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Ethiopia is among the African nations most susceptible to climate change because of its frequent droughts and heavy rainfall. Therefore, hydrological and water management problems require an investigation of regional variability and extreme rainfall patterns. This study analyzed the spatiotemporal trends of extreme rainfall in the Lake Tana sub-basin (LTSB) of Ethiopia's upper Blue Nile basin (UBNB) between 1981 and 2019. The trend and geographic patterns of ten extreme rainfall indices are evaluated using high-resolution data from Climate Hazards Group InfraRed Precipitation Stations (CHIRPS). The researcher used RClimDex, an R software tool, to analyze the ten severe rainfall indices. The variability of the extreme rain indices was also assessed by applying the standard anomaly index (SAI). The trend analysis shows that the majority of rainfall indices decreased in the majority of station locations. Among the rainfall locations, the decreasing trend was only significant in 40% consecutive wet days (CWD), 13.33% (R95p and R99p), and 6.66% highest rainfall amount in a 1-day period (RX1day). In contrast, significant positive patterns were revealed in the incidence of rainfall events of number of heavy precipitation days (R10mm), annual total wet day rainfall (PRCPTOT), and consecutive dry days (CDD), with significant positive trends of 26.66% (R10mm) and 40% (PRCPTOT). Furthermore, a spatial distribution result of extreme rainfall trends reveals considerable variations between stations location. Thus, these findings point to the necessity of creating adaptation and mitigation plans for climate change variability within the sub-basin., (© 2024. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

China has been suggested to be the country with the largest vegetation greenness over the last four decades. In this study, we investigated the change in the speed of canopy development and senescence as well as its linkage with climate at monthly scale across temperate China, using satellite-derived Normalized Difference Vegetation Index (NDVI) data from 1982 to 2015. A significant increase in mean monthly NDVI occurred across all growing-season months except June and November, but this greening trend was mainly contributed by the faster speed of canopy development in April and the slower speed of canopy senescence in October. The average of V NDVI (the difference in NDVI between 2 consecutive months) over temperate China is significantly increased only in April (7.75 × 10 -4  year -1 , p < .05) and October (4.98 × 10 -4  year -1 , p < .05). In contrast, V NDVI in November is significantly decreased, indicating an increasing trend in the magnitude of leaf fall in the last month of the growing season due to both increase in season maximum greenness and slower canopy senescence in October. We also found clear seasonal differences in the correlations between V NDVI and climatic factors, especially temperature. In April and October, the correlations between V NDVI and temperature were generally positive, while they were negative in June. V NDVI in spring (early growing season) and summer (middle growing season) was also positively correlated with precipitation in semiarid regions. Such seasonally distinct climatic controls on V NDVI should be considered in modelling vegetation responses to climate change. Overall, our findings can help quantify the contribution of different climatic drivers on the shifts in canopy development and senescence and better elucidate the change in vegetation greenness under future climate change., (© 2022 John Wiley & Sons Ltd.)

We analyzed trends of air temperature across the Cévennes National Park in Southern France, a mid-altitude coastal mountain experiencing a rapid spread of forests at the expense of rangelands and submitted to Mediterranean Sea influences and so, impacted by local and regional processes of climate change. Since 1980, April to June warming trend reached a maximum temperature increase of + 0.124 °C year -1 and uniform whatever the altitude. Minimum temperature increased by + 0.058 °C year -1 at 500 m altitude and + 0.089 °C year -1 at 1500 m. Concomitantly, forest cover is increasing by + 0.51% year -1 . Using an intrinsic biophysical mechanism model, we demonstrated that, at monthly scale, the forest surface is 1.7-3.1 °C cooler than that of nearby grasslands. As a result, the decrease in albedo corresponding to the conversion from grasslands to dense forests, translates into a cooling of maximum air temperatures of 0.023 °C year -1 which contributes to slow down the warming rate enhancement. Spring warming trends co-varied with negative WeMO phases associated with a low in the Gulf of Cádiz and an anticyclone in Central Europe. An east to west pressure gradient increases atmospheric humidity leading to a strong water vapor feedback, enhancing the forcing of thermal long wave radiations and hence the rise in temperature., (© 2022. The Author(s).)

