Why do heat waves have such significant impacts on ecosystems and human health? Are climate migrations caused by heat waves on Earth? Are there any strategies to prevent climate migrations? Will human habitability be severely affected by the increase in heat waves?

However, the contribution of soil moisture effects to increased exposure is unknown. We use data from four climate models, in which two experiments are used to separate the effects of soil moisture and thus to examine the predicted changes in the contribution of soil moisture to the predicted increase in heat wave events. The contribution of soil moisture to population exposure to future heat waves is also examined. Combining the effects of soil moisture with global warming, the longest annual heat waves increase by up to 20 days, intensify by up to 2°C in mean temperature, and increase in frequency by up to 15% (percentage of total days for a year) in most mid-latitude drylands by 2040–2070 under the SSP585 high-emission scenario. Furthermore, changes in soil moisture are found to play a significant role in the projected increase in regional-scale heat wave characteristics compared to global land area, contributing to the increase in the global population exposed to heat waves.

Introduction: Recent decades have seen an increasing trend in heatwave duration, frequency, and intensity worldwide (Cowan et al., 2014; Miralles et al., 2014; Perkins‐Kirkpatrick & Lewis, 2020; Perkins et al., 2012; Seneviratne et al., 2021). In the 2020 Northern Hemisphere summer, across the south‐western and north‐eastern US, eastern Canada, and northern Russia, the seasonal temperature was at least 2.0°C above average (“State of the Climate: Global Climate Report for August 2020,” 2020). In 2022, extreme heat events continued throughout India and Pakistan, where the temperature in Pakistan recorded 49°C (Zachariah et al., 2023). For the Southern Hemisphere, the summer from December 2019 to February 2020 was the hottest on record for Australia. During December 2019, 11 days, in which the national area‐averaged maximum was 40°C or above, were among this unusually extended period of heatwave over much of Australia. In the context of global land, the year 2020 ranked the warmest year in the 141‐year record and had an average land temperature of +1.59 K above pre‐industrial levels (“State of the Climate: Global Climate Report for Annual, 2020,” 2020). Effects of soil moisture in amplifying hot temperature extremes in past and present climate have been widely explored in recent decades (Dirmeyer et al., 2021; Lorenz et al., 2016; Seneviratne et al., 2010, 2013; Zhang et al., 2011). Most previous studies on soil moisture‐temperature (hereafter S‐T) coupling during heatwaves had a regional focus (Fischer et al., 2007; Geirinhas et al., 2022; Hauser et al., 2016; Lewis & King, 2015; Vogel et al., 2018; Zhang et al., 2011). So far, research comparing the mechanisms of heatwaves across regions using a consistent methodology is more limited (Lorenz et al., 2016). As the spatial heterogeneity of land surface conditions (e.g., vegetation, soil moisture, and land use), precipitation variability, and atmospheric dynamics contributes to regional differences in S‐T interactions and heatwaves (A. L. Hirsch & King, 2020; A. L. Hirsch et al., 2019; Seneviratne et al., 2010; Teuling et al., 2010; S. Zhou & Yuan, 2022), the quantification and distinction of how much regional land surface contributions to heatwaves vary globally, is of primary importance. In particular, it is necessary to avoid generalizing land surface contributions when it has been demonstrated that effects of projected soil moisture changes on heat extremes vary spatially (Seneviratne et al., 2010) and land feedbacks are teleconnected in propagating heat extremes (Miralles et al., 2019; S. Zhou & Yuan, 2022). As great decreases of summer soil moisture and water limitation are widely found across the world (Denissen et al., 2022; McKinnon et al., 2021) and the impacts from heatwaves are projected to increase under future warming (Miralles et al., 2019; Perkins‐Kirkpatrick & Lewis, 2020; Perkins et al., 2012; Seneviratne et al., 2006, 2021), investigating future changes in soil moisture effects on hot temperature extremes could help to better understand regional differences in projected heatwave characteristics. Different management strategies are required to cope with shorter and more intense heatwaves or longer and less intense heatwaves (Perkins‐Kirkpatrick & Lewis, 2020). Investigations on different heatwave characteristics can facilitate more appropriate combinations of future community adaption methods and mitigation policies against heatwave impacts. Existing research is often focused on the population exposure to heatwaves per se and its influences on the mortality and morbidity of people (Jones et al., 2018; Mishra et al., 2017). Yang et al. (2021) found that heat‐ related excess mortality is projected to increase from 1.9% in the 2010s to 5.5% (0.5%–9.9%) in the 2090s under Representative Concentration Pathway 8.5 (RCP8.5) in China (Yang et al., 2021). With different heatwave definitions used, heatwave‐related mortality risks increased by a range of 3%–16% globally (Xu et al., 2016). However, population exposure to heatwaves is rarely investigated from a soil moisture perspective, despite its relevance to temperatures on timescales ranging from heatwave events to long‐term temperature trends. As soil moisture effects tend to amplify heatwaves (Miralles et al., 2014; Qiao et al., 2023) and more severe heatwaves lead to higher population exposure to heat stress and related health risks (Jones et al., 2018; Mora et al., 2017), which highlights the importance of considering soil moisture when assessing heatwave risks for human populations. In this study we investigate the contribution of soil moisture changes, that is, the soil moisture variability and trend beyond its climatological value of the recent historical climate as induced both by S‐T interactions and climate change, to increases of heatwave characteristics and how this relates to future changes in population exposure to extreme heat.

