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The northeastern United States (NEUS) and the adjacent Northwest Atlantic Shelf (NWS) have emerged as warming hotspots, but the connection between them remains unexplored. Here we use gridded observational and reanalysis datasets to show that the twentieth-century surface air temperature increase along the coastal NEUS is exceptional on the continental and hemispheric scale and is induced by a combination of two factors: the sea surface temperature (SST) increase in the NWS associated with a weakening Atlantic Meridional Overturning Circulation (AMOC), and atmospheric circulation changes associated with a more persistent positive North Atlantic Oscillation. These connections are important because AMOC slowdown and NWS warming are projected to continue. A survey of climate model simulations indicates that realistic SST representation at high spatial resolution might be a minimum requirement to capture the observed pattern of coastal warming, suggesting that prior projection-based assessments may not have captured key features in this populous region.

Extreme weather and climate events and their impacts can occur in complex combinations, an interaction shaped by physical drivers and societal forces. In these situations, governance, markets and other decision-making structures—together with population exposure and vulnerability—create nonphysical interconnections among events by linking their impacts, to positive or negative effect. Various anthropogenic actions can also directly affect the severity of events, further complicating these feedback loops. Such relationships are rarely characterized or considered in physical-sciences-based research contexts. Here, we present a multidisciplinary argument for the concept of connected extreme events, and we suggest vantage points and approaches for producing climate information useful in guiding decisions about them.

In an interconnected world, simultaneous extreme weather events in distant regions could potentially impose high-end risks for societies1,2. In the mid-latitudes, circumglobal Rossby waves are associated with a strongly meandering jet stream and might cause simultaneous heatwaves and floods across the northern hemisphere3–6. For example, in the summer of 2018, several heat and rainfall extremes occurred near-simultaneously7. Here we show that Rossby waves with wavenumbers 5 and 7 have a preferred phase position and constitute recurrent atmospheric circulation patterns in summer. Those patterns can induce simultaneous heat extremes in specific regions: Central North America, Eastern Europe and Eastern Asia for wave 5, and Western Central North America, Western Europe and Western Asia for wave 7. The probability of simultaneous heat extremes in these regions increases by a factor of up to 20 for the most severe heat events when either of these two waves dominate the circulation. Two or more weeks per summer spent in the wave-5 or wave-7 regime are associated with 4% reductions in crop production when averaged across the affected regions, with regional decreases of up to 11%. As these regions are important for global food production, the identified teleconnections have the potential to fuel multiple harvest failures, posing risks to global food security8.

Present and future freshwater availability and drought risks are physically tied to the responses of surface vegetation to increasing CO2. A single‐model large ensemble identifies the occurrence of colocated warming‐ and CO2‐induced leaf area index increases with summer soil moisture declines. This pattern of “greening” and “drying,” which occurs over 42% of global vegetated land area, is largely attributable to changes in the partitioning of precipitation at the land surface away from runoff and toward terrestrial vegetation ecosystems. Changes in runoff and ecosystem partitioning are inversely related, with changes in runoff partitioning being governed by changes in precipitation (mean and extremes) and ecosystem partitioning being governed by ecosystem water use and surface resistance to evapotranspiration (ET). Projections show that warming‐influenced and CO2‐enriched terrestrial vegetation ecosystems use water that historically would have been partitioned to runoff over 48% of global vegetated land areas, largely in Western North America, the Amazon, and Europe, many of the same regions with colocated greening and drying. These results have implications for how water available for people will change in response to anthropogenic warming and raise important questions about model representations of vegetation water responses to high CO2. Using a large ensemble of simulations from a state‐of‐the‐art Earth System Model, we show that 42% of global vegetated land areas are projected to have “greening” in the form of additional vegetation growth at the same time as “drying” in the form of reduced soil moisture in a business‐as‐usual world. Simultaneous greening and drying is curious and suggests that future ecosystems—which could demand more water due to warmer and longer growing seasons and CO2fertilization—siphon water that historically would have become the runoff that fills rivers and streams, termed “blue water.” We show that warming and changes in plant growth from CO2creates an explicit water trade‐off in which future vegetation directly diminishes runoff relatively or absolutely for nearly half of global land areas. Our results have important implications for future water availability, but also point to the crucial importance of resolving model uncertainties associated with terrestrial vegetation and its response to increasing CO2. Vegetation “greening” and soil “drying” cooccurs over 42% of global vegetated land in a large GCM ensembleThe greening and drying pattern is driven by changes in precipitation partitioning at the land surfaceThese results highlight an intrinsic future trade‐off between terrestrial vegetation (green water) versus total runoff (blue water)