The global warming on Earth during the last century has been discussed in many studies. The most significant factors of climate change are the increase in the atmospheric concentration of greenhouse gases, catastrophic eruptions of volcanoes, and variations in the solar activity. In this paper, we consider the character of climate change and its possible relation to solar-activity variations using the data of the global network of meteorological stations on temperature variations in different regions across the globe from 1880 and information about variations in the relative sunspot number over the last 300 years and temporal variations in the total solar irradiation. We found that the annual mean sunspot number increased on average by about 0.2% per year in both 11-year and secular cycles. The increase in the globally averaged surface air temperature in the period 1880–2004 was Δ t = 0.61 ± 0.04 °C. The difference in Δ t calculated for periods with different solar-activity levels in 11-year cycles was estimated. This difference was most clearly revealed over land at high latitudes of the northern hemisphere. The medians of the distributions of the annual mean surface air temperature over land, ocean, and over the entire globe in years with high solar activity in the secular cycle are significantly greater than the corresponding values related to the years of low solar activity. Noticeable falls in temperature (by ∼0.1–0.2°C) through ∼1900–1920 and 1945–1980 are likely to be associated with the radiation balance perturbation caused by a large number of catastrophic volcanic eruptions during these periods. A considerable warming during the last three decades is most probably due to the substantial growth in the rate of carbon dioxide input to the atmosphere and the corresponding large increase in its concentration. The importance of this factor of global warming becomes even greater if we bear in mind that the solar activity in the secular cycle declines after 1970. [ABSTRACT FROM AUTHOR]

Global warming has significantly altered plant phenology by advancing the timing of leaf emergence, impacting vegetation productivity and adaptability. Winter and spring temperatures have commonly been used to explain spring phenology shifts, but we still lack a solid understanding of the effects of interactions between conditions in different seasons. This study utilizes normalized difference vegetation index (NDVI) and meteorological data to examine the effects of changes in winter and spring temperatures and precipitation on the start of the vegetation growing season (SOS) at high latitudes in China from 1982 to 2015. We found that SOS in Northeast China, as a whole, showed a weak advancing trend (moving earlier in the year), but with obvious regional differences. Even within the same vegetation type, changes in SOS were faster in the cold north (1.9 days/decade) and the cold and dry northwest (1.6 days/decade) than the regional averages for deciduous needleleaf forests (DNF; 1.2 days/decade) and grasslands (0.6 days/decade). Increases in spring temperatures dominate forest SOS advancement, while grassland SOS is mainly influenced by winter and spring precipitation. Decreases in winter minimum temperature (Tmin) enhance the spring temperature sensitivity of SOS. The way that winter precipitation regulates the spring temperature sensitivity of SOS differs among vegetation types: increasing sensitivity in grasslands but suppressing it in DNF. The moderating effects of winter conditions account for the greatest part of the regional differences in the magnitude of change in SOS. Our findings highlight that, although rising spring temperatures significantly affect SOS, winter Tmin and precipitation are crucial for understanding spatial SOS differences, particularly in cold, arid high-latitude regions. Winter conditions play an essential role in regulating the response of vegetation SOS to spring climate at high latitudes. These results suggest that considering the moderating effect of winter climate can facilitate more accurate predictions of temperature-driven phenological changes under future climate change. [ABSTRACT FROM AUTHOR]

The Warm Arctic‐Cold North American (WACNA) pattern features opposing surface temperature anomalies, with centers over the Chukchi‐Bering Seas (CBS) and the North American Great Plains. The pattern is mainly driven by meridional heat transport and damped by the generation of available potential energy through diabatic heating. The Canadian Earth System Model CanESM5, part of the Coupled Model Intercomparison Project Phase 6 (CMIP6), reasonably reproduces this pattern and its formation mechanisms. Future projections under the Shared Socioeconomic Pathway 8.5 (SSP5‐8.5) suggest a significant weakening of WACNA with global warming. Notable changes in pattern intensity and spatial structure are anticipated, particularly a decrease in intensity over the CBS. WACNA changes are also found in 31 CMIP6 multi‐model ensemble simulations. These changes are attributed to Arctic amplification associated with global warming, which diminishes the equator‐to‐pole temperature gradient and consequently reduces meridional heat transport, particularly over the CBS region. Plain Language Summary: The Warm Arctic‐Cold North American (WACNA) pattern shows warming in the Arctic and cooling in North America, with hot spots in the Chukchi‐Bering Seas (CBS) and the Great Plains. This happens because heat moves from one place to another along temperature differences, balanced mainly by the interaction of temperature and diabatic heating. We explore how global warming affects this pattern using the latest Canadian Earth System Model CanESM5 and other 30 CMIP6 models. These models simulate this pattern well and why it occurs. According to climate simulations under a future scenario called SSP5‐8.5, the WACNA pattern will become weaker as the Earth keeps warming up, especially over the CBS. These changes are due to a phenomenon called Arctic amplification, where warming happens faster in the Arctic than in other places. This reduces the movement of heat from the equator to the Arctic, especially in the CBS region, causing changes in the WACNA pattern. Key Points: WACNA variability decreases with global warming, with a faster decline in the Chukchi‐Bering Seas than North AmericaWACNA variability is supported by meridional heat transport and damped by generation of available potential energy through diabatic heatingWACNA changes are driven by changes in meridional heat transport, related to global warming and its featured Arctic amplification [ABSTRACT FROM AUTHOR]

