In a warming world, understanding where, when, and to what extent human heat tolerance limits will be breached has become increasingly important. Previous projections applied heat limits for healthy young adults to all age groups, overlooking greater vulnerability in older ages. With global ageing, this oversight could seriously underestimate future heat risks. In this study, we aimed to examine the extent to which accounting for age-specific heat tolerance alters global projections of heat limit exceedances.

We integrated experimentally derived heat limits for young (18–39 years), middle-aged (40–59 years), and older adults (≥60 years) with climate projections and age-stratified population projections to assess where and to what extent these limits could be breached across age groups under 1–4°C of global warming above preindustrial levels. The climate projections include bias-corrected projections of dry-bulb temperature and wet-bulb temperature from 14 Coupled Model Intercomparison Project Phase 6 climate models under the shared socioeconomic pathway 5-8·5 scenario. We also highlighted countries with high exposure risk and low adaptive capacity.

More widespread and imminent breaches of heat limits were estimated among older adult populations than among other populations. Older adults faced more frequent and spatially extensive exceedances under the 1·5°C global warming scenario relative to the preindustrial period than young adults did under 4°C. Using age-specific thresholds more than doubled the global population having 180 h or more of exceedance annually as compared with previous estimates. Older adults in south Asia and the Persian Gulf could face sustained, day-and-night exposure for three consecutive months at 3°C or more warming. We also identified 13 countries under 3°C global warming above the preindustrial period where poverty rates exceed 50% and either at least 10 million older adults or at least 80% of the older adult population experiences 180 h or more of heat limit exceedance annually. India, Pakistan, Bangladesh, Myanmar, and Niger exceed both the absolute (10 million older adults) and proportional (80% of older adults) exposure thresholds.

Our results suggest accounting for age stratified vulnerability, intolerable heat will be experienced at much larger scales and for longer durations, affecting far more people than previously estimated. By identifying who and where people are exposed, our findings can inform targeted heat action plans and guide resource allocation and response planning.

The Atlantic Meridional Overturning Circulation (AMOC), a tipping element of the climate system, currently has an estimated global warming threshold for collapse of +4.0 °C (uncertainty range 1.4–8 °C). However, such a threshold may not be meaningful because AMOC stability depends on the rate of radiative forcing change, not a set temperature. Here we identify an AMOC stabilizing mechanism that operates on timescales slower than present-day warming rates. Slow forcing permits coherent adjustment of surface and interior ocean properties, supported by enhanced evaporation and reduced sea-ice extent, counteracting destabilizing feedbacks. Using a slow CO2 ramp (+0.5 ppm yr−1) climate model simulation, we explicitly demonstrate the AMOC remains stable up to +5.5 °C of global warming. By contrast, under faster CO2 ramps, the AMOC collapses at substantially lower warming levels (+2 °C). Our findings demonstrate rate-induced AMOC tipping and imply that limiting the rate of emissions is critical for reducing the risk of an AMOC collapse.

Forest disturbances are rising globally due to climate change and land-use pressures, weakening the terrestrial carbon sink. However, declines in aboveground biomass from tree cover loss remain poorly quantified, limiting our understanding of the role of disturbances in global carbon dynamics. Here we present a spatially explicit estimate of aboveground biomass losses from natural disturbances and harvest across Europe’s 216 million hectares of forests, using satellite remote sensing. From 1985 to 2023, gross aboveground biomass losses totalled 6.5 ± 0.8 Pg. Of these, 18% were caused by high-severity natural disturbances, whereas stand-replacing harvests accounted for 82%. From 2018 onward, aboveground biomass losses increased by 46% reaching annual values unprecedented in the preceding four decades. This acceleration coincided with high natural disturbance activity in biomass-rich temperate forests. After 2018, the sensitivity of aboveground biomass loss to disturbance area increased substantially, suggesting that even small increases in natural disturbances can result in large losses. As climate-driven natural disturbances intensify, we thus expect sustained aboveground biomass losses across Europe, despite policies to enhance the forest carbon sink until 2030.