Figure 1. Spatial distributions of changes in soil moisture effects on different heatwave characteristics. (a, c, e, and g), Differences between CTL and pdLC for the heatwave characteristics such as heatwave mean intensity (a; HWMt), peak intensity (c; HWAt), frequency (e; HWF), and duration (g; HWD) is depicted in 2000–2030 (PRE). (b, d, f, and h), Differences between CTL and pdLC for the heatwave characteristics such as heatwave mean intensity (b; HWMt), peak intensity (d; HWAt), frequency (f; HWF), and duration (h; HWD) in 2040–2070 (FUT). The data within this figure are of SSP585. Hatching indicates where the differences between CTL and pdLC are statistically significant (KS test, p value < 0.05). The analysis is limited to the summer‐centering periods. The extensions in color bars indicate values beyond the limits. Oceans have been masked in white.

Figure 2. Projected changes in global and regional heatwave characteristics. (a–d) Increases of heatwave mean intensity (a; HWMt), peak intensity (b; HWAt), frequency (c; HWF), and duration (d; HWD) induced by soil moisture changes and other factors under SSP585 are displayed across two 30‐year periods. The global and regional values of the projected changes shown here are area‐weighted. The changes are between 2040–2070 and 2000–2030. The orange part indicates contributions of soil moisture changes; the blue part indicates other factors. The domains are delineated in both Figures 1 and 3, the oceans are excluded for these domains. Horizontal bars are added to this figure to help readers more easily compare the increases of global and regional heatwave characteristics with or without contributions from soil moisture.

Figure 3. Projected changes of population exposure to heatwaves. Increases of heatwaves' impacts on the human population induced by soil moisture changes and other factors under SSP585 are shown in the form of four different heatwave characteristics, which includes heatwave mean intensity (HWMt), peak intensity (HWAt), frequency (HWF), and duration (HWD). The changes are between present period (2040–2070) and future period (2000–2030) and are calculated between the population percentage that is above the threshold (HWMt, 21.5°C; HWAt, 32°C; HWF, 5%; HWD, 9 days) in both 2000–2030 and 2040–2070. The population data for the current period are of 2000–2030, while those in the future period are of 2040–2070. The map indicates the synthetical increases of the population exposure based on the four heatwave characteristics. In the bar plots, the orange part indicates contributions from soil moisture changes; the blue part indicates other factors. Oceans and other regions without population data available have been masked in white.

Conclusions:

