Clara Deser

Wildfire frequency, extent, and duration in the western United States are projected to increase throughout the twenty-first century as dry and warm conditions become more frequent, widespread, and persistent. However, there is limited knowledge about how changes in mean climate and changes in climate variability, individually and together, contribute to these increases. To disentangle these effects, we use a 100-member ensemble of climate simulations produced with the Community Earth System Model, version 2, under historic and shared socioeconomic pathway (SSP) 3-7.0 forcing from 1980 to 2100. Using the Canadian forest fire weather index (FWI) to quantify fire-related meteorological conditions in the simulations, we select extreme FWI thresholds relative to a baseline distribution centered at 1980 (“fixed threshold”) and an evolving distribution (“moving threshold”) to identify spatiotemporally connected extreme fire weather events. By 2100, the frequency, area, and number of event days increase when considering a fixed threshold, and events are up to 4°C warmer on average. Moreover, events over the Pacific Coast expand northwestward, while those over the Four Corners region expand westward and northeastward. Changes in event frequency are small and not significant when considering a moving baseline but become spatiotemporally more connected and widespread, reflecting nonthermodynamically driven changes. Increases in the mean of maximum temperature, as well as changes in both the variability and mean of relative humidity, drive the largest increases in extreme fire weather event area and number of event days. By quantifying how changes in the mean and variability of climate variables impact wildfire conditions in the western United States, our study has implications for increasing resiliency to wildfire risk under climate change.

Sea surface temperature (SST) cooling trends observed in the Southern Ocean over the recent decades are widely hypothesized to be driven by intensified westerly winds associated with Southern Hemisphere stratospheric ozone depletion. Standard coarse-resolution Earth system models are unable to simulate these observed cooling trends. We show that a 10-member ensemble of the high-resolution Community Earth System Model (CESM-HR) can indeed reproduce Southern Ocean cooling. We then investigate the compensating role of ocean eddy-induced meridional heat transport and meridional overturning circulation in shaping the ozone-driven response. We hypothesize that while the ozone-driven intensification of the westerlies initiates the surface cooling in the Southern Ocean through enhanced equatorward Ekman transport, the persistence and amplification of this cooling depend critically on the extent of oceanic eddy compensation, which regulates wind-driven atmosphere-ocean feedback. Unlike standard low-resolution Earth system models that parameterize eddy heat transport, the CESM-HR, which explicitly represents eddies, exhibits much weaker compensation. This allows the wind-driven circulation to dominate the meridional overturning, enhancing upwelling and the northward transport of cooler waters in the Southern Ocean, thereby strengthening the wind-driven atmosphere-ocean feedback and producing a more pronounced Southern Ocean cooling trend that aligns more closely with observations.

Observed sea-surface temperature (SST) trends over recent decades feature cooling in the tropical eastern Pacific and the Southern Ocean (SO). Growing evidence suggests that tropical cooling may partly stem from remote impacts of the SO. Using a hierarchy of multimodel simulations, we demonstrate that these teleconnections are robustly modulated by the mean-state intertropical convergence zone (ITCZ): Models with a more realistic ITCZ simulate a stronger tropical SST response to SO forcing via stronger wind-evaporation-SST feedback. When realistic Antarctic meltwater forcing is included, correcting a model’s tropical mean-state bias yields a stronger tropical cooling response to meltwater-driven SO cooling, improving the agreement between simulated and observed SST trends. Our results suggest that the SO’s contribution to tropical warming patterns is systematically underestimated due to model mean-state biases. Improving representations of the mean-state climate is therefore critical for accurately assessing large-scale climate responses associated with historical and future warming patterns.

Global surface air temperature reached unprecedented heights in early boreal fall 2023, surpassing the previous record for year-to-year temperature increase by a significant margin. We attribute most of this temperature jump to the onset and maturing stages of the 2023 El Niño, with some contributions from the North Atlantic. Using a process-based analysis from multiple observational datasets, we show that the uniqueness of the 2023 event arose from the La Niña-like ocean-atmosphere state on which it developed. This background favoured (1) a steep year-to-year increase of Sea Surface Temperature, particularly in mean atmospheric subsidence regions, reducing low-cloud cover and giving rise to a record-breaking change in the radiative budget; (2) anomalously sustained precipitation over high sea surface temperatures in the Western Pacific, fuelling unusual diabatic heating compared to canonical El Nino events. This altogether led to an exceptionally early increase in tropical tropospheric temperature in boreal fall, ultimately influencing the jump in temperature at the global scale.