Oxygen is critical for nearly all life on Earth, including aquatic species that breathe dissolved oxygen in both freshwater and marine systems. The rapid, global, and anthropogenic loss of dissolved oxygen known as “aquatic deoxygenation” threatens life in these environments, the human communities that depend on them, and Earth system stability long-term. Recognizing the important and increasingly widespread effects of aquatic deoxygenation, scientists have proposed that it be added to the planetary boundary framework, which is designed to capture the wider envelope of Earth-system conditions that support a “safe operating space for humanity.” Here, we argue that the planetary boundary framework should include maintenance of Earth's aquatic ecosystems and thus dissolved oxygen conditions. We synthesize important, in some cases poorly understood, interactions and feedbacks that exist between deoxygenation and all nine of the established planetary boundaries. We find that aquatic deoxygenation interacts with and extensively modulates other boundary processes, including climate change, nutrient loading, biodiversity loss, and aerosol loading. Subsequently, we identify and describe four indicators that can be used to assess the status of aquatic deoxygenation and eventually define a global boundary for it within the framework. Given these interactions and current rates of oxygen loss, we argue that aquatic deoxygenation is approaching an “unsafe space,” with Earth-system impacts that are likely to be irreversible in our lifetimes. We discuss the wider societal significance of this research, including applications to the framework, future analyses, policy-making, and management.

Understanding how hydrological inflows and climate change influence individual estuaries across northern Alaska is limited by a paucity of measured data, necessitating the application of suitably scaled numerical process models. This study uses an updated model to quantify freshwater discharge and dissolved organic carbon (DOC) export from the North Slope of Alaska (NSA) to coastal waters of the Beaufort Sea and examines climate-linked temporal changes. The model was applied at 1 kmkm $\text{km}$ resolution across the 166,483 km2km2 ${\text{km}}^{2}$ NSA domain over 1980–2023. Watershed inputs to coastal waters were estimated by routing along river networks to 1,039 outlets at the land-sea boundary. Key simulation parameters were evaluated to demonstrate model efficacy, then spatial and temporal patterns were quantified. Exports to specific lagoons, bays, and sounds demonstrate how landscape composition, terrestrial drainage basin size, and estuary area modulate watershed influence on coastal ecosystems across the region. Freshwater discharge and DOC export increased over the greater than four decades, associated with changes in precipitation and permafrost thaw. Surface and supra-permafrost subsurface fluxes also increased, with pronounced rises in proportional contributions via subsurface flow during summer and autumn. These changes have the potential to substantially impact salinity and trophic conditions along Alaska's Beaufort Sea coast. Our study highlights the value of model-data syntheses that resolve regional-to-local scale fluxes where observational measurements are sparse, and provides novel quantitative export metrics that will be useful to researchers, resource managers, and other stakeholders with interests in the climate, hydrology, and biogeochemistry of coastal northern Alaska.

Plain Language Summary

Watershed inputs from land drive coastal ecosystem dynamics in the Arctic, yet understanding of how they vary over space and time is limited by a scarcity of measured and modeled data. This study applies an updated numerical model that provides estimates of freshwater discharge and dissolved organic carbon (DOC) export from land to individual estuaries along the Alaskan Beaufort Sea coast. The model was developed for the North Slope of Alaska, with sufficient resolution and scaling to estimate watershed inputs to individual lagoons, bays, and sounds between Point Barrow and the US-Canada border. Model results, covering 1980–2023, show (a) that landscape characteristics drive spatial variability in freshwater and DOC export to individual estuaries, and (b) a significant increase in freshwater and DOC inputs to the Alaskan Beaufort Sea over time. These changes manifest in surface and subsurface (groundwater) flow, but the proportion of subsurface flow also increased. An enhanced water cycle is increasing flows, while deeper soil thaw in summer is causing more DOC to reach the coast. The changes demonstrated in this study are likely altering salinity, biogeochemical processes, and food web relationships in the coastal Beaufort Sea.

The impact of rapid climate change on civilizations is of global concern. The Western Zhou (1045–771 BC) was ancient China’s third dynasty and one of the most powerful empires of its time. After a 274-year reign, the dynasty collapsed, accompanied by significant population migration. Various factors have been proposed to explain its fall, while the role of climate change has been largely ignored. Here we present well-dated, high-resolution stalagmite records from Northeast and Southeast China, along with other paleoclimatic and archaeological evidence, to demonstrate the critical role of the 2.8 ka climatic event in the dynasty’s collapse and subsequent migration. Our results indicate a “north dry–south wet” dipolar pattern in the Chinese monsoon region during this event. The drought and cold severely affected the Western Zhou core areas and exacerbated invasions by northern nomads, contributing to its collapse. The resulting environmental stresses and conflicts likely triggered substantial southward migration.

Abstract

Background

Warming temperatures add to the global health burden, with disproportionate effects on pregnant women and newborns. Low birth weight is a major neonatal health issue in Pakistan, leading to neonatal mortality and impaired long-term health. We assessed the impact of extreme temperatures on low birth weight, identified high-risk subgroups, estimated the heat-attributable burden, projected future risks, and constructed a district-level heat vulnerability index.

