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Studies have identified elevation-dependent warming trends, but investigations of such trends in fire danger are absent in the literature. Here, we demonstrate that while there have been widespread increases in fire danger across the mountainous western US from 1979 to 2020, trends were most acute at high-elevation regions above 3000 m. The greatest increase in the number of days conducive to large fires occurred at 2500–3000 m, adding 63 critical fire danger days between 1979 and 2020. This includes 22 critical fire danger days occurring outside the warm season (May–September). Furthermore, our findings indicate increased elevational synchronization of fire danger in western US mountains, which can facilitate increased geographic opportunities for ignitions and fire spread that further complicate fire management operations. We hypothesize that several physical mechanisms underpinned the observed trends, including elevationally disparate impacts of earlier snowmelt, intensified land-atmosphere feedbacks, irrigation, and aerosols, in addition to widespread warming/drying. Elevation-dependent warming trends have been previously identified, but its effect on fire danger is still unclear. Here the authors show that there has been widespread increases in fire danger across the mountainous western US from 1979 to 2020 with most acute trends at high-elevation regions above 3000 m.

Between 15,000 and 18,000 years ago, large amounts of ice and meltwater entered the North Atlantic during Heinrich stadial 1. This caused substantial regional cooling, but major climatic impacts also occurred in the tropics. Here, we demonstrate that the height of this stadial, about 16,000 to 17,000 years ago (Heinrich event 1), coincided with one of the most extreme and widespread megadroughts of the past 50,000 years or more in the Afro-Asian monsoon region, with potentially serious consequences for Paleolithic cultures. Late Quaternary tropical drying commonly is attributed to southward drift of the intertropical convergence zone, but the broad geographic range of the Heinrich event 1 megadrought suggests that severe, systemic weakening of Afro-Asian rainfall systems also occurred, probably in response to sea surface cooling.

Large volcanic eruptions can have major impacts on global climate, affecting both atmospheric and ocean circulation through changes in atmospheric chemical composition and optical properties. The residence time of volcanic aerosol from strong eruptions is roughly 2–3 y. Attention has consequently focused on their short-term impacts, whereas the long-term, ocean-mediated response has not been well studied. Most studies have focused on tropical eruptions; high-latitude eruptions have drawn less attention because their impacts are thought to be merely hemispheric rather than global. No study to date has investigated the long-term effects of high-latitude eruptions. Here, we use a climate model to show that large summer high-latitude eruptions in the Northern Hemisphere cause strong hemispheric cooling, which could induce an El Niño-like anomaly, in the equatorial Pacific during the first 8–9 mo after the start of the eruption. The hemispherically asymmetric cooling shifts the Intertropical Convergence Zone southward, triggering a weakening of the trade winds over the western and central equatorial Pacific that favors the development of an El Niño-like anomaly. In the model used here, the specified high-latitude eruption also leads to a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) in the first 25 y after the eruption, followed by a weakening lasting at least 35 y. The long-lived changes in the AMOC strength also alter the variability of the El Niño–Southern Oscillation (ENSO).

During the last deglaciation, the Indian summer monsoon failed during periods of cooling in the North Atlantic. Sediment records suggest that the concomitant cooling of the surface of the Arabian Sea contributed to the monsoon weakening. The Indian monsoon, the largest monsoon system on Earth, responds to remote climatic forcings, including temperature changes in the North Atlantic 1 , 2 . The monsoon was weak during two cool periods that punctuated the last deglaciation—Heinrich Stadial 1 and the Younger Dryas. It has been suggested that sea surface cooling in the Indian Ocean was the critical link between these North Atlantic stadials and monsoon failure 3 ; however, based on existing proxy records 4 it is unclear whether surface temperatures in the Indian Ocean and Arabian Sea dropped during these intervals. Here we compile new and existing temperature proxy data 4 , 5 , 6 , 7 from the Arabian Sea, and find that surface temperatures cooled whereas subsurface temperatures warmed during both Heinrich Stadial 1 and the Younger Dryas. Our analysis of model simulations shows that surface cooling weakens the monsoon winds and leads to destratification of the water column and substantial subsurface warming. We thus conclude that sea surface temperatures in the Indian Ocean are indeed the link between North Atlantic climate and the strength of the Indian monsoon.