Sea-level rise (SLR) poses a range of threats to natural and built environments, making assessments of SLR-induced hazards essential for informed decision making. We develop a probabilistic model that evaluates the likelihood that an area will inundate (flood) or dynamically respond (adapt) to SLR. The broad-area applicability of the approach is demonstrated by producing 30 × 30 m resolution predictions for more than 38,000 km2of diverse coastal landscape in the northeastern United States. Probabilistic SLR projections, coastal elevation and vertical land movement are used to estimate likely future inundation levels. Then, conditioned on future inundation levels and the current land-cover type, we evaluate the likelihood of dynamic response versus inundation. We find that nearly 70% of this coastal landscape has some capacity to respond dynamically to SLR, and we show that inundation models over-predict land likely to submerge. This approach is well suited to guiding coastal resource management decisions that weigh future SLR impacts and uncertainty against ecological targets and economic constraints.

Future coastal flood risk will be strongly influenced by sea-level rise (SLR) and changes in the frequency and intensity of tropical cyclones. These two factors are generally considered independently. Here, we assess twenty-first century changes in the coastal hazard for the US East Coast using a flood index (FI) that accounts for changes in flood duration and magnitude driven by SLR and changes in power dissipation index (PDI, an integrated measure of tropical cyclone intensity, frequency and duration). Sea-level rise and PDI are derived from representative concentration pathway (RCP) simulations of 15 atmosphere–ocean general circulation models (AOGCMs). By 2080–2099, projected changes in the FI relative to 1986–2005 are substantial and positively skewed: a 10th–90th percentile range 4–75 times higher for RCP 2.6 and 35–350 times higher for RCP 8.5. High-end FI projections are driven by three AOGCMs that project the largest increases in SLR, PDI and upper ocean temperatures. Changes in PDI are particularly influential if their intra-model correlation with SLR is included, increasing the RCP 8.5 90th percentile FI by a further 25%. Sea-level rise from other, possibly correlated, climate processes (for example, ice sheet and glacier mass changes) will further increase coastal flood risk and should be accounted for in comprehensive assessments.

Once‐per‐year (annual) maximum temperature extremes in North American Regional Climate Change Assessment Program (NARCCAP) models are projected to increase more (less) than mean daily maximum summer temperatures over much of the eastern (western) United States. In contrast, the models almost everywhere project greater warming of once‐per‐year minimum temperatures as compared to mean daily minimum winter temperatures. Under projected changes associated with extremes of the temperature distribution, Baltimore's maximum temperature that was met or exceeded once per year historically is projected to occur 17 times per season by midcentury, a 28% increase relative to projections based on summer mean daily maximum temperature change. Under the same approach, historical once‐per‐year cold events in Baltimore are projected to occur once per decade. The models are generally able to capture observed geopotential height anomalies associated with temperature extremes in two subregions. Projected changes in extreme temperature events cannot be explained by geopotential height anomalies or lower boundary conditions as reflected by soil moisture anomalies or snow water equivalent. Extremes warm ∼2°C more than meanOrder of magnitude recurrence changesRCMs capture dynamics