Climate change is rapidly modifying biodiversity across the Arctic, driving a shift from Arctic to more boreal ecosystem characteristics. This phenomenon, known as borealization, is mainly described for certain functional groups along sub-Arctic inflow shelves (Barents and Chukchi Seas). In this review, we evaluate the spatial extent of such alterations across the Arctic, as well as their effects on ecosystem-level processes and risks. Along the inflow shelves, borealization is driven by long-term strengthened inflow of increasingly warm waters from the south and punctuated by advection and low sea ice extreme events. A growing body of literature also points to an emerging borealization of the other Arctic shelf ecosystems, through a "spillover" effect, as local changes in environmental conditions enable movement or transport of new species from inflow shelves. These modifications are leading to changes across functional groups, although many uncertainties remain regarding under-sampled groups, such as microbes, and technical challenges of consistent, regular monitoring across regions. There is also clear consensus that borealization is affecting phenology, species composition, community traits, population structure and essential habitats, species interactions, and ecosystem resilience. Non-dynamic environmental factors, such as depth and photoperiod, are thought to limit the complete borealization of the system, and may lead to intermediate, "hybrid" ecosystems in the future. We expect current borders of Arctic and boreal ecosystems to progress further northward and ultimately reach an equilibrium state with seasonal borealization. Risks to the system are difficult to estimate, as adaptive capacities of species are poorly understood. However, ice-associated species are clearly most at risk, although some might find temporary refuge in areas with a slower rate of change. We discuss the likely character of future Arctic ecosystems and highlight the uncertainties. Those changes have implications for local communities and the potential to support Blue Growth in the Arctic. Addressing these issues is necessary to assess the full scale of Arctic climate impacts and support human mitigation and adaptation strategies. [ABSTRACT FROM AUTHOR]

Over recent decades, the Arctic has begun facing a range of climate-related challenges, from rising temperatures to melting ice caps and permafrost thaw, with significant implications for ecosystems and human well-being. Addressing the health impacts of these issues requires a comprehensive approach, integrating scientific research, community engagement, and policy interventions. This study conducts a literature review to assess the effects of climate change on human health in northern latitudes and to compile adaptation strategies from the Arctic countries. A literature search was performed between January and April 2024 for papers published after 2000, using the electronic databases Web of Science, Pubmed, Science Direct, Scopus, Google Scholar, and eLibrary.RU, with specific questions formulated to direct the search: (i) What are the climate changes? (ii) How does climate change affect human health? (iii) What adaptation measures and policies are required? The key phrases "climate change", "human health", "adaptation practices", and "Arctic" were employed for searching. Ultimately, 56 relevant studies were identified, reviewing health risks such as infectious diseases, mental health issues, and diseases connected with extreme weather events; wildfires and their associated pollution; permafrost degradation; pure water; and food quality. The paper also examines mitigation and adaptation strategies at all levels of governance, emphasizing the need for international cooperation and policy action to combat negative health outcomes, investments in healthcare infrastructure, emergency preparedness, and public health education. Incorporating diverse perspectives, including Indigenous knowledge, Community-Based Adaptation, EcoHealth and One Health approaches, is crucial for effectively addressing the health risks associated with climate change. In conclusion, the paper proposes adaptation strategies to mitigate the health impacts of climate change in the Arctic. [ABSTRACT FROM AUTHOR]

The Arctic rapidly transforms due to global warming and increased human activities, triggering complex changes at unprecedented speeds that challenge conventional institutional responses. We analyse these changes through the lenses of social, political, and environmental boundaries and investigate their impacts on both inhabitants' livelihoods and the region's political framework. Employing an interdisciplinary approach, we highlight the complexities of understanding the interplay among global, regional, and local dynamics in an era where human and non-human aspects are entwined. Our analysis concentrates on three areas: definition of the Arctic; legal disputes concerning the waters around the Svalbard Archipelago; evolving natural hazards and societal risk perceptions in Longyearbyen. Through these examples, we underscore the intricate nature of social, political, and ecological changes and how they challenge current boundary-making processes. By combining knowledge from different systems and scales, our research reveals the interplay between policy-driven science, science-influenced policy, and performative behaviors in reshaping today's Arctic borders and boundaries. We particularly emphasize how climate change is challenging borders and advocate for a departure from static definitions, towards the formulation of environmentally conscious, socially just, and politically viable policies, acknowledging the new biophysical realities of the Anthropocene. [ABSTRACT FROM AUTHOR]