The winter and summer monsoons in Southeast Asia are important but highly variable sources of rainfall. Current understanding of the winter monsoon is limited by conflicting proxy observations, resulting from the decoupling of regional atmospheric circulation patterns and local rainfall dynamics. These signals are difficult to decipher in paleoclimate reconstructions. Here, we present a winter monsoon speleothem record from Southeast Asia covering the Holocene and find that winter and summer rainfall changed synchronously, forced by changes in the Pacific and Indian Oceans. In contrast, regional atmospheric circulation shows an inverse relation between winter and summer controlled by seasonal insolation over the Northern Hemisphere. We show that disentangling the local and regional signal in paleoclimate reconstructions is crucial in understanding and projecting winter and summer monsoon variability in Southeast Asia. Distinguishing local hydrological, cave internal, and regional monsoon signals in speleothem records resolves disagreements among proxy reconstructions and illuminates the Holocene evolution of summer and winter monsoon in Southeast Asia.

Previous studies on future scenarios identified two key effects of increasing CO2${\\text{CO}}_{2}$on the African summer monsoon (ASM): Rising CO2${\\text{CO}}_{2}$leads to an enhancement in moisture supply, favoring an increase in ASM precipitation (the thermodynamic effect). However, it also results in a weakening in mean atmospheric flow, thus facilitating a dryness across the ASM region (the dynamic effect). Therefore, the ultimate change in ASM precipitation stems from the balance of both the thermodynamic and dynamic effects. This study further examines the impact of rising CO2${\\text{CO}}_{2}$on ASM rainfall, by taking into account various climate states. Our results suggest that an increase in CO2${\\text{CO}}_{2}$during warm interglacial periods has a stronger influence from thermodynamic factors than from dynamic factors, resulting in an enhancement in ASM rainfall. In contrast, if CO2${\\text{CO}}_{2}$increases under cold glacial climate backgrounds, its dynamic impact dominates a reduction of rainfall in most ASM region. Plain Language Summary The increase in carbon dioxide (CO2${\\text{CO}}_{2}$ ) levels influences African monsoon rainfall mainly via two processes. First, the warming induced by rising CO2${\\text{CO}}_{2}$leads to enhanced evaporation, thereby increasing atmospheric water vapor content, which provides a greater moisture supply for precipitation formation (the “thermodynamic effect”). Second, rising CO2${\\text{CO}}_{2}$also weakens the tropical circulation, which influences the dynamics of monsoon rainfall (the “dynamic effect”). Consequently, the final changes in monsoon rainfall are determined by the combined effects of these two processes. In this study, through a set of sensitivity simulations, we investigated the impact of rising CO2${\\text{CO}}_{2}$on African summer monsoon (ASM) rainfall during different climatic periods. Although the response in monsoon precipitation is not spatially uniform, our results generally indicate that increased CO2${\\text{CO}}_{2}$levels exert a greater thermodynamic impact than a dynamic one during relatively warm interglacial periods, resulting in strengthened monsoon rainfall. Conversely, a dryness over most of the ASM region is dominated by the dynamic effect when CO2${\\text{CO}}_{2}$increases during cool glacial periods. This research contributes to our understanding of the complex interplay between CO2${\\text{CO}}_{2}$levels, thermodynamic processes, and dynamic processes in shaping African summer monsoon rainfall. It also underscores the importance of considering climatic periods and the relative strengths of different mechanisms when assessing the impact of CO2${\\text{CO}}_{2}$on monsoon systems. Key Points The impact of rising CO2${\\text{CO}}_{2}$on African summer monsoon rainfall is climate dependent Under interglacial climate background, a rise in CO2${\\text{CO}}_{2}$can enhance northern Africa precipitation by increasing atmospheric moisture content In glacial times, a rise in CO2${\\text{CO}}_{2}$facilitates a dryness over most northern Africa, primarily due to a weakened tropical circulation

