Thomas Lukas Frölicher

In 2023, the North Atlantic Ocean experienced a marine heatwave of unprecedented intensity and duration, raising the question of whether substantially more severe outcomes were physically plausible under comparable present-day preconditioning, and which processes could produce them. Here, we apply ensemble boosting to a coupled Earth system model to generate physically consistent alternative evolutions of a 2023-like event. The boosted ensemble produced marine heatwaves lasting up to 579 days, reaching maximum intensities of 4.5 °C above climatology and cumulative intensities of 717 °C·days, compared with 240 days, 2.9 °C and 224 °C·days in observations. Detailed analyses of the longest and most intense events highlight contrasting roles for persistent Euro-Atlantic high pressure, reduced turbulent heat loss, enhanced shortwave radiation and upper-ocean heat redistribution. Finally, we place these present-day storylines in the context of future warming, showing that events comparable to the observed 2023 event become increasingly common as global warming progresses.

This paper is motivated by growing concern about tipping dynamics and processes that can cause abrupt, self-amplifying, and irreversible changes across the Earth system and societies. It assesses current scientific understanding of the most severe climate outcomes, and proposes ways to address the deep uncertainty surrounding them. We find substantial, though difficult-to-quantify, probabilities of major disruptions to large-scale ocean circulation, including the Atlantic Meridional Overturning Circulation and the Southern Ocean Abyssal Circulation, with the potential to cause global climate shifts. The probability of ice sheet instability in vulnerable sectors, particularly Antarctica's marine-based sectors, is already high and could accelerate sea level rise over centennial to multi-centennial timescales. Permafrost thaw is underway, with the potential for accelerated greenhouse gas release. Tropical forests face increasing risks of widespread tree mortality, with major consequences for biodiversity, regional climate, and water availability. Coral reef dieback and mountain glacier loss are also underway, while severe compound marine events may trigger irreversible ecosystem changes. Even gradual climate change can produce nonlinear, abrupt impacts when hazards breach physiological or socioecological thresholds, disrupting health, food, or economic systems. It is essential to determine how the probability of such outcomes depends on the level and rate of warming. Current modelling systems challenge our ability to reliably quantify these risks. Sustained investment in observations, process understanding and next-generation earth system modelling is critical to identify precursors and improve early warning. Decision frameworks are needed to support proactive, flexible, anticipatory adaptation. Effective mitigation reduces risks of these highly disruptive outcomes.

Climate extremes are increasingly disrupting marine ecosystems and fisheries. However, evidence on extreme ocean temperature effects on fish stocks and fisheries is mixed, and the combined impacts of heat and productivity extremes remain unclear. Using three decades of global data encompassing 6659 time-series for 1246 species across 254 regions, we conduct a risk-based analysis to quantify how local extreme high temperatures and low ocean productivity that these species are exposed to, alone and together, affect local fisheries catches. These events, particularly when compounded, sharply increase the likelihood of local extreme low catch events, especially in tropical and subtropical regions. Species critical to food security and conservation are disproportionately affected, with widespread risks in socio-economically vulnerable countries. Without adaptation, extreme events’ risks are projected to strongly intensify by the mid-21st century. Strengthening monitoring and climate-responsive fisheries management is urgently needed to build resilience.

The world is on the brink of exceeding the 1.5°C global temperature limit of the Paris Agreement. By limiting peak warming and subsequently reversing global warming, temperatures could return below 1.5°C by 2100. However, the health consequences of a temporary temperature ‘overshoot’ of 1.5°C remain poorly understood. Here, we assess changes in heat-related mortality for 874 locations across 55 countries under all overshoot scenarios of the IPCC AR6 scenario database. We estimate impacts by sampling from large climate model ensembles, and validate our results with dedicated Earth system model runs. We find that irrespective of the temperature trajectory, heat-related mortality increases linearly with overshoot-degree-years – the cumulative duration and magnitude of GMST exceeding 1.5°C. Heat-related mortality increases by 1.2 %/°C-year (country range: 0.4-4.1 %/°Cy) across the countries covered in our dataset, with highest increases in the Caribbean and (sub-)tropical countries. The linear relationship highlights that each year and fraction of a degree above 1.5°C increases heat-mortality as compared to scenarios that stabilize at 1.5°C. Correspondingly, heat-mortality decreases again in a cooling phase following the peak of the overshoot. Minimising peak warming and reversing warming in the long term is critical for limiting heat-mortality under overshoot pathways.

The coastal ocean and its vital ecosystem services are threatened by human perturbations of both global climate and nutrient cycles, leading to severe regional warming, acidification, deoxygenation, and algae blooms (greening). Globally however, the large-scale deterioration of the coastal environment has remained inadequately assessed due to methodological limitations. Using a comprehensive model and observational basis, we demonstrate warming, acidification, greening, and deoxygenation occurring in the global coastal ocean since the 1900s and an acceleration of these trends over the last four decades. Coastal changes critically exceed global open ocean changes since the 1980s (1.6-fold for warming, 1.8-fold for acidification, 7.8-fold for biological productivity, and 2.4-fold for deoxygenation), with 76% of the global coastal ocean subject to at least three stressors with perturbations larger than open ocean means. Our study underlines the need to emphasize constraining coastal ocean changes within assessments of global ocean health and ecosystem resilience.