Current global warming results in an uptake of heat by the Earth system, which is distributed among the different components of the climate system. However, current‐generation climate models deliver heat inventory and partitioning estimates of Earth system components that differ from recent observations. Here we investigate the global heat distribution under warming by using fully‐coupled CMIP6 Earth system model experiments, including a version of the MPI‐ESM with a deep land model component, accommodating the required space for more realistic terrestrial heat storage. The results show that sufficiently deep land models exert increased subsurface land heat uptake, leading to a heat uptake partitioning among the Earth system components that is closer to observational estimates. The results are relevant for the understanding of Earth's heat partitioning and highlight the importance of the land heat sink in the Earth heat inventory. Plain Language Summary: Global warming is associated with heat accumulation in the Earth system due to the intensification of the greenhouse effect. The available heat is distributed unevenly throughout the climate subsystems: the ocean, land, atmosphere, and cryosphere. Overall, the current generation of climate models captures this partitioning well but, on average, shows an overestimation of the ocean heat uptake and an underestimation of the land heat uptake. Previous studies have shown that the lack of heat input into the land comes from structural limitations in the land model components used. In this study, we account for these shortcomings, which greatly improve the land heat uptake in simulations of future climate scenarios. We find that, as a result, the fraction of simulated heat taken up by the ocean is reduced. This leads to a heat distribution among the climate subsystems that is closer to observational estimates. Our results highlight that land heat uptake is relevant for the Earth system heat distribution and that future research should consider modeling approaches including a more realistic land heat sink. Key Points: Considering a sufficient land depth in current‐generation Earth system models facilitates realistic land heat uptake estimates under warmingIncreased land heat uptake leads to an adjusted global heat distribution and a reduced overestimation of simulated ocean heat uptakeThe land heat sink is relevant for the projection of the global energy inventory and its partitioning under global warming [ABSTRACT FROM AUTHOR]

While a poleward shift of the near-surface jet and storm track in response to increased greenhouse gases appears to be robust, the magnitude of this change is uncertain and differs across models, and the mechanisms for this change are poorly constrained. An intermediate complexity GCM is used in this study to explore the factors governing the magnitude of the poleward shift and the mechanisms involved. The degree to which parameterized subgrid-scale convection is inhibited has a leading-order effect on the poleward shift, with a simulation with more convection (and less large-scale precipitation) simulating a significantly weaker shift, and eventually no shift at all if convection is strongly preferred over large-scale precipitation. Many of the physical processes proposed to drive the poleward shift are equally active in all simulations (even those with no poleward shift). Hence, we can conclude that these mechanisms are not of leading-order significance for the poleward shift in any of the simulations. The thermodynamic budget, however, provides useful insight into differences in the jet and storm track response among the simulations. It helps identify midlatitude moisture and latent heat release as a crucial differentiator. These results have implications for intermodel spread in the jet, hydrological cycle, and storm track response to increased greenhouse gases in intermodel comparison projects. [ABSTRACT FROM AUTHOR]

A fundamental divide exists between previous studies that conclude that polar amplification does not occur without sea ice and studies that find that polar amplification is an inherent feature of the atmosphere independent of sea ice. We hypothesize that a representation of climatological ocean heat transport is key for simulating polar amplification in ice-free climates. To investigate this, we run a suite of targeted experiments in the slab ocean aquaplanet configuration of CESM2-CAM6 with different profiles of prescribed ocean heat transport, which are invariant under CO2 quadrupling. In simulations without climatological ocean heat transport, polar amplification does not occur. In contrast, in simulations with climatological ocean heat transport, robust polar amplification occurs in all seasons. What is causing this dependence of polar amplification on ocean heat transport? Energy-balance model theory is incapable of explaining our results and in fact would predict that introducing ocean heat transport leads to less polar amplification. We instead demonstrate that shortwave cloud radiative feedbacks can explain the divergent polar climate responses simulated by CESM2-CAM6. Targeted cloud locking experiments in the zero ocean heat transport simulations are able to reproduce the polar amplification of the climatological ocean heat transport simulations, solely by prescribing high-latitude cloud radiative feedbacks. We conclude that polar amplification in ice-free climates is underpinned by ocean–atmosphere coupling, through a less negative high latitude shortwave cloud radiative feedback that facilitates enhanced polar warming. In addition to reconciling previous disparities, these results have important implications for interpreting past equable climates and climate projections under high-emissions scenarios. Significance Statement: Polar amplification is a robust feature of climate change in the modern-day climate. However, previous climate modeling studies fundamentally do not agree on whether polar amplification occurs in ice-free climates. In this study, we find in a state-of-the-art climate model that, if ocean heat transport is neglected, the response to an increase in CO2 is not polar amplified, whereas robust polar amplification occurs if ocean heat transport is included. Using targeted model experiments, we diagnose cloud radiative effects as the driver of this divergent behavior. We conclude that polar amplification is a robust feature of the atmosphere–ocean system. Our results have important implications for interpreting past warm climates and future projections under high-emissions scenarios. [ABSTRACT FROM AUTHOR]