Our study shows the influences from the changes in soil moisture on heatwave conditions by isolating soil moisture influences and comparing the future and present responses of extreme indices to soil moisture. The influences of soil moisture include both the effects of short‐term variability and climate change‐induced long‐ term changes of soil moisture on heatwaves. This study focused on the near future period (2040–2070) to project changes from current period (2000–2030), reflecting approaching challenges faced for implementing mitigation strategies and global policies against imminent heatwave impacts. It is found that heatwaves are projected to increase in their frequency, duration, mean intensity, and peak intensity for most mid‐latitude land regions by the end of the 21st century (Figure S4 in Supporting Information S1). Moreover, greater increases are found over tropical and extra‐tropical regions for heatwave frequency and duration, while for heatwave magnitude and amplitude, greater increases happen in the high‐latitude regions (Figure S4 in Supporting Information S1). It is found that soil moisture effects are stronger in US, SA, WE, East Europe, northern Asia, eastern Asia, SCAF, and AU (Figure 1). The regions with the largest responses of heatwave characteristics to S‐T coupling are majorly distributed in mid‐latitude areasin both hemispheres(Figure 1), which coincide with most of those identified with strong impacts from projected changes of soil moisture (Figure S4 in Supporting Information S1; Seneviratne et al., 2013). The areas in our study that are identified with strong contributions of soil moisture to heatwaves (Figure 1) tend to agree with some observationally derived estimates (M. Hirschi et al., 2010; Mueller & Seneviratne, 2012). Besides, some former modeling studies have also indicated regional differences in S‐T coupling (Knist et al., 2017; Seneviratne et al., 2006). Soil moisture changes constitute a strong component in the exacerbation of heatwaves in ESCA, AU, US, WE, EEWA, SCAF, and SA. Compared with global average, more contributions from soil moisture changes are found over most of these domains (Figure 2). It confirms the results of some existing research that removing S‐T coupling will significantly weaken the temperature extremes over most land surfaces (Lorenz et al., 2016; Vogel et al., 2017). The intensification of heatwaves due to soil moisture is more concentrated in densely populated regions(Figures 1 and 2), which makes these dynamics especially salient both in real‐world scenarios and in their simulation. In our investigations on changes of population exposure to heatwaves and how the soil moisture influences those, we found large increases (more than 50%) of population exposure to heatwaves in northern and northeastern parts of the US, southern part of the SA, SCAF excluding the eastern part, WE excluding the Mediterranean part, EEWA excluding the Arabian Peninsula, ESCA excluding Southeast Asia, and AU (Figure 3). These regions excluding AU are also located in the regions where soil moisture effects are strong (Figure 1) and most of them are densely populated regions. However, soil moisture effects sometimes don't match in their contributions to changes of heatwave characteristics and those of population exposure to heatwaves. For example, in AU, although soil moisture effects are not strong enough to make a dominant component in contributing to increases of heatwave amplitude and frequency, they make significant impacts on increasing population exposure to both heatwave characteristics. One of the reasons is that the calculation of population exposure to heatwaves utilize only population count even though population exposure to heatwaves incorporates both population count and heatwave metrics. While comparatively less population count is distributed in some subregions with less soil moisture effects, other parts in the same domain where population count that is above the respective threshold tends to change greatly are sometimes limited to the regions with stronger soil moisture effects (Figure 1), which makes the contributions of soil moisture to changes of population exposure greater than those to projected changes of heatwave characteristics. Using percentage of population count to express exposure to heatwave cause underestimation of heatwave impacts such that for specific regions increases of population count exposed to heatwaves alone don't necessarily induce increases in percentage of population count exposed to heatwaves. That said, increases of population exposure to heatwave frequency and duration are found to be strong, which indicates that different heatwave characteristics can be incorporated in heatwave evaluation and more emphases should be put on these two heatwave characteristics in the future when considering policies addressing heatwave impacts on humans. In summary, understanding soil moisture dynamics and its influence on heatwaves is essential for effective heatwave management and public health strategies. Better simulation of soil moisture data can improve heatwave predictions, thus allowing for better public health preparedness. As SSP126 represents a low emission scenario, which serves as a more optimistic future reference compared with the challenging SSP585. It is helpful in more completely understanding results of incorporating soil moisture variability and trend on heatwave impacts by comparing both scenarios. Even though S‐T coupling is generally less strong (Figure 1 and Figure S1 in Supporting Information S1), it generally makes up relatively higher percentage (Figure 2 and Figure S2 in Supporting Information S1) of less projected changes of all the heatwave characteristics (Figures S4 and S5 in Supporting Information S1) in SSP126 in contrast to SSP585. Contribution from soil moisture effects to increases of population exposure to different heatwave characteristics are relatively similar in SSP126 and SSP585, while projected increases of population exposure to heatwave characteristics are different in the two scenarios (Figure 3 and Figure S3 in Supporting Information S1). One of the reasonsis that the change measured for population exposure is in percentage based on the four heatwave characteristics rather than in population count and increases of the population exposed to heatwaves across the thresholds in SSP585 happen less than those in SSP126 (Figures S6 and S7 in Supporting Information S1), which makes the increases of population exposure in SSP585 less than those in SSP126. Some land management can be implemented to mitigate the impacts of heatwaves on human population. In particular, as irrigation can greatly change local soil moisture, it can be seen as one of the plausible approaches to alleviate heat extremes (A. L. Hirsch et al., 2017; Thiery et al., 2020). Irrigation can substantially reduce human exposure (0.79–1.29 billion) to heatwaves (Thiery et al., 2020). However, although irrigation is found to decrease temperature in some places (A. L. Hirsch et al., 2017; Thiery et al., 2020), it is proved not to attenuate or even increase moist heat stress while increasing air humidity (Krakauer et al., 2020; Wouters et al., 2022). In addition, ample water supply is a challenge especially at some hot regions where water availability is sometimes limited (Barker et al., 2021; He et al., 2021), which raises practical issuesfor using irrigation as an adaption option against heatwaves. Apart from irrigation, some other land management, such as land radiative management and forestation can be taken into consideration (Gormley‐Gallagher et al., 2022; Kala et al., 2022; Portmann et al., 2022; Seneviratne et al., 2018), for example, the study conducted by A. L. Hirsch et al. (2017) has indicated the collective effects of both irrigation and crop albedo were robust in cooling the land surface, which can reduce hot temperature extremes by more than 2°C in North America, Eurasia, and India compared to a scenario in which no land management is implemented (A. L. Hirsch et al., 2017). The land radiative management was found to help counteract hot extremes especially in densely populated and some agricultural regions (Seneviratne et al., 2018). Regional land radiative management reduces average temperature anomaly during heatwaves by 0.8–1.2°C and heatwave frequency by 10–20 days over Europe and North America (Kala et al., 2022). Future investigations on influences from sole background warming trend are needed. Climatic warming trend may dominate changes of regional temperature compared to soil moisture, land use changes, or other land‐ atmosphere interactions (Perkins‐Kirkpatrick & Lewis, 2020; Pitman et al., 2011). Land‐atmosphere coupling regime and its variability are also one of the future research focuses (Denissen et al., 2022; Hsu & Dirmeyer, 2023). Furthermore, occurrence and severity of heatwaves and their future changes under multiple compounding drivers deserve further research. In addition to soil moisture effects, heatwave evolution and variability are also related to natural climate variability such as El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). These potential drivers will evolve over time and alter the risk of future heatwaves. Taking climate indices as covariates or using sensitivity experiments based on model simulations can shed light on the effects of changes in natural variability and their corresponding impacts on occurrence and severity of future heatwaves.The subregions divided from the domains need to be investigated in the future research to improve evaluation of population exposure to heatwaves. Different regional thresholds in measuring population exposure to heatwaves need to be considered in the future studies. Different effects from soil moisture on subregions are required for further studies, for example, southern and southeastern Africa and middle Africa have contrasting trends of the soil moisture effects on heatwave characteristics, the Arabian Peninsula and East Europe have similar phenomena (Figure 1). In the meantime, Southeast Asia, northern Asia, eastern Asia, middle Asia, northern India, and southern India all have different patterns of heatwaves from soil moisture effects, and among these subregions, S‐ T coupling on different heatwave characteristics is varied if diving into a more local scale. For subregions such as Arctic regions in Russia, Far East in Russia, Tibetan Plateau, the impacts of heatwaves measured through increases of population exposure are mild from a population perspective as these regions have low population count. However, the exacerbation of heatwave conditions to these regions will still cause catastrophic impacts on the local ecosystems as the ecosystems in these regions are fragile and extremely vulnerable to climate change. More future studies on regional differences of heatwave impacts, which may integrate finer spatial resolutions of climate model results and recent historical simulations on spatial distributions of soil moisture effects, heatwave characteristics, and human populations, are needed to better find local thresholds of heatwave characteristics and account for the local changes of human exposure to heatwaves. Differentregions also require tailored analysis periods. A pronounced seasonal pattern occurs over higherlatitudes, with heat extremesmore common in June through August(JJA)in the Northern Hemisphere and Decemberthrough February (DJF) in the Southern Hemisphere. On the other hand, tropical regions experience a more uniform distribution of extreme heat eventsthroughout the year, with the most intense dry heat occurring in varying months, as noted by Rogers et al. (2021). Tomasini et al. (2022) observed that during April 2010, the West Sahel experienced its peak season for heatwaves coinciding with the culmination of the dry season, when the impacts ofsoil moisture on temperature through land‐atmosphere feedbacks might be small (Tomasini et al., 2022). Additionally, recognizing distinctseasonsfor heatwavesin variousregions, along with the patterns of monsoon, aidsin examining how soil moisture might interact with humid heat extremes in future studies (Birch et al., 2022; Rogers et al., 2021). Contrast between urban area and non‐urban area is also a focal point to be considered. Rapid urbanization process is expected in the future, which tends to make urban heatwaves more frequent and increases the population exposure to these events through urban heat island and some other feedbacks(Han et al., 2021; Wang et al., 2021). At the urban scale the feedbackssuch as urban heat island effectsthat cause urban heatwaves are not well resolved by most current global‐scale climate models. These feedbacks are not negligible as they can augment heatwave conditions and add uncertainty to our estimations (Zheng et al., 2021). Moreover, populations are not equally exposed to heatwaves, population in some regions may have access to advanced cooling systems, well‐insulated buildings, or green spaces which reduce surrounding temperature, while the others may not. Such inequality of access to cooling services are exacerbated by the decreases of the cooling services per se. For example, study carried out by Dong et al. (2022) showed that approximately 93.3% of the cities they investigated had significant decreasing trends of the urban green spaces (Dong et al., 2022). Same heatwave temperature threshold (T90)throughout the present period of 2000–2030 and future period of 2040– 2070 contributes partly to the situation where all the selected heatwave characteristics increased significantly (Figure 2, Figures S4 and S5 in Supporting Information S1). Changes in both spatial and temporal dimensions of heatwaves strongly depend on the selected threshold as strong increasesin heatwave characteristics are detected if time‐invariant climatic thresholds are adopted while minor changes are found in these characteristics when moving thresholds are defined (Vogel et al., 2020). 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