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Heat extremes have severe implications for human health, ecosystems, and the initiation of wildfires. While they are mostly introduced by atmospheric circulation patterns, the intensity of heat extremes is modulated by terrestrial evaporation associated with soil moisture availability. Thereby, ecosystems provide evaporative cooling through plant transpiration and soil evaporation, which can be reduced under water stress. While it has been shown that regional ecosystem water limitation is projected to increase in the future, the respective repercussions on heat extremes remain unclear. In this study, we use projections from 12 Earth system models to show that projected changes in heat extremes are amplified by increasing ecosystem water limitation in regions across the globe. We represent the ecosystem water limitation with the ecosystem limitation index (ELI) and quantify temperature extremes through the differences between the warm-season mean and maximum temperatures. We identify hotspot regions in tropical South America and across Canada and northern Eurasia where relatively strong trends towards increased ecosystem water limitation jointly occur with amplifying heat extremes. This correlation is governed by the magnitude of the ELI trends and the present-day ELI which denotes the land–atmosphere coupling strength determining the temperature sensitivity to evaporative cooling. Many regions where ecosystem functioning is predominantly energy-limited or transitional in the present climate exhibit strong trends towards increasing the water limitation and simultaneously experience the largest increases in heat extremes. Sensitivity of temperature excess trends to ELI trends is highest in water-limited regions, such that in these regions relatively small ELI trends can amount to drastic temperature excess trends. Therefore, considering the ecosystem's water limitation is key for assessing the intensity of future heat extremes and their corresponding impacts. [ABSTRACT FROM AUTHOR]

Heat extremes have severe implications for human health, ecosystems and the initiation of wildfires. Whereas they are mostly introduced by atmospheric circulation patterns, the intensity of heat extremes is modulated by vegetation functioning associated with soil moisture availability. Thereby, vegetation provides evaporative cooling through transpiration, which can be reduced under water stress. While it has been shown that regional ecosystem water limitation is projected to increase in the future, the respective repercussions on heat extremes remain unclear. In this study we use projections from eight Earth system models to show that projected changes in heat extremes are amplified by increasing ecosystem water limitation in regions across the globe. We represent ecosystem water limitation with the Ecosystem Limitation Index (ELI) and quantify temperature extremes through the differences between warm-season mean and maximum temperatures. We identify hotspot regions in tropical South America and across Northern Eurasia where relatively strong trends towards increased ecosystem water limitation jointly occur with amplifying heat extremes. This correlation is governed by the magnitude of the ELI trends and the present-day ELI which denotes the land-atmosphere coupling strength determining the temperature sensitivity to evaporative cooling. Many regions where vegetation functions are predominantly energy-limited or transitional in present climate exhibit strong trends towards increasing water limitation and simultaneously experience the largest increases in heat extremes. Therefore, considering the ecosystem's water limitation is key for assessing the intensity of future heat extremes and their corresponding impacts. [ABSTRACT FROM AUTHOR]

In 2018 and 2019, heatwaves set all‐time temperature records around the world and caused adverse effects on human health, agriculture, natural ecosystems, and infrastructure. Often, severe impacts relate to the joint spatial and temporal extent of the heatwaves, but most research so far focuses either on spatial or temporal attributes of heatwaves. Furthermore, sensitivity of heatwaves characteristics to the choice of the heatwave thresholds in a warming climate are rarely discussed. Here, we analyze the largest spatiotemporal moderate heatwaves—that is, three‐dimensional (space‐time) clusters of hot days—in simulations of global climate models. We use three different hazard thresholds to define a hot day: fixed thresholds (time‐invariant climatological thresholds), seasonally moving thresholds based on changes in the summer means, and fully moving thresholds (hot days defined relative to the future climatology). We find a substantial increase of spatiotemporally contiguous moderate heatwaves with global warming using fixed thresholds, whereas changes for the other two hazard thresholds are much less pronounced. In particular, no or very little changes in the overall magnitude, spatial extent, and duration are detected when heatwaves are defined relative to the future climatology using a temporally fully moving threshold. This suggests a dominant contribution of thermodynamic compared to dynamic effects in global climate model simulations. The similarity between seasonally moving and fully moving thresholds indicates that seasonal mean warming alone can explain large parts of the warming of extremes. The strong sensitivity of simulated future heatwaves to hazard thresholds should be considered in the projections of potential future heat‐related impacts. Plain Language Summary: Heatwaves, such as the worldwide events that occurred in 2018 and 2019, can be associated with substantial impacts on humans, ecosystems, and the economy. These impacts are often caused by the joint temporal and spatial dimensions of the events. However, these properties of the heatwaves are rarely studied jointly. Furthermore, heatwaves are often defined based on exceedance above a fixed threshold, which might overestimate future heatwave impacts in a warming climate. Therefore, we compute here moderate heatwaves considering their true time‐space structure based on three hazard thresholds. We then investigate future changes of these moderate heatwaves for different warming levels. We detect a strong increase in the overall magnitude, spatial extent, and duration of heatwaves if we use fixed hazard thresholds. In the case with seasonally and fully moving hazard thresholds, we find no or only little changes of heatwave characteristics. This strong threshold sensitivity should be considered when estimating impacts associated with future heatwaves in a warming climate. Key Points: Changes in spatiotemporal moderate heatwaves strongly depend on the hazard thresholdsWe detect strong increase in projected heatwave area, duration, and magnitude if based on time‐invariant climatological thresholdsNo/minor changes are modeled in projected heatwaves when defined relative to the future climatology and changes in summer means (moving thresholds) [ABSTRACT FROM AUTHOR]

The frequency and intensity of climate extremes is expected to increase in many regions due to anthropogenic climate change. In central Europe extreme temperatures are projected to change more strongly than global mean temperatures, and soil moisture-temperature feedbacks significantly contribute to this regional amplification. Because of their strong societal, ecological and economic impacts, robust projections of temperature extremes are needed. Unfortunately, in current model projections, temperature extremes in central Europe are prone to large uncertainties. In order to understand and potentially reduce the uncertainties of extreme temperature projections in Europe, we analyze global climate models from the CMIP5 (Coupled Model Intercomparison Project Phase 5) ensemble for the business-as-usual high-emission scenario (RCP8.5). We find a divergent behavior in long-term projections of summer precipitation until the end of the 21st century, resulting in a trimodal distribution of precipitation (wet, dry and very dry). All model groups show distinct characteristics for the summer latent heat flux, top soil moisture and temperatures on the hottest day of the year (TXx), whereas for net radiation and large-scale circulation no clear trimodal behavior is detectable. This suggests that different land- atmosphere coupling strengths may be able to explain the uncertainties in temperature extremes. Constraining the full model ensemble with observed present-day correlations between summer precipitation and TXx excludes most of the very dry and dry models. In particular, the very dry models tend to overestimate the negative coupling between precipitation and TXx, resulting in a warming that is too strong. This is particularly relevant for global warming levels above 2 °C. For the first time, this analysis allows for the substantial reduction of uncertainties in the projected changes of TXx in global climate models. Our results suggest that long-term temperature changes in TXx in central Europe are about 20% lower than those projected by the multi-model median of the full ensemble. In addition, mean summer precipitation is found to be more likely to stay close to present-day levels. These results are highly relevant for improving estimates of regional climate-change impacts including heat stress, water supply and crop failure for central Europe. [ABSTRACT FROM AUTHOR]