical West Africa, Indonesia, and the Tibetan Plateau as persistent regional climate change hotspots throughout the 21st century of the RCP8.5 and RCP4.5 forcing pathways. In addition, areas of southern Africa, the Mediterranean, the Arctic, and Central America/western North America also emerge as prominent regional climate change hotspots in response to intermediate and high levels of forcing. Comparisons of different periods of the two forcing pathways suggest that the pattern of aggregate change is fairly robust to the level of global warming below approximately 2 °C of global warming (relative to the late-20th-century baseline), but not at the higher levels of global warming that occur in the late-21st-century period of the RCP8.5 pathway, with areas of southern Africa, the Mediterranean, and the Arctic exhibiting particular intensification of relative aggregate climate change in response to high levels of forcing. Although specific impacts will clearly be shaped by the interaction of climate change with human and biological vulnerabilities, our identification of climate change hotspots can help to inform mitigation and adaptation decisions by quantifying the rate, magnitude and causes of the aggregate climate response in different parts of the world.Electronic supplementary materialThe online version of this article (doi:10.1007/s10584-012-0570-x) contains supplementary material, which is available to authorized users.

Terrestrial ecosystems have encountered substantial warming over the past century, with temperatures increasing about twice as rapidly over land as over the oceans. Here, we review the likelihood of continued changes in terrestrial climate, including analyses of the Coupled Model Intercomparison Project global climate model ensemble. Inertia toward continued emissions creates potential 21st-century global warming that is comparable in magnitude to that of the largest global changes in the past 65 million years but is orders of magnitude more rapid. The rate of warming implies a velocity of climate change and required range shifts of up to several kilometers per year, raising the prospect of daunting challenges for ecosystems, especially in the context of extensive land use and degradation, changes in frequency and severity of extreme events, and interactions with other stresses.

We find that elevated greenhouse gas concentrations dramatically increase heat stress risk in the Mediterranean region, with the occurrence of hot extremes increasing by 200 to 500% throughout the region. This heat stress intensification is due to preferential warming of the hot tail of the daily temperature distribution, with 95th percentile maximum and minimum temperature magnitude increasing more than 75th percentile magnitude. This preferential warming of the hot tail is dictated in large part by a surface moisture feedback, with areas of greatest warm‐season drying showing the greatest increases in hot temperature extremes. Fine‐scale topographic and humidity effects help to further dictate the spatial variability of the heat stress response, with increases in dangerous Heat Index magnified in coastal areas. Further, emissions deceleration substantially mitigates heat stress intensification throughout the Mediterranean region, implying that emissions reductions could reduce the risk of increased heat stress in the coming decades.

California has experienced devastating autumn wildfires in recent years. These autumn wildfires have coincided with extreme fire weather conditions during periods of strong offshore winds coincident with unusually dry vegetation enabled by anomalously warm conditions and late onset of autumn precipitation. In this study, we quantify observed changes in the occurrence and magnitude of meteorological factors that enable extreme autumn wildfires in California, and use climate model simulations to ascertain whether these changes are attributable to human-caused climate change. We show that state-wide increases in autumn temperature (∼1 °C) and decreases in autumn precipitation (∼30%) over the past four decades have contributed to increases in aggregate fire weather indices (+20%). As a result, the observed frequency of autumn days with extreme (95th percentile) fire weather—which we show are preferentially associated with extreme autumn wildfires—has more than doubled in California since the early 1980s. We further find an increase in the climate model-estimated probability of these extreme autumn conditions since ∼1950, including a long-term trend toward increased same-season co-occurrence of extreme fire weather conditions in northern and southern California. Our climate model analyses suggest that continued climate change will further amplify the number of days with extreme fire weather by the end of this century, though a pathway consistent with the UN Paris commitments would substantially curb that increase. Given the acute societal impacts of extreme autumn wildfires in recent years, our findings have critical relevance for ongoing efforts to manage wildfire risks in California and other regions.