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AbstractCompound extremes—particularly hot and dry years—can reduce crop yields and result in acute water scarcity. These risks are particularly pronounced in the Upper Nile Basin, a chronically water stressed agricultural region that includes western Ethiopia, South Sudan, and Uganda. While the causes of humanitarian crises in the Nile Basin are complex and involve governance, conflict, and climate, we demonstrate that nearly all recent regional crop failures have occurred amid hot and dry conditions and low runoff supplies. Using an observational ensemble, we find that such hot and dry years have been more frequent in recent decades, driven by increasing regional temperatures. This trend is likely to continue despite climate model projections of increasing regional precipitation. By the late 21st century, the frequency of hot and dry years may rise by a factor of 1.5–3, even if warming is limited to 2 °C. Regional water scarcity will continue to be a chronic issue for the Upper Nile from population growth alone, but runoff deficits during future hot and dry years will amplify this effect, leaving an additional 5–15% of the future population facing water scarcity. Climate change, along with the region's complex water politics, dependence on subsistence agriculture, and history of geopolitical instability, places the region at risk of severe food and water shortages as hot and dry years become more frequent. Adaptation and climate‐resilient water management policies informed by an understanding of compound extremes will be essential to manage these risks.

Thermal power plants use fossil fuels or nuclear material to generate most of the world’s electricity. On hot days, when electricity demand peaks, the ambient air and water used to cool these plants can become too warm, forcing operators to curtail electricity output. Using all available observed daily-scale plant outage data, we estimate the observed dependence of thermal plant curtailment on temperature and runoff and use this relationship to quantify curtailments due to global warming. Climate change to date has increased average thermal power plant curtailment in nuclear, coal, oil, and natural gas fired plants by 0.75–1 percentage points; with each degree Celsius of additional warming, we project curtailment to increase by 0.8–1.2 percentage points during peak demand, requiring an additional 18–27 GW of capacity, or 40–60 additional average-sized power plants, to offset this global power loss. Relative to policy scenarios with global transitions to renewable portfolios or that allow aging plants to retire, thermal power generation is a systemically disadvantaged means of electricity production in a warming world. Our results point to the crucial need for additional operational data across a diversity of thermal power plants to better constrain the risks warming poses to our electricity supply.

Abstract Nonlinear increases in warm season temperatures are projected for many regions, a phenomenon we show to be associated with relative surface drying. However, negative human health impacts are physiologically linked to combinations of high temperatures and high humidity. Since the amplified warming and drying are concurrent, the net effect on humid-heat, as measured by the wet bulb temperature (T W), is uncertain. We demonstrate that globally, on the hottest days of the year, the positive effect of amplified warming on T W is counterbalanced by a larger negative effect resulting from drying. As a result, the largest increases in T W and T x do not occur on the same days. Compared to a world with linear temperature change, the drying associated with nonlinear warming dampens mid-latitude T W increases by up to 0.5 °C, and also dampens the rise in frequency of dangerous humid-heat (T W > 27 °C) by up to 5 d per year in parts of North America and Europe. Our results highlight the opposing interactions among temperature and humidity changes and their effects on T W, and point to the importance of constraining uncertainty in hydrological and warm season humidity changes to best position the management of future humid-heat risks.

AbstractUS maize and soy production have increased rapidly since the mid-20th century. While global warming has raised temperatures in most regions over this time period, trends in extreme heat have been smaller over US croplands, reducing crop-damaging high temperatures and benefiting maize and soy yields. Here we show that agricultural intensification has created a crop-climate feedback in which increased crop production cools local climate, further raising crop yields. We find that maize and soy production trends have driven cooling effects approximately as large as greenhouse gas induced warming trends in extreme heat over the central US and substantially reduced them over the southern US, benefiting crops in all regions. This reduced warming has boosted maize and soy yields by 3.3 (2.7–3.9; 13.7%–20.0%) and 0.6 (0.4–0.7; 7.5%–13.7%) bu/ac/decade, respectively, between 1981 and 2019. Our results suggest that if maize and soy production growth were to stagnate, the ability of the crop-climate feedback to mask warming would fade, exposing US crops to more harmful heat extremes.

As a result of global increases in both temperature and specific humidity, heat stress is projected to intensify throughout the 21st century. Some of the regions most susceptible to dangerous heat and humidity combinations are also among the most densely populated. Consequently, there is the potential for widespread exposure to wet bulb temperatures that approach and in some cases exceed postulated theoretical limits of human tolerance by mid- to late-century. We project that by 2080 the relative frequency of present-day extreme wet bulb temperature events could rise by a factor of 100–250 (approximately double the frequency change projected for temperature alone) in the tropics and parts of the mid-latitudes, areas which are projected to contain approximately half the world's population. In addition, population exposure to wet bulb temperatures that exceed recent deadly heat waves may increase by a factor of five to ten, with 150–750 million person-days of exposure to wet bulb temperatures above those seen in today's most severe heat waves by 2070–2080. Under RCP 8.5, exposure to wet bulb temperatures above 35 °C—the theoretical limit for human tolerance—could exceed a million person-days per year by 2080. Limiting emissions to follow RCP 4.5 entirely eliminates exposure to that extreme threshold. Some of the most affected regions, especially Northeast India and coastal West Africa, currently have scarce cooling infrastructure, relatively low adaptive capacity, and rapidly growing populations. In the coming decades heat stress may prove to be one of the most widely experienced and directly dangerous aspects of climate change, posing a severe threat to human health, energy infrastructure, and outdoor activities ranging from agricultural production to military training.

Abstract The frequency, intensity and duration of heat waves are all expected to increase as the climate warms in response to increasing greenhouse gas concentrations. The focus of this study is on another dimension of heat waves, their spatial extent, something that has not been studied systematically by researchers but has important implications for associated impacts. Of particular interest are spatially contiguous heat wave regions, examined here over the conterminous US for the May–September season in both the current climate and climate model projections from the CMIP5 archive (11 models total) using the RCP4.5 and RCP8.5 radiative forcing scenarios. Given their myriad impacts, heat waves are defined using multiple temperature variables, one which includes atmospheric moisture. In addition to their spatial extent, several other physical attributes are computed across contiguous heat wave regions, including a proxy for energy use. An estimate of the human population exposed to current and future heat waves is also evaluated. We find that historical climate model simulations, in aggregate, show good fidelity in capturing key characteristics of heat waves in the current climate while projections show a substantial increase in spatial extent and other attributes by mid-century under both scenarios, though generally less for RCP4.5, as expected. Overall, the study presents a framework for examining the behavior, and associated impacts, of a frequently overlooked aspect of heat waves. The projected increases in the spatial extent and other attributes of heat waves reported here provides a new perspective on some of the potential consequences of the continued increase in atmospheric greenhouse gas concentrations.