Climate extremes are increasingly disrupting marine ecosystems and fisheries. However, evidence on extreme ocean temperature effects on fish stocks is mixed, and the combined impacts of heat and productivity extremes remain unclear. Using three decades of global data encompassing 6,659 time-series for 1,246 species across 254 regions, we conducted a risk-based analysis to quantify how local extreme high temperatures and low ocean productivity that these species were exposed to, alone and together, affect local fisheries catches. These events, particularly when compounded, sharply increase the likelihood of local extreme low catch events, especially in tropical and subtropical regions. Species critical to food security and conservation are disproportionately affected, with widespread risks in socio-economically vulnerable countries. Without adaptation, extreme events’ risks are projected to strongly intensify by the mid-21st century. Strengthening monitoring and climate-responsive fisheries management is urgently needed to build resilience.Main Text

Monitored seabird populations have declined by up to 70% worldwide since the 1950s, yet little is known about population sizes or long-term trends prior to anthropogenic impacts. This is particularly true for the Southern Ocean, where seabirds and many other species are currently facing multiple environmental threats. Here, we identify shifts in seabird populations over the last 8100 years by applying a mercury (Hg) flux and isotope signature technique as a novel tracer of population size via guano input into peatlands on sub-Antarctic Bird Island (South Georgia). Our results show that seabirds have colonized Bird Island between 6800 and 6100 years ago, which pre-dates evidence for colonization of other sub-Antarctic islands by more than 1000 years. There are five periods when seabird populations were at maxima during the mid- to late-Holocene, coeval with periods of lower Southern Hemisphere westerly wind intensity. Our study provides unprecedented insights into cyclicity in seabird population sizes as a consequence of regional to large-scale environmental change.

Climate-induced distribution changes are particularly challenging for fisheries targeting fish populations shared between exclusive economic zones (EEZs) and the high seas, known as straddling stocks. Here, we combine multiple datasets and ecosystem modeling to identify the presence of straddling stocks worldwide and consider the management implications of climate change–driven shifts. We identify 347 straddling stocks across 67 species, including both highly migratory and less mobile species. Our results suggest that regardless of the climate change scenario, at least 37% and 54% of stocks are projected to shift between EEZs and the high seas, by 2030 and 2050, respectively. More stocks are expected to shift toward the high seas, and highly migratory stocks are projected to shift across Regional Fisheries Management Organizations (RFMOs). Coastal states and RFMOs require to revise governance frameworks to support sustainable and equitable adaptation to shifting stocks. Developing such strategies requires cooperation between management bodies within EEZs and the high seas.

Exceeding 1.5 °C of global warming above pre-industrial levels has become a distinct possibility, yet the consequences of such an overshoot for mountain glaciers and their contribution to raising sea levels and impacting water availability are not well understood. Here we show that exceeding and then returning to below 1.5 °C will have irreversible consequences for glacier mass and runoff over centuries. Global climate and glacier simulations project that a 3.0 °C peak-and-decline scenario will lead to 11% more global glacier mass loss by 2500 compared with limiting warming to 1.5 °C without overshooting. In basins where glaciers regrow after peak temperature, glacier runoff reduces further than if the glaciers stabilize, a phenomenon we call ‘trough water’. Half the studied glaciated basins show reduced glacier runoff with overshoot compared with without for decades to centuries after peak warming. These findings underscore the urgency of near-term emissions reductions and limiting temperature overshoot.

Marine species are increasingly threatened by extreme and compound events, as warming, deoxygenation, and acidification unfold. Yet, the surface and especially the subsurface distribution and evolution of such compound events remain poorly understood. We present the current and projected distributions of compound marine heatwave (MHW), low oxygen (LOX), and high acidity (OAX) events throughout the water column, using observation‐based data from 2004 to 2019 and large ensemble Earth system model simulations from 1890 to 2100. Our findings reveal that compound MHW‐OAX and OAX‐LOX events are prevalent in the low to mid latitudes at the ocean surface. At 200 and 600 m, MHW‐OAX and MHW‐LOX events are frequent in the high latitudes and parts of the tropics, while OAX‐LOX events occur globally. Subsurface compound events are often associated with vertical displacements of water masses, with the climatological vertical gradients of ecosystem stressors typically explaining their occurrence patterns. Projections show a strong rise in compound event frequency over the historical period and under continued global warming, primarily driven by shifts in mean oceanic conditions. The portion of the top 2,000 m affected by extreme or compound events rises from 20 to 98 under 2°C of global warming in a high emissions scenario using a preindustrial baseline, and to 30 using a shifting‐mean baseline. However, physical and biogeochemical changes may also lead to regional decreases in subsurface events, highlighting complexities in how warming, deoxygenation, and acidification unfold in the ocean interior. Increasing compound event frequency poses a major threat to marine ecosystems, potentially disrupting food webs and biodiversity.