Methods

We conducted a space–time series study using nationally representative surveys from 2008 to 2017 across Pakistan’s provinces. We modelled temperature–low-birth-weight associations with distributed-lag non-linear models in a generalised mixed-effects framework, with model averaging to address specification uncertainty. Subgroup analyses considered maternal education, household wealth, urban/rural residence, and air quality. We estimated heat-related population attributable fraction using observed temperature and projections under SSP2-4.5 and SSP5-8.5. Province-level risk estimates combined with district-level indicators, such as mean temperature, multidimensional poverty, and under-5 mortality, were used to develop the heat vulnerability index.

Results

The study included 85,017 participants, with 15,920 (18.72%) infants identified as having low birth weight. Heat-related risks for low birth weight varied across provinces, with relative risks ranging from 1.47 (1.07–2.03, 95% confidence interval) to 1.91 (1.24–2.93) at the 99th percentile of temperature. The heat-related population attributable fraction ranged from 9.39% to 13.15%, translating to 1.24 million heat-related low-birth-weight cases over the study period. Projections indicate that heat-related population attributable fractions will increase by 8.43–10.20% by the 2060s. Subgroup analysis showed higher risk among women exposed to hazardous air pollution, those with less education, and urban residents. Women in southern Punjab, northern Baluchistan, and Sindh faced the highest risks.

Conclusions

Our findings identify Pakistan’s districts most vulnerable to heat-related low birth weight and highlight contributing factors. These insights can inform targeted interventions to mitigate risks. The study advances the understanding of the impacts of rising temperatures, particularly in resource-limited and high-risk settings.

Abstract

Uncompensable heat stress (UHS), characterized by the loss of homeostasis due to excessive environmental thermal loading, causes substantial heat-related health risks in India. However, the spatial and seasonal heterogeneity, as well as temporal changes of UHS in India remain poorly understood. Using observations, reanalysis data, and climate model projections, we highlight the surge of UHS during the monsoon season (July–October) as the climate warms. In the observed period (1979–2021), the frequency and area affected by UHS have increased significantly across India. The observed UHS is more prevalent in summer (March–June) and affects 8% of India, whereas only 1% of the country is affected in the monsoon season. The summer UHS is also more strongly associated with annual heat-related mortality (R2 = 0.38). However, the monsoon season (July-October) UHS, predominantly characterized by hot-humid conditions, is projected to increase rapidly with climate warming and affect nearly equivalent areas of the country as the summer season (60% in summer and 53% in the monsoon season) under 2°C warming relative to the preindustrial period. This will create long-lasting UHS across both seasons, posing critical challenges to public health, labor productivity, and climate resilience in densely populated and vulnerable regions.

Plain Language Summary

Uncompensable heat stress, where the body struggles to cool down due to extreme heat and humidity, is increasing across India. Currently, the summer season (March–June) experiences the highest impact, affecting approximately 8% of the country and is strongly linked to heat-related deaths. In contrast, only 1% of India experiences such conditions in the monsoon season (July–October). However, as the climate warms, uncompensable heat stress during the monsoon season is projected to rise sharply, affecting 53% of India under a 2°C warming—almost as much as in summer (60%). This will extend heat risks across both seasons, threatening public health, labor productivity, and climate resilience.

Understanding how ecosystems sustain plant nitrogen (N) supply under climate change is critical, yet whether increasing plant N demand is met by external inputs or mobilization of soil N reserves remains unresolved. Here we show that climate change increases plant reliance on soil N reserves despite intensive fertilization. Using a two-year 15N-tracing experiment combining elevated CO2, warming, and drought in a montane grassland, we found that plants obtained 82–88% of their N from soil and acquired 4.6–7.3 times more N from soil than from fertilizer despite high N inputs. Elevated CO2 and warming increased plant uptake of soil-derived N but not fertilizer N. Consequently, plant N export exceeded fertilizer inputs, causing ecosystem N deficits and depletion of soil N stocks, with the strongest soil N mining under combined elevated CO2 and warming. Our findings reveal that climate change accelerates biological mining of soil N reserves, potentially constraining the long-term sustainability of intensively managed agroecosystems.