Global warming is destabilizing the cryosphere, with consequences for glaciers, permafrost, sea ice and lake ice. Polar lakes have short ice‐free seasons, and small changes in ice cover duration have the potential to provoke alterations to ecosystem structure. However, these lakes are understudied, and the consequences for mixing regimes, thermal structures and biogeochemical processes remain unclear. We measured three annual cycles of dissolved oxygen, temperature and specific conductivity in a lake at ∼83°N to investigate limnological processes and their interannual variability. There were sharp interannual contrasts in lake dynamics, with state shifts in mixing, stratification and oxygen regimes due to air temperature variability and meteorological events. We also observed unusual thermal profiles that were associated with solute gradients. These striking differences underscore the sensitivity of high Arctic lakes to interannual variations in meteorological forcing, and their susceptibility to regime shifts in response to ongoing global change. Plain Language Summary: Lakes are biodiversity refuges, climate regulators and providers of ecosystem services. High Arctic lakes experience brief summer ice‐free periods and their annual dynamics remain largely unexplored despite their location in one of the most rapidly warming regions on Earth. In particular, patterns of thermal structure and oxygen dynamics have rarely been measured. We studied three complete annual cycles in a coastal lake less than 100 km south of the northernmost land on Earth. The observed limnological dynamics highlight the potential for rapid regime shifts in polar lakes and underline their sensitivity to interannual differences in environmental conditions. Key Points: Pronounced shifts in lake oxygen and temperature dynamics occur due to interannual air temperature variation and meteorological episodesDivergent annual trajectories in lake structure and functioning highlight the vulnerability of high Arctic lakes to climate change [ABSTRACT FROM AUTHOR]

Arctic Ocean warming and sea ice loss are closely linked to increased ocean heat transport (OHT) into the Arctic and changes in surface heat fluxes. To quantitatively assess their respective roles, we use the 100-member Community Earth System Model, version 2 (CESM2), Large Ensemble over the 1920–2100 period. We first examine the Arctic Ocean warming in a heat budget framework by calculating the contributions from heat exchanges with atmosphere and sea ice and OHT across the Arctic Ocean gateways. Then we quantify how much anomalous heat from the ocean directly translates to sea ice loss and how much is lost to the atmosphere. We find that Arctic Ocean warming is driven primarily by increased OHT through the Barents Sea Opening, with additional contributions from the Fram Strait and Bering Strait OHTs. These OHT changes are driven mainly by warmer inflowing water rather than changes in volume transports across the gateways. The Arctic Ocean warming driven by OHT is partially damped by increased heat loss through the sea surface. Although absorbed shortwave radiation increases due to reduced surface albedo, this increase is compensated by increasing upwelling longwave radiation and latent heat loss. We also explicitly calculate the contributions of ocean–ice and atmosphere–ice heat fluxes to sea ice heat budget changes. Throughout the entire twentieth century as well as the early twenty-first century, the atmosphere is the main contributor to ice heat gain in summer, though the ocean's role is not negligible. Over time, the ocean progressively becomes the main heat source for the ice as the ocean warms. Significance Statement: Arctic Ocean warming and sea ice loss are closely linked to increased ocean heat transport (OHT) into the Arctic and changes in surface heat fluxes. Here we use 100 simulations from the same climate model to analyze future warming and sea ice loss. We find that Arctic Ocean warming is primarily driven by increased OHT through the Barents Sea Opening, though the Fram and Bering Straits are also important. This increased OHT is primarily due to warmer inflowing water rather than changing ocean currents. This ocean heat gain is partially compensated by heat loss through the sea surface. During the twentieth century and early twenty-first century, sea ice loss is mainly linked to heat transferred from the atmosphere; however, over time, the ocean progressively becomes the most important contributor. [ABSTRACT FROM AUTHOR]