Anthropogenic climate change is currently driving environmental transformation on a scale and at a pace that exceeds historical records. This represents an undeniably serious challenge to existing social, political, and economic systems. Humans have successfully faced similar challenges in the past, however. The archaeological record and Earth archives offer rare opportunities to observe the complex interaction between environmental and human systems under different climate regimes and at different spatial and temporal scales. The archaeology of climate change offers opportunities to identify the factors that promoted human resilience in the past and apply the knowledge gained to the present, contributing a much-needed, long-term perspective to climate research. One of the strengths of the archaeological record is the cultural diversity it encompasses, which offers alternatives to the solutions proposed from within the Western agro-industrial complex, which might not be viable cross-culturally. While contemporary climate discourse focuses on the importance of biodiversity, we highlight the importance of cultural diversity as a source of resilience.

We use a high-resolution regional climate model to investigate the changes in Atlantic tropical cyclone (TC) activity during the period of the mid-Holocene (MH: 6000 years BP) with a larger amplitude of the seasonal cycle relative to today. This period was characterized by increased boreal summer insolation over the Northern Hemisphere, a vegetated Sahara and reduced airborne dust concentrations. A set of sensitivity experiments was conducted in which solar insolation, vegetation and dust concentrations were changed in turn to disentangle their impacts on TC activity in the Atlantic Ocean. Results show that the greening of the Sahara and reduced dust loadings (MHGS+RD) lead to a larger increase in the number of Atlantic TCs (27 %) relative to the pre-industrial (PI) climate than the orbital forcing alone (MHPMIP; 9 %). The TC seasonality is also highly modified in the MH climate, showing a decrease in TC activity during the beginning of the hurricane season (June to August), with a shift of its maximum towards October and November in the MHGS+RD experiment relative to PI. MH experiments simulate stronger hurricanes compared to PI, similar to future projections. Moreover, they suggest longer-lasting cyclones relative to PI. Our results also show that changes in the African easterly waves are not relevant in altering the frequency and intensity of TCs, but they may shift the location of their genesis. This work highlights the importance of considering vegetation and dust changes over the Sahara region when investigating TC activity under a different climate state.

Extensive lockdowns during the COVID‐19 pandemic caused a remarkable decline in human activities that have influenced urban climate, especially air quality and urban heat islands. However, the impact of such changes on local climate based on long term ground‐level observations has hitherto not been investigated. Using air pollution measurements for the four major Canadian metropolitan areas (Toronto, Montreal, Vancouver, and Calgary), we find that PM2.5 markedly decreased during and after lockdowns with peak reduction ranging between 42% and 53% relative to the 2000–2019 reference period. Moreover, we show a substantial decline in canopy urban heat island intensity during lockdown and in the post lockdowns periods with peak reduction ranging between 0.7°C and 1.6°C in comparison with the 20‐year preceding period. The results of this study may provide insights for local policymakers to define the regulation strategies to facilitate air quality improvement in urban areas. Plain Language Summary The global response to the COVID‐19 outbreak has resulted in unprecedented reductions in human activity. Our findings indicate that lockdowns have reduced the concentration of particulate matter levels in 2020 compared to the period of 2000–2019, by almost 60% across Canada. Additionally, the reduction in anthropogenic heat release, brought about by the closure of road transportation and the shutdown of nonessential businesses and industrial activities, has also contributed to a significant reduction in the urban heat island effect across the four major Canadian metropolitan areas. Key Points The drastic reduction in human activity during COVID‐19 lockdown led to a significant decrease in both air pollution and the CUHI intensity The PM2.5 average was reduced up to almost 2 S.D. throughout the lockdown and after the lockdown period in comparison with the long‐term mean The CUHI intensity dropped up to almost 1.5 S.D. during both the lockdown and post‐lockdown periods in comparison with the historical mean