Yansong Guan

Anthropogenic activities, including anthropogenic climate change (ACC) and historical water and land management (HWLM), have intensified hydrological extremes in recent decades, with potential increases in flood risk. However, uncertainties in hydrological models and a lack of consideration for HWLM in climate models have limited the estimation and attribution of the changes in global‐scale flood risks. Based on a multi‐model ensemble of daily discharge simulations and projections from the latest Inter‐Sectoral Impact Model Intercomparison Project (ISIMIP 3), our estimation shows that flood risks significantly increased (slightly decreased) in the north (south) of 50°N during 1971–2019 when considering the combined effects of ACC and HWLM. Under continuing global warming (from 1.5°C to 3°C), 59.6%–69.4% of the global land area would witness an increasing flood risk, with 71.9%–87.5% of basin‐level risks attributable to anthropogenic activities, particularly in arid/tropical zones. Meanwhile, the flood risk in 38.8% of the global land area shows the anthropogenic signals before 2065. This increase is enlarged by greater flood magnitude and variability in the future, with flood magnitude being the dominant factor in most regions. We suggest that hydrological services and policymakers should implement adaptive strategies that consider anthropogenic increases in flood magnitude and variability, utilizing detected signals to guide global flood‐risk mitigation.

Global warming enhances soil moisture (SM)‐temperature (SM‐T) coupling, exacerbating extreme climates. However, the mechanisms and future trends of this enhancement remain inadequately understood. Analysis of multiple reanalysis data sets shows that during summer seasons of 1981–2020, approximately 45.6% of global lands exhibit a significant intensifying SM‐T coupling trend, with hotspots in Northeast Asia, Central Asia, and southwestern North America. Ridge regression reveals the drying SM (contribution: 50.2%) dominates the enhanced SM‐T coupling, followed by greening vegetation (28.3%) and decreasing cloud cover (21.5%). Furthermore, pre‐summer vegetation greening has the effect of intensifying the summer SM‐T coupling. Despite discrepancies in climate models' simulations of SM‐T coupling trend, the optimal multi‐model ensemble mean well reproduces SM‐T coupling's spatiotemporal characteristics. Based on this multi‐model ensemble mean, the enhancement of SM‐T coupling is attributed to anthropogenic forcings and projected to further intensify under future emission scenarios.

Terrestrial water availability sustains livelihoods, socioeconomic development, and ecosystems. Despite an understanding of contributions of oceanic moisture to terrestrial hydroclimatic extremes, whether surpluses of terrestrial water availability migrate directly and contiguously from the ocean and the influence of climate change on this process remain unclear. Here, we use a coherent feature-tracking method to identify ocean-to-land water availability surpluses (OWASs), characterized by spatiotemporally contiguous migration of excess atmospheric freshwater (precipitation-minus-evapotranspiration) from ocean to land. Over the past several decades, especially in northern mid-high latitudes (NMHL; above 48°N), OWASs have exhibited longer persistence, wider areal extent, and greater intensity than those developed solely over land. These landward migrations are associated with seasonal Atlantic teleconnection and Pacific circulation shift. Under the business-as-usual scenario, these two processes are projected to be enhanced, markedly increasing OWAS characteristics in NMHL driven by thermodynamic atmospheric responses to future warming. Intensified OWASs may not only help alleviate long-term droughts but also have the potential to accentuate pluvial risks.

Compound hot extremes (CHEs)—the concurrence of daytime and nighttime heat—have been increasing under anthropogenic warming, causing serious damage to human society and ecosystems. However, the anthropogenic fingerprint in past and future changes in daily CHEs and the corresponding population exposure remain unclear. Here, using a fingerprint‐based detection and attribution method, we quantify contributions of different external forcings to the historical increase in CHEs by defining three daily scale metrics: the probability ratio (PR) of CHEs and the proportion of CHEs in the number of extreme hot days/nights (PTday/PTnight). All three metrics increased significantly by 117.9%, 128.7%, and 133.4% globally from 1950 to 2014. It is very likely (>90%) for human to leave fingerprints in increasing daily CHEs. The increases in PR, PTday, and PTnight that are attributable to greenhouse gas emissions are 121.2%, 123.1%, and 121.5%, respectively. By the end of the 21st century under a high‐emission scenario, fingerprint‐constrained projections show the PR of CHEs will be about 60 times higher than the climatological period, with around 41% of hot days and 49% of hot nights as CHEs, reducing the magnitude compared to raw projections. In some regions such as mid and high‐latitudes, almost all daytime or nighttime extreme‐heat events would become CHEs. The growing difference between CHE days per person and per unit of land area suggests that CHEs will disproportionately affect densely populated areas. Our results indicate that adaptive measures are required to alleviate the increasing proportion of CHEs and the disproportionate population exposure in densely populated areas.

Anthropogenic warming has exacerbated atmospheric heatwaves globally, yet the transboundary migration of heatwaves between land and ocean, along with the anthropogenic influence on this process, remain unclear. Here, we employ a Lagrangian tracking approach to identify and track spatiotemporally contiguous warm-season heatwaves in both reanalyses and simulations. This way, we show that land-ocean transboundary heatwaves, especially in the tropics, exhibit longer persistence, wider areal extent, and greater intensity than those confined to land or ocean. These transboundary migrations are primarily driven by the movement of high-pressure systems (such as the westward extension of subtropical highs) and the propagation of Rossby waves. Associated with increasing greenhouse gas concentrations, the frequency of tropical heatwave migrations has increased over the past four decades, and is projected to accelerate further in the twenty-first century under the high-emissions scenario. Anthropogenically-driven landward migrations are amplified by stronger landward winds that drive heat advection, while oceanward processes are likely intensified by increased land-ocean temperature gradient. These intensified transboundary heatwaves not only accentuate humid heat risks for humans but also threaten ecosystems.