Cold exposure remains a significant public health concern, particularly in the Arctic regions prone to extremely cold weather. While the physical health impacts of cold exposure are well documented, understanding the social vulnerability aspects is crucial for effective mitigation and policy development. This study investigates the multifaceted dimensions of social vulnerability in the face of cold temperatures across various communities in Alaska. Alaska, renowned for its extreme cold temperatures and harsh environmental conditions, poses unique challenges to its residents, particularly in the context of social vulnerability. Drawing on a combination of quantitative data analysis and qualitative insights, we examine the factors contributing to social vulnerability, including demographic, economic, geographic, and infrastructural elements, in terms of the Extremely Cold Social Vulnerability Index, for seven Public Health Regions in Alaska. The Universal Thermal Climate Index in two very cold categories (<−27 °C) was used to identify cold exposure. Factors such as income, housing quality, health status, and resilience of the population play crucial roles in determining an individual or community's sensitivity to, and ability to cope with, cold temperatures. Our analysis reveals that social vulnerability in Alaska is not uniform but varies significantly among regions. The research findings highlight the importance of considering factors of both sensitivity and adaptivity in understanding and addressing social vulnerability, thereby informing the development of targeted strategies and policies to enhance the resilience of Alaskan communities. As cold temperatures are projected to continue to challenge the region, addressing social vulnerability is essential for ensuring the well-being and safety of Alaska's diverse populations. [ABSTRACT FROM AUTHOR]

Under the background of global warming, the Arctic region has warmed faster than the Antarctic, which is referred to as asymmetric Arctic and Antarctic warming. The new generation of model simulations from the CMIP6 offers an opportunity to identify the major factors contributing to the asymmetric warming and its intermodel spread. In this study, the preindustrial and abrupt-4 × CO2 experiments from 18 CMIP6 models are examined to extract the asymmetric warming and its intermodel spread. A climate feedback-response analysis method is applied to reveal the contributions of external and internal feedback processes to the asymmetric warming and its intermodel spread, by decomposing total warming into the partial temperature changes caused by individual factors. It is found that a seasonal energy transfer mechanism (SETM) dominates in both polar warmings. The direct consequence of the sea ice declining in response to the anthropogenic forcing is an increase in the effective heat capacity of the ocean surface layer. Such an increase in the effective heat capacity temporally withholds most of the extra solar energy absorbed during summer and then releases it during winter, contributing to stronger warming in winter. However, the background oceanic circulation in the Southern Ocean, namely, the Antarctic Circumpolar Current, continually transports energy equatorward, resulting in a suppressed SETM and surface warming in the Antarctic. The key factor that accounts for intermodel spread in the asymmetric warming is the difference in their strengths of SETM. The poleward atmospheric transport and water vapor feedback also contribute to the intermodel spread. Significance Statement: The asymmetric Arctic and Antarctic warming, as a response to the increase in CO2, can regulate global atmospheric and oceanic circulations via meridional temperature gradient. Previous studies have all ascribed the key role of the ocean in the asymmetric warming over the two poles with a lack of comprehensive understanding of the roles of other feedback processes. This study emphasizes that oceanic circulation is the root of the asymmetric warming via suppressing sea ice retreat and the associated SETM in the Antarctic, instead of increasing ocean heat uptake and equatorward transport only. For the intermodel spread in the asymmetric warming over two poles, the differences in the strength of all processes involving the SETM among models are the prominent issue. [ABSTRACT FROM AUTHOR]

Thermal comfort refers to the suitability of meteorological conditions for humans with the environment. Temperature is the main meteorological variable, which determines the thermal comfort as expressed in various climate indices. This study aims to analysis the distribution of thermal comfort, and to identify environmental factors influencing the comfort situated in North Sumatra, Indonesia. We applied the Universal Thermal Climate Index (UTCI) to determine the heat stress level for 2011 - 2020. The higher UTCI value indicates more uncomfortable conditions related to the higher heat stress. The results showed that the average value of UTCI in North Sumatra was categorised at moderate heat stress. Densely urbanised area in the eastern region contributed to high heat stress, whereas mountainous areas in central to west regions were at low level. Our findings found that climate season affected the distribution of heat stress level. The low heat stress occurred in December-February, while high heat stress happened in June-August. Further, findings revealed that altitude and land cover have contributed to UTCI variation by more than 75% variance. [ABSTRACT FROM AUTHOR]

