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Global warming is leading to increased heat stress in many regions around the world. An extensive number of heat stress indicators (HSIs) has been developed to measure the associated impacts on human health. Here we calculate eight HSIs for global climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We compare their future trends as function of global mean temperature, with particular focus on highly populated regions. All analyzed HSIs increase significantly (p < 0.01) in all considered regions. Moreover, the different HSIs reveal a substantial spread ranging from trends close to the rate of global mean temperature up to an amplification of more than a factor of two. Trends change considerably when normalizing the HSIs by accounting for the different scales on which they are defined, but the large spread and strong trends remain. Consistently, exceedances of impact-relevant thresholds are strongly increasing globally, including in several densely populated regions, but also show substantial spread across the selected HSIs. The indicators with the highest exceedance rates vary for different threshold levels, suggesting that the large indicator spread is associated both to differences in trend magnitude and the definition of threshold levels. These results highlight the importance of choosing indicators and thresholds that are appropriate for the respective impact under consideration. Additionally, further validation of HSIs regarding their capability to quantify heat impacts on human health on regional-to-global scales would be of great value for assessing global impacts of future heat stress more reliably.

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Multiproxy reconstructions of monthly and seasonal surface temperature fields for Europe back to 1500 show that the late 20th- and early 21st-century European climate is very likely (>95% confidence level) warmer than that of any time during the past 500 years. This agrees with findings for the entire Northern Hemisphere. European winter average temperatures during the period 1500 to 1900 were reduced by ~0.5°C (0.25°C for annual mean temperatures) compared to the 20th century. Summer temperatures did not experience systematic century-scale cooling relative to present conditions. The coldest European winter was 1708/1709; 2003 was by far the hottest summer.

Historical simulations of models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) are evaluated over 10 Australian regions for their performance in simulating extreme temperatures, among which three models with initial‐condition large ensembles (LEs) are used to estimate the effects of internal variability. Based on two observational data sets, the Australian Water Availability Project (AWAP) and the Berkeley Earth Surface Temperatures (BEST), we first analyze the models' abilities in simulating the probability distributions of daily maximum and minimum temperature (TX and TN), followed by the spatial patterns and temporal variations of the extreme indices, as defined by the Expert Team on Climate Change Detection and Indices (ETCCDI). Overall, the CMIP6 models are comparable to CMIP5, with modest improvements shown in CMIP6. Compared to CMIP5, the CMIP6 ensemble tends to have narrower interquartile model ranges for some cold extremes, as well as narrower ensemble ranges in temporal trends for most indices. Over southeast, tropical, and southern regions, both CMIP ensembles generally exhibit relatively large deficiencies in simulating temperature extremes. We also confirm that internal variability can affect the trends of the extremes and there is uncertainty in representing the irreducible variability among different LEs in CMIP6. Furthermore, the evaluation based on Perkins' skill score (PSS) and root‐mean‐square error (RMSE) in the three LEs does not directly correlate with the ranges of the trends for extreme temperatures. The findings of this study are useful in informing and interpreting future projections of temperature‐related extremes over Australia. Key Points The assessment on the probability distributions of daily maximum and minimum temperature makes the evaluation of extremes more robust Temperature extremes over Australia are broadly similar in CMIP5 and CMIP6 There are differences in estimating internal variability across multiple CMIP6 models

Stratospheric Aerosol Injection (SAI) is a proposed method of climate intervention aiming to reduce the impacts of human‐induced global warming by reflecting a portion of incoming solar radiation. Many studies have demonstrated that SAI would successfully reduce global‐mean surface air temperatures; however the vast array of model scenarios and strategies result in a diverse range of climate impacts. Here we compare two SAI strategies—a quasi‐ equatorial injection and a multi‐latitude off‐equatorial injection—simulated with the UK Earth System Model (UKESM1), both aiming to reduce the global‐mean surface temperature from that of a high‐end emissions scenario to that of a moderate emissions scenario. We compare changes in the surface and stratospheric climate under each strategy to determine how the climate response depends on the injection location. In agreement with previous studies, an equatorial injection results in a tropospheric overcooling in the tropics and a residual warming in the polar regions, with substantial changes to stratospheric temperatures, water vapor and circulation. Previous comparisons of equatorial versus off‐equatorial injection strategies are limited to two studies using different versions of the Community Earth System Model. Our study evaluates how the climate responds in UKESM1 under these injection strategies. Our results are broadly consistent with previous findings, concluding that an off‐equatorial injection strategy can minimize regional surface temperature and precipitation changes relative to the target. We also present more in‐depth analysis of the associated changes in Hadley Circulation and regional temperature changes, and call for a new series of inter‐model SAI comparisons using an off‐equatorial strategy. Plain Language Summary Stratospheric Aerosol Injection (SAI) is a method to tackle the impacts of global warming and involves reflecting some of the sun's rays away from Earth. Different strategies for implementing SAI can have various effects on the climate. This study compares two strategies—one injecting at the equator and the other at different latitudes. Both strategies successfully lower global temperatures, but they also lead to different regional climate changes. The equatorial strategy cools the tropics too much and doesn't cool the poles enough. Whereas the off‐equatorial strategy minimizes some of the negative impacts seen in the equatorial strategy. In summary, injecting aerosols away from the equator avoids unfavorable climate impacts. Key Points We compare the climate impacts of equatorial and multi‐latitude Stratospheric Aerosol Injection (SAI) strategies under the Geoengineering Model Intercomparison Project G6 framework We demonstrate that an off‐equatorial multi‐latitude injection strategy minimizes unfavorable climate impacts This research highlights the importance of injection location in determining the impacts of SAI on the climate