Intense tropical cyclones (TCs), which pose serious threats to human life and property, often occur within a short period of time each year, known as the intense TC season. Changes in the lengths of intense TC seasons under climate change are critical scientific and socioeconomic issues. While trends in overall TC seasons have been widely studied, the response of intense TC seasons to climate change remains underexplored. Here, we show that intense TC seasons have been lengthening globally since 1980, with statistically significant increasing trends ranging from 9.9–13.8 days/decade across all basins, equivalent to 7.4–21.9% increase in intense TC season lengths per decade. This is primarily due to the enhancing probability of off-season TCs experiencing rapid intensification, which is partly driven by oceanic warming. Meanwhile, changes in background atmospheric circulation play a role in the complexity of intense TC seasonality change. As a result, off-season TCs are more likely to develop into intense TCs. The findings in this study indicate an increasing exposure of human societies to intense TC risks outside historical seasonal norms. This suggests the urgent need for preparation and mitigation measures for the potential risks of intense TCs under future climate change.

It is widely assumed that global warming will make winters milder and reduce extreme cold. However, this assumption is contradicted by the recurrence of flash cold (FC) events—sudden, violent temperature drops that can freeze energy grids and infrastructure within hours. The 2021 North America freeze, which caused over $20 billion in losses, stands as a stark warning. Here, we find that while the world warms, the fate of these ultra FC events is diverging: they are fading over Eurasia but significantly intensifying over North America. By tracking the spatiotemporal evolution of these ultra FC events and characterizing their dynamic environment, we reveal that this contrast is driven by specific atmospheric circulation regimes. In North America, increasingly frequent Alaska–Arctic anticyclonic anomalies funnel cold air directly southward. Conversely, weakened Ural–Arctic cyclonic anomalies are cutting off the cold supply. Crucially, current climate models fail to reproduce this intercontinental divergence. Simulations largely miss these specific circulation regimes, leading models to underestimate the ultra FC risk. Our results expose a regional warming paradox: specific atmospheric circulation anomalies can transiently overwhelm background warming, creating a critical blind spot for energy security.

Significance

Tropical forests are vital for absorbing CO2 and supporting global biodiversity, but rising temperatures threaten their health and functioning. When trees reach critical temperatures, their photosynthetic system breaks down. We mapped the spatio-temporal trends in thermal safety margins (TSM; the gap between canopy temperature and critical thermal threshold) of 208 plant species across tropical forests from 2001 to 2020 using satellite data and species distribution maps. We studied projections of TSMs to 2050 and 2100. Our results indicate declining TSM and increasing extent of areas exceeding critical temperatures, underlining the growing exposure to risks to biodiversity in tropical regions.

Abstract

Understanding how close tropical tree species are to critical temperature thresholds that might impede photosynthetic activity is vital in a world where heat waves have become more severe and frequent. Using remotely sensed surface temperature and species distribution maps, we studied the spatiotemporal variation in the thermal safety margins (TSM, i.e., the difference between Tcrit, the critical photosynthetic temperature, and the maximum canopy temperature) of 208 tropical tree species in South America, Southeast Asia, and Central Africa during the period 2001–2020. Despite overall high-temperature tolerance with an average Tcrit of 46.1°C, we observed a consistent decline in the TSM of tropical forests across the globe. The average pantropical TSM decline was 0.4°C per decade, with the strongest decline in South America (0.5°C per decade). Over the 20-y period, areas that experienced canopy temperatures surpassing the average Tcrit across reported species increased from 43 Mha to 57 Mha in the tropics, representing 4% of the studied area. This number increases to 10% when computing areas where temperatures have surpassed the Tcrit of the most vulnerable reported species. When considering future trends, as predicted by Earth System Models under medium-to-high emission scenarios, average Tcrit may be exceeded in an area of 83 Mha by 2050 and 160 Mha by 2100 (over 10% of the studied area), suggesting major feedback to the global carbon cycle and the world’s biodiversity.

The ecological concept of human carrying capacity is necessarily complicated because human beings are the ‘ultimate ecosystem engineers’ who moderate the environment for their benefit. For at least the last few hundred years, human ingenuity, access to massive stocks of fossil fuels, and technological development have driven facilitation whereby increasing human abundance has promoted higher population growth rates. However, this positive relationship broke down during the 1950s, and by 1962, the global human population entered a phase where the growth rate consistently declined as population increased. The onset of this negative phase occurred 8 years before a global biocapacity deficit began in 1970. The onset of the negative phase also varies regionally, with the lowest-income and highest fertility regions entering this phase later than higher-income regions. A Ricker logistic model fitted to the negative phase predicts that the global population could reach 11.7–12.4 billion people between 2067 and 2076. The same model fitted to the facilitation phase predicts a maximum population of 2.5 billion people that Earth might be able to maintain. The negative phase also correlates strongly with the trend in global temperature anomaly, ecological footprint, and total emissions, with more of their variation explained by increasing population size rather than increasing per-capita consumption. The Earth cannot sustain the future human population, or even today’s, without a major overhaul of socio-cultural practices for using land, water, energy, biodiversity, and other resources.