Extreme humid heat events have seen rapid increases globally in recent decades, but regional changes and higher‐order temporal characteristics, such as interannual and intra‐annual clustering, have not been widely explored. Using ERA5 reanalysis data from 1979 to 2019, we find increasing trends of varying magnitudes in extreme wet‐bulb temperatures at the Intergovernmental Panel on Climate Change Sixth Assessment Report (IPCC AR6) regional scale. In many locations, interannual variations in extremes show a strong relationship with the El Niño‐Southern Oscillation. The temporal proximity of precipitation events to humid heat days in arid regions suggests that local moisture effects may lead to clustering. Knowledge of these spatial and temporal patterns aids in understanding how potential heat stress is increasing, as well as facilitates the development of regionally specific adaptation and mitigation strategies for combating the associated societal impacts. Extreme humid heat, or the combination of high temperature and humidity, poses a more severe threat to human health than does dry heat alone. Though extremes are particularly dangerous, even moderate levels of humid heat can lead to a variety of health and socioeconomic effects. Motivated by the growing demand for regional, decision‐relevant climate information, we calculate historical changes in the intensity of humid heat extremes in the regions used in the Intergovernmental Panel on Climate Change Sixth Assessment Report. Humid heat extremes have intensified in most regions, though some areas have experienced greater increases than others. The timing of extremes also affects their impacts, and thus we additionally analyze how humid heat extremes are distributed, both within the year and across all years. In many locations across the world, extreme humid heat is more common during strong El Niño episodes. In some typically dry regions, extremes tend to occur near each other within a given year and around the same time as rainfall events. Our results help advance the understanding of potential heat stress and the development of regionally specific strategies for combating its impacts. From 1979 to 2019, extreme wet‐bulb temperatures intensified in most IPCC AR6 regions, though to varying degreesPatterns in interannual clustering of extreme humid heat show signals of the overall warming trend and the El Niño‐Southern OscillationIn some arid regions, extreme humid heat days often occur in close temporal proximity to one another and to precipitation events From 1979 to 2019, extreme wet‐bulb temperatures intensified in most IPCC AR6 regions, though to varying degrees Patterns in interannual clustering of extreme humid heat show signals of the overall warming trend and the El Niño‐Southern Oscillation In some arid regions, extreme humid heat days often occur in close temporal proximity to one another and to precipitation events

Global average temperatures have been rising for the past half-century, and the warming trend has accelerated in recent decades. Further warming is expected over the next few decades, with significant regional variations. These warming trends will probably result in more frequent, intense and persistent periods of hot temperatures in summer, and generally higher temperatures in winter. Daily death counts in cities increase markedly when temperatures reach levels that are very high relative to what is normal in a given location. Relatively cold temperatures also seem to carry risk. Rising temperatures may result in more heat-related mortality but may also reduce cold-related mortality, and the net impact on annual mortality remains uncertain. Here we use 16 downscaled global climate models and two emissions scenarios to estimate present and future seasonal patterns in temperature-related mortality in Manhattan, New York. All 32 projections yielded warm-season increases and cold-season decreases in temperature-related mortality, with positive net annual temperature-related deaths in all cases. Monthly analyses showed that the largest percentage increases may occur in May and September. These results suggest that, over a range of models and scenarios of future greenhouse gas emissions, increases in heat-related mortality could outweigh reductions in cold-related mortality, with shifting seasonal patterns.