The International Maritime Organization (IMO) introduced new regulations on the sulfur content of shipping emissions in 2020 (IMO2020). Estimates of the climatic impact of this global reduction in anthropogenic sulfate aerosols vary widely. Here, we contribute to narrowing this uncertainty with two sets of climate model simulations using UKESM1. Using fixed sea‐surface temperature atmosphere‐only simulations, we estimate an IMO2020 global effective radiative forcing of 0.139 ± 0.019 Wm−2 and show that most of this forcing is due to aerosol‐induced changes to cloud properties. Using coupled ocean‐atmosphere simulations, we note significant changes in cloud top droplet number concentration and size across regions with high shipping traffic density, and—in the North Atlantic and North Pacific—these microphysical changes translate to a decrease in cloud albedo. We show that IMO2020 increases global annual surface temperature on average by 0.046 ± 0.010°C across 2020–2029; approximately 2–3 years of global warming. Furthermore, our model simulations show that IMO2020 helps to explain the exceptional warming in 2023, but other factors are needed to fully account for it. The year 2023 also had an exceptionally large decrease in reflected shortwave radiation at the top‐of‐atmosphere. Our results show that IMO2020 made that more likely, yet the observations are within the variability of simulations without the reduction in shipping emissions. To better understand the climatic impacts of IMO2020, a model intercomparison project would be valuable whilst the community waits for a more complete observational record. Plain Language Summary In 2020, the International Maritime Organization introduced new regulations decreasing the sulfur content of shipping emissions (IMO2020). Since sulfur is a pollutant, it is expected that IMO2020 will improve air quality and health outcomes. These emissions, however, also lead to the formation of tiny particles in the air which brighten clouds, resulting in more sunlight reflected to space which helps cool the planet. Hence, by reducing sulfur emissions, IMO2020 will lead to planetary warming, yet the magnitude of this effect is hotly debated. In this work, we use a state‐of‐the‐art Earth system model to assess the warming impact of IMO2020. We find that IMO2020 increases the global average temperature by around 0.05°C; the equivalent to 2–3 years of global warming. Thus, IMO2020 helps to explain the exceptional warmth observed in 2023, yet other factors are needed to fully account for it. The year 2023 also had a record decrease in reflected sunlight contributing to the record temperatures, and our results show that IMO2020 made that more likely. Finally, we emphasize that IMO2020 has simply brought forward the warming from reductions in pollutants that are factored in favorable future climate scenarios. Key Points Recent regulations on the sulfur content of ship emissions has accelerated global warming by approximately 2–3 years Reduced ship emissions induce responses in cloud properties, top‐of‐atmosphere radiation, and surface temperatures The regulations contribute to the exceptional warming observed in 2023, yet other factors are needed to fully account for it

Changes in the global water cycle can cause major environmental and socioeconomic impacts. As the average global temperature increases, it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase. Paradoxically, terrestrial observations over the past 50 years show the reverse. Here, we show that the decrease in evaporation is consistent with what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration.

The late Pleistocene history of seawater temperature and salinity variability in the western tropical Pacific warm pool is reconstructed from oxygen isotope (δ18O) and magnesium/calcium composition of planktonic foraminifera. Differentiating the calcite δ18O record into components of temperature and local water δ18O reveals a dominant salinity signal that varied in accord with Dansgaard/Oeschger cycles over Greenland. Salinities were higher at times of high-latitude cooling and were lower during interstadials. The pattern and magnitude of the salinity variations imply shifts in the tropical Pacific ocean/atmosphere system analogous to modern El Niño-Southern Oscillation (ENSO). El Niño conditions correlate with stadials at high latitudes, whereas La Niña conditions correlate with interstadials. Millennial-scale shifts in atmospheric convection away from the western tropical Pacific may explain many paleo-observations, including lower atmospheric CO2, N2O, and CH4 during stadials and patterns of extratropical ocean variability that have tropical source functions that are negatively correlated with El Niño.

Sea surface temperatures (SSTs) in the cold tongue of the eastern equatorial Pacific exert powerful controls on global atmospheric circulation patterns. We examined climate variability in this region from the Last Glacial Maximum (LGM) to the present, using a SST record reconstructed from magnesium/calcium ratios in foraminifera from sea-floor sediments near the Galápagos Islands. Cold-tongue SST varied coherently with precession-induced changes in seasonality during the past 30,000 years. Observed LGM cooling of just 1.2°C implies a relaxation of tropical temperature gradients, weakened Hadley and Walker circulation, southward shift of the Intertropical Convergence Zone, and a persistent El Niño-like pattern in the tropical Pacific. This is contrasted with mid-Holocene cooling suggestive of a La Niña-like pattern with enhanced SST gradients and strengthened trade winds. Our results support a potent role for altered tropical Pacific SST gradients in global climate variations.