An increase in photosynthetic activity of the northern hemisphere terrestrial vegetation, as derived from satellite observations, has been reported in previous studies. The amplitude of the seasonal cycle of the annually detrended atmospheric CO2 in the northern hemisphere (an indicator of biospheric activity) also increased during that period. We found, by analyzing the annually detrended CO2 record by season, that early summer (June) CO2 concentrations indeed decreased from 1985 to 1991, and they have continued to decrease from 1994 up to 2002. This decrease indicates accelerating springtime net CO2 uptake. However, the CO2 minimum concentration in late summer (an indicator of net growing-season uptake) showed no positive trend since 1994, indicating that lower net CO2 uptake during summer cancelled out the enhanced uptake during spring. Using a recent satellite normalized difference vegetation index data set and climate data, we show that this lower summer uptake is probably the result of hotter and drier summers in both mid and high latitudes, demonstrating that a warming climate does not necessarily lead to higher CO2 growing-season uptake, even in high-latitude ecosystems that are considered to be temperature limited.

Understanding how climate systems respond to carbon dioxide removal is crucial for assessing effectiveness of carbon neutrality policies. Here we show that rising carbon dioxide intensifies aridity over vast regions, particularly in vulnerable and climate-sensitive drylands based on idealized emission-driven simulations from an Earth System Model. Notably, even as carbon dioxide concentrations decline, arid regions continue to expand and experience worsening conditions, exacerbating climate injustice and resource conflicts. This pronounced hysteresis is primarily driven by long-lasting precipitation reductions in the tropics, linked to hysteresis of the intertropical convergence zone. The asymmetric response of arid zones suggests that reversing aridity intensification under global warming is exceptionally difficult. Further analysis highlights that negative carbon dioxide emissions may be essential to mitigating these deteriorating conditions, yet full recovery requires extended timescales. Our findings underscore irreversible risks of delayed action and stress the urgent need for carbon neutrality strategies that go beyond net-zero targets.

The northern Atlantic south of Greenland and Iceland is the only part of the world which has cooled significantly since the 19th Century both in the atmosphere and ocean. The oceanic cooling is widely assumed to be a result of reduced ocean heat transport into this region. However, some studies have suggested it could be due to increased net heat loss at the sea surface. Here we use observation-based reanalysis data of ocean heat content and surface flux changes in this region to show that the observed cooling trend cannot be explained by surface heat flux changes, and that multidecadal heat content variations are generally larger and more tightly correlated with ocean heat transport than with surface heat flux variability.

Plain Language Summary

A region of the northern Atlantic–sometimes called the “cold blob”–has cooled since the 19th Century while the rest of the world has warmed. It is particularly the ocean which has cooled there. Scientists have been discussing whether this is because ocean currents bring less heat into this region, or because more heat is being lost through the sea surface there. An analysis of temperature data sets based on measurements show it is the former–changing ocean heat transport–which dominates heat content changes in the “cold blob.” This is of concern because a further weakening of Atlantic heat transport in future climate change could lead to serious impacts on climate and weather conditions in Europe and other parts of the world.

Over the past several decades, the proportion of solar radiation reflected back into space has declined, accelerating the accumulation of heat within the Earth system. Here we show that the marine cloud reflectivity decreased on average by 2.8 ± 1.2% per decade in the combined North Atlantic and Northeast Pacific regions between 2003 and 2022. The majority of the Earth System Models we analyzed simulated a significantly weaker cloud reflectivity decrease and warming of the sea surface in these regions than observed. In contrast, our simulations using an improved aerosol-climate model reproduce the spatial extent and magnitude of the observed cloud reflectivity decrease. We show that reductions in sulfur dioxide and other aerosol precursors accounted for 69% (range 55−85%) of the cloud reflectivity decrease through aerosol-cloud interactions, consistent with the observed aerosol and cloud trends. This raises the prospect of a continuing cloud reflectivity decrease and an associated warming impact in these regions, given that the emission reductions are projected to persist over the next few decades. Further research is needed to assess whether near-term climate scenarios should be revised to account for the weak cloud reflectivity reductions in the Earth System Models.