Continued melting of Antarctic ice sheets and shelves adds freshwater to the Southern Ocean (SO), enhancing stratification and inducing surface cooling. This cooling influences tropical climate through coupled atmosphere–ocean interactions, though model responses vary. Using coordinated coupled model experiments with idealized Antarctic meltwater forcing, we assess the remote impacts of SO surface cooling. All 11 models simulate equatorial surface cooling and a northward Intertropical Convergence Zone shift, but show discrepant responses in the equatorial Pacific zonal temperature gradient and Atlantic meridional dipole. When normalized by SO cooling amplitude, these tropical metrics are positively correlated with shortwave cloud feedback strength. Surface energy budget analysis indicates that the previously proposed teleconnection mechanisms in the eastern Pacific are not robust across models. The timescale of tropical cooling and the relative roles of wind‐driven latent heat and shortwave fluxes differ across models and basins, highlighting the uncertainty in SO–tropics teleconnections.

The equatorial Pacific sea surface temperature (SST) zonal gradient has worldwide impacts and is expected to be highly sensitive to future climate change. However, biases in climate models call the reliability of future SST gradient projections into question. Here, we combine multiple climate model Large Ensembles to show that equatorial precipitation and cloud feedbacks have a controlling influence on the future Pacific SST gradient. An “SST gradient sensitivity” parameter is computed for each model, which shows that models with stronger historical equatorial precipitation have systematically higher sensitivities (more El Nino-like changes). This arises from the stronger negative SST-shortwave radiation feedback, which then creates a wind response that favors El Nino–like warming. Notably, when simulated historical deep convection is sufficiently strong, a “saturation” effect occurs that tends to inhibit this effect. These results imply that models likely underestimate future El Nino–like changes but that the “true” magnitude of changes may be predictable.

Changes in sea surface temperature (SST) patterns have recently been recognized as a major feedback affecting the sensitivity of climate to increases in greenhouse gases ^1–3 . Over recent decades, while most of Earth’s surface warmed, the eastern tropical Pacific and Southern Ocean unexpectedly cooled. These regional SST cooling trends are not reproduced by most global climate models (GCMs) ⁴ , leading to systematic biases in estimates of global climate sensitivity ^1,2 . While Antarctic ozone depletion has been proposed as a potential driver of the cooling ⁵ , its influence has previously been considered too weak ^6,7 . Here we provide novel evidence that suggests Antarctic ozone depletion can indeed quantitatively account for this observed SST cooling. Using ~4,000 years of simulation from eight GCMs, we construct a multiple linear regression model that isolates the intrinsic relationship between Antarctic ozone and SST, capturing robust short-timescale coupling while avoiding biases from the lack of resolved ocean eddies and their long-timescale adjustments in GCMs ^8–10 . We calculate that the ozone-driven SST pattern effect strengthened the global radiative feedback by 17-21% (0.49-0.83 W m ^-2 K ^-1 ) during 1979-2000, thereby reducing effective climate sensitivity. As Antarctic ozone starts to recover ^11,12 , the stabilizing influence has begun to wane, leading to a more warming-prone climate.

Human-induced warming is amplified in the Arctic, but its causes and consequences are not precisely known. Here, we review scientific advances facilitated by the Polar Amplification Model Intercomparison Project. Surface heat flux changes and feedbacks triggered by sea-ice loss are critical to explain the magnitude and seasonality of Arctic amplification. Tropospheric responses to Arctic sea-ice loss that are robust across models and separable from internal variability have been revealed, including local warming and moistening, equatorward shifts of the jet stream and storm track in the North Atlantic, and fewer and milder cold extremes over North America. Whilst generally small compared to simulated internal variability, the response to Arctic sea-ice loss comprises a non-negligible contribution to projected climate change. For example, Arctic sea-ice loss is essential to explain projected North Atlantic jet trends and their uncertainty. Model diversity in the simulated responses has provided pathways to observationally constrain the real-world response.

Continued melting of Antarctic ice sheets and shelves introduces freshwater into the Southern Ocean (SO), enhancing stratification and surface cooling. This cooling influences tropical climate through coupled atmosphere–ocean pathways, though the magnitude of the response varies across models. Using a coordinated suite of coupled model experiments with idealized Antarctic meltwater forcing, we assess the remote impacts of SO surface cooling. All seven models simulate equatorial surface cooling and a northward displacement of the Intertropical Convergence Zone, but the equatorial Pacific zonal sea surface temperature gradient and Atlantic meridional dipole exhibit substantial intermodel spread. When normalized by each model’s SO cooling amplitude, these tropical metrics are positively correlated with shortwave cloud feedback strength. The timescale of tropical cooling and the relative roles of wind-driven latent heat and shortwave fluxes differ across models and basins, underscoring the need for further investigation of SO–tropics teleconnections.