Arctic warming under increased CO2 peaks in winter, but is influenced by summer forcing via seasonal ocean heat storage. Yet changes in atmospheric heat transport into the Arctic have mainly been investigated in the annual mean or winter, with limited focus on other seasons. We investigate the full seasonal cycle of poleward heat transport modeled with increased CO2 or with individually applied Arctic sea‐ice loss and global sea‐surface warming. We find that a winter reduction in dry heat transport is driven by Arctic sea‐ice loss and warming, while a summer increase in moist heat transport is driven by sub‐Arctic warming and moistening. Intermodel spread in Arctic warming controls spread in seasonal poleward heat transport. These seasonal changes and their intermodel spread are well‐captured by down‐gradient diffusive heat transport. While changes in moist and dry heat transport compensate in the annual‐mean, their opposite seasonality may support non‐compensating effects on Arctic warming. Plain Language Summary: The Arctic is warming much faster than the rest of the planet in response to rising greenhouse gas concentrations. Because Arctic warming peaks in winter, many studies have focused on the wintertime processes amplifying Arctic warming. However, others have found that summer atmospheric heating also contributes to winter warming by melting sea ice and storing heat in the ocean until it is released to the atmosphere in winter. Here we study changes in all seasons for one source of atmospheric heating in the Arctic—atmospheric heat transport from lower latitudes. Using climate model simulations, we find that heat and moisture are transported away from the regions that warm and moisten the most in response to rising greenhouse gas concentrations. The Arctic warms more than lower latitudes in winter, which reduces heat transport to the Arctic in winter. Atmospheric moisture increases most in late summer at lower latitudes, driving increased moisture transport in late summer from lower latitudes to the Arctic. We suggest that changes in heat and moisture transport may impact Arctic warming differently due to their opposite seasonality: by producing a larger change in surface solar absorption, summer changes in moisture transport may outweigh winter changes in heat transport. Key Points: Sea‐ice loss reduces dry heat transport to the Arctic in winter; sub‐Arctic warming increases latent heat transport to the Arctic in summerIntermodel spread in Arctic warming controls intermodel spread in seasonal heat transport changesThe seasonal pattern of poleward heat transport change is well‐captured by down‐gradient diffusion of temperature and moisture anomalies [ABSTRACT FROM AUTHOR]

Water vapor in the Arctic can be attributed to local evaporation and transport from mid‐latitudes. Here we investigate the response of Arctic water vapor to declining sea ice in a climate model, using an idealized warming scenario in which the Arctic becomes ice free year‐round. We find distinct monthly patterns in water vapor change with warming, with a strong influence of sea ice loss on changes to evaporation, except in summer when transports dominate. Before the Arctic becomes ice‐free, total column water increases at a rate slower than that indicated by the Clausius‐Clapeyron relationship; once ice‐free, total column water increases at a rate equal to Clausius‐Clapeyron. The exception is summer when the increase is initially faster then becomes slower when ice‐free. The boundary layer structure changes, under global warming, from stratified to well mixed in all seasons except summer, which remains largely unchanged throughout the ice‐free transition. Plain Language Summary: Surface evaporation from the Arctic Ocean is limited by the presence of sea ice, which creates a natural barrier between the ocean and atmosphere above. We show the loss of Arctic sea ice, in a climate model affects atmospheric water vapor and find that this varies throughout the year. The greatest increase in atmospheric water vapor is in winter, during the Arctic night, whilst there is very little change in summer. There are two reasons why sea ice loss does not impact water vapor in the summer: first, the summer air temperature barely changes between ice‐covered and ice‐free, whereas in winter the loss of ice cover can increase air temperatures by at least 20°C; second the summer Arctic water budget is dominated by transport of water vapor from midlatitudes rather than local evaporation. We also show changes in the vertical structure of the near‐surface atmosphere and cloud types, with a transition from stable to turbulent regimes in autumn and winter. Key Points: As Arctic sea ice declines with increased warming, Arctic evaporation and water vapor increase most in autumn and winterArctic sea ice limits water vapor availability, but once the Arctic is ice‐free, it increases with the Clausius‐Clapeyron relationshipArctic boundary layer changes from stably stratified to well mixed, during winter and autumn, but less change in spring and summer [ABSTRACT FROM AUTHOR]

Climate change does not occur in the same way worldwide; its effects display regional differences. Some regions with unique characteristics may experience dramatic changes, leading to significant indications for the global climate. The Siberian high, a system of high atmospheric pressure, is formed on the Central Siberian Plateau, affecting a significant part of the northern hemisphere from November to February. Climate changes in this region have significant influences on the global climate cycle. Hence, determining the temperature trends of this region will yield key indicators for climate change studies. Remote sensing provides useful databases for climate change studies, surface temperatures, temporal and spatial resolutions, and numerous advantages. In the present study, we aimed to determine the temporal and spatial surface temperature trends of the Central Siberian Plateau. As our data source, we used MODIS (Aqua and Terra) satellite images for 8 days between 2002- and 2021. The measurements from the region were arranged as monthly and annual values, presented as mean, minimum, maximum, and standard deviation. Then, using these data, we obtained the mean values for the region and performed Mann–Kendall trend analyses. Accordingly, there was an overall increase of more than 2 °C in the study area. We performed a pixel-based Mann–Kendall trend test to reveal the mean annual temperatures and detect local changes. Our findings showed significant rises in temperature in the northern part of the study area. [ABSTRACT FROM AUTHOR]