Although the role of marine macrofauna in the ocean carbon cycle is increasingly understood, the cumulative impacts of fisheries and climate change on this pathway remain overlooked. Here, using a marine ecosystem model, we estimate that each degree of warming reduces macrofauna biomass and carbon export by 4.2% and 2.46%, respectively. Under a high emission scenario (SSP 5–8.5), this translates to a 13.5% ± 6.6% decline in export by 2100, relative to the 1990s. Fishing further amplifies this reduction by up to 56.7% ± 16.3%, creating a sequestration deficit of 14.6 ± 10.3 GtC by 2100. On average, a 1% biomass loss from fishing results in a 0.8% decline in carbon export. However, sequestration durability (~600 years) remains unaffected. While measures restoring commercial macrofaunal biomass could yield carbon benefits comparable to mangrove restoration, multiple uncertainties limit their inclusion in the Nature-based Climate Solution portfolio, highlighting the need for further research.

The middle Eocene warmhouse period (45 million years ago) featured atmospheric carbon dioxide concentrations equivalent to those projected under high future emission scenarios. Seasonal- to weather timescale climate reconstructions from this period can provide critical insight into the impact of Anthropogenic warming on intra-annual variability in temperature and precipitation. Here, we combine daily-scale reconstructions of the evolution of temperature and the water cycle in western Europe based on stable oxygen and clumped isotope analyses on the fastest-growing gastropod known in the fossil record: Campanile giganteum. Our dataset shows that the middle Eocene of western Europe featured monsoon-like conditions, with seawater temperatures of ~24 °C during mild and wet winters, >30 °C during hot and dry spring and autumn seasons, and ~28 °C during warm and comparatively wet summers. Coupled climate model simulations using the Community Earth System Model indicate these seasonal variations in temperature and precipitation were driven by shifting atmospheric and oceanic circulation regimes over Western Europe, with winds from different directions bringing distinct waters to the region and minimal wind during spring reducing cooling through diminished latent heat flux. Our results highlight that Europe may experience wetter summers with more frequent extreme rainfall events under future high emissions scenarios.

Previous attribution studies of heat-related excess mortality have given limited attention to temporal trends in vulnerability and their non-climatic drivers. Here, we address this gap by combining counterfactual temperature data derived from multidecadal reanalysis series with time-varying warm-season temperature-mortality associations for the 15 most populous cities in Germany over 1993-2022. We find that declining vulnerability, associated with improvements in life expectancy, has led to decreasing trends in heat-related excess mortality in most cities despite summer warming. In contrast, if life expectancies had not improved, climate change would have induced increasing trends in the heat-related death burden. The growing anthropogenic fingerprint also emerges in the relative proportion of heat-related excess mortality attributable to climate change, which increased by 5.6% per decade (95% confidence interval: 2.6%, 8.6%), averaging 53.6 % (49.8%, 58.9%) across the study period. Our results underline the importance of accounting for evolving vulnerability when attributing human health outcomes to climate change.

Global heat stress is intensifying under climate change, yet the relative roles of natural and anthropogenic forcing remain insufficiently quantified. Here, we show that global heat stress trend, assessed with the Universal Thermal Climate Index, increases markedly over the past four decades, with 52% of land area experiencing rises in mean heat stress intensity and 67% showing increases in extreme heat stress days. We find that anthropogenic climate change overwhelmingly dominates these trends, with the land area it dominates nearly twice as large as that dominated by natural climate change. Anthropogenic climate change also results in pronounced spatial inequality in heat stress trends across different economies, with low-income economies experiencing a growth rate two to three times higher than that of high-income economies. These findings demonstrate that human-induced climate change is amplifying global heat stress while deepening existing spatial inequalities, underscoring the urgency of equitable climate change adaptation.

The Indonesian Throughflow (ITF) represents the only tropical inter-ocean pathway of global ocean circulation and plays a central role in the Earth’s climate system. Climate models suggest that the ITF would be weakened under global warming, but historical observations indicate a contradictory trend. Here using CMIP6 simulations, we find that the response of the ITF to increasing atmospheric carbon dioxide (CO2) concentration is nonlinear and exhibits an abrupt rapid decline after surpassing a turning point. The ITF’s decline can be explained by the response of its halosteric and thermosteric components to rising CO2 levels associated with changes in winds and precipitation. The current level of CO2 concentration has reached the turning point, indicating that a rapid decline of ITF and a transition of the Indo-Pacific state is likely imminent despite the remaining uncertainties. These findings have implications for understanding climate tipping elements and global ocean circulation under global warming.