Humid heat presents a major societal challenge through its impacts on human health, energy demand, and economic productivity, underlined by the projected emergence of conditions beyond human tolerance. However, systematic assessment of what drives the most extreme humid heat worldwide has been lacking. Here, we investigate factors determining the location and magnitude of humid‐heat extremes, framing our analysis around the four regions with the highest values: the southern Persian Gulf, north‐central Pakistan, eastern South Asia, and the western Amazon. We find that strong boundary‐layer moisture fluxes, together with stability that inhibits moist convection, explain well the timing and location of near‐surface humid‐heat extremes. These favorable conditions are achieved through regionally distinct factors, including shallow sea breezes in the Persian Gulf and large‐scale subsidence in eastern South Asia. Our results demonstrate some of the principal controls on the most intense humid heat, both globally and for particular regions and heat events. Combinations of high temperatures and high humidity are a primary threat of climate change because they can be not only uncomfortable but deadly, and are growing rapidly in frequency and intensity. In this study, we present a general theory for where the most extreme such events occur and describe the precise meteorological conditions that favor them. We are therefore able to state with new confidence the reasons why extreme humid heat events take place where they do, and why. Our results also highlight the regional distinctions in the factors (such as winds or ocean temperatures) that contribute to these events, increasing the strength of the evidence that some regions are much more sensitive to certain factors than others—information that can be used to guide better forecasts as well as climate‐change projections. A systematic assessment of humid‐heat hotspots reveals the dual importance of vertical stability and low‐level moisture sourcesThese two conditions are achieved through markedly different meteorological processes and in markedly different geographical contextsUnderstanding the most extreme humid heat requires applying global principles to detailed regional analyses across space and time scales A systematic assessment of humid‐heat hotspots reveals the dual importance of vertical stability and low‐level moisture sources These two conditions are achieved through markedly different meteorological processes and in markedly different geographical contexts Understanding the most extreme humid heat requires applying global principles to detailed regional analyses across space and time scales

Extreme heat research has largely focused on dry‐heat, while humid‐heat that poses a substantial threat to human‐health remains relatively understudied. Using hourly high‐resolution ERA5 reanalysis and HadISD station data, we provide the first spatially comprehensive, global‐scale characterization of the magnitude, seasonal timing, and frequency of dry‐ and wet‐bulb temperature extremes and their trends. While the peak dry‐ and humid‐heat extreme occurrences often coincide, their timing differs in climatologically wet regions. Since 1979, dry‐ and humid‐heat extremes have become more frequent over most land regions, with the greatest increases in the tropics and Arctic. Humid‐heat extremes have increased disproportionately over populated regions (∼5.0 days per‐person per‐decade) relative to global land‐areas (∼3.6 days per‐unit‐land‐area per‐decade) and population exposure to humid‐heat has increased at a faster rate than to dry‐heat. Our study highlights the need for a multivariate approach to understand and mitigate future harm from heat stress in a warming world. Combined high temperature and humidity can be more dangerous to humans and wildlife relative to high temperature alone. There are known physiological limits to humid‐heat and adverse impacts on human‐health and performance can be felt well below those limits. Thus, it is important we understand their climatological characteristics and recent changes. Prior research on heat extremes has largely focused on dry‐heat. Using two high‐spatial resolution datasets, we provide a global‐scale characterization of the magnitude, seasonal timing, and frequency of dry‐ and humid‐heat extremes and their historical trends. While dry‐ and humid‐heat extremes occur at the same time of the year in most regions, their timing differs by multiple months in tropical regions with high rainfall. Over the past four decades, dry‐ and humid‐heat extremes have become more frequent over most land regions, with the greatest increases in the tropics and Arctic. Since 1979, each person, on average, has experienced an increase of approximately five additional extreme humid‐heat days per‐decade. These increases are concentrated over densely populated regions in the tropics and sub‐tropics, where humid‐heat levels are already high. Our results emphasize the increasing risk from dangerous heat stress, particularly in vulnerable regions of the world. Global patterns and trends in magnitude, timing, and frequency of dry and humid‐heat extremes in ERA5 compare well with HadISD station dataLargest increases in frequency, intensity, and months of occurrence of dry‐ and humid‐heat extremes in tropical and Arctic regionsStronger trends in population exposure to extreme humid‐heat relative to dry‐heat and greater amplification during recent El Niños Global patterns and trends in magnitude, timing, and frequency of dry and humid‐heat extremes in ERA5 compare well with HadISD station data Largest increases in frequency, intensity, and months of occurrence of dry‐ and humid‐heat extremes in tropical and Arctic regions Stronger trends in population exposure to extreme humid‐heat relative to dry‐heat and greater amplification during recent El Niños