Predicting the impact of marine ecosystem warming on the timing and magnitude of phytoplankton production is challenging. For example, warming can advance the progression of stratification thereby changing the availability of nutrients to surface phytoplankton, or influence the surface mixed layer depth, thus affecting light availability. Here, we use a time series of sea surface temperature (SST) and chlorophyll remote sensing products to characterize the response of the phytoplankton community to increased temperature in the Northeast US Shelf Ecosystem. The rate of change in SST was higher in the summer than in winter in all ecoregions resulting in little change in the timing and magnitude of the spring thermal transition compared to a significant change in the autumn transition. Along with little phenological shift in spring thermal conditions, there was also no evidence of a change in spring bloom timing and duration. However, we observed a change in autumn bloom timing in the Georges Bank ecoregion, where bloom initiation has shifted from late September to late October between 1998 and 2020—on average 33 d later. Bloom duration in this ecoregion also shortened from ∼7.5 to 5 weeks. The shortened autumn bloom may be caused by later overturn in stratification known to initiate autumn blooms in the region, whereas the timing of light limitation at the end of the bloom remains unchanged.  These changes in bloom timing and duration appear to be related to the change in autumn thermal conditions and the significant shift in autumn thermal transition. These results suggest that the spring bloom phenology in this temperate continental shelf ecosystem may be more resilient to thermal climate change effects than blooms occurring in other times of the year. [ABSTRACT FROM AUTHOR]

Polar amplification of climate change has the potential to cause large‐scale shifts in the dissolved oxygen (DO) dynamics of Arctic lakes, with implications for fish survival, greenhouse gas production, and drinking water quality. While DO is also a sentinel of environmental changes of physical, chemical, and biological nature (e.g., ice cover, temperature, dissolved organic carbon, photosynthesis, and respiration), no synthesis exists of current knowledge of DO dynamics across the diverse freshwater systems of the Arctic. We thus conducted a systematic review of the literature that yielded DO data from 167 sites north of the Subarctic limit (based on vegetation zones), spanning 76 years and including 40 sites with time series. The compilation revealed insufficient observations for adequate representativeness of oxygen dynamics over Arctic ecosystem gradients. We described the main processes controlling DO budgets of Arctic lakes and tested relationships of summer oxygen depletion with maximum depth and latitude. The meta‐analysis showed that most sites with low O2 concentrations were shallow (<10 m) and situated toward the southern end of the latitudinal gradient. Permanently stratified lakes with deep, perennially anoxic basins were located toward the northern end of the gradient. By way of a conceptual model, we identified the direct and indirect drivers and mechanisms that lead to changes in oxygen budgets in the context of the warming Arctic. This comprehensive update on available data allowed us to suggest future research directions and recommend the use of moored instruments for continuous all‐season observations, combined with modeling, remote sensing, and paleo‐reconstructions. Plain Language Summary: Lakes are dominant features of the Arctic landscape, and they have great cultural and environmental significance. The amplified effects of global warming observed in the Arctic represent a threat to the integrity of these systems. Levels of dissolved oxygen (DO) in lake water have implications for fish survival, greenhouse gas production, and drinking water quality. To summarize the state of knowledge about DO in Arctic lakes, we reviewed literature and extracted data from 167 sites. We describe the main processes known to control the oxygen balance and classified the lakes according to their bottom oxygen concentrations during summer. Most lakes that had low oxygen were shallow and situated toward the southern part of our study region. Lakes that never mix, and thus preserve permanent oxygen depletion in their deep waters, were located toward the northern end of the gradient. With a conceptual model, we illustrate how climate change is likely to influence oxygen balance by the way of direct and indirect drivers. We suggest future research directions including the collection of continuous data throughout all seasons, modeling, remote sensing, and studies of sediment cores to reconstruct past conditions. Key Points: Our systematic review located oxygen data in the Arctic from 167 field sites spanning 76 years, including 40 lakes with time‐seriesMeta‐analysis showed that summer oxygen depletion occurred mostly in shallow lakes toward the southern end of the latitudinal gradientWe formulated conceptual models of oxygen sinks and sources relevant to Arctic lakes and their responses to ongoing climate change [ABSTRACT FROM AUTHOR]

Climate change in the Arctic has substantial impacts on human life and ecosystems both within and beyond the Arctic. Our analysis of CMIP6 simulations shows that some climate models project much larger Arctic climate change than other models, including changes in sea ice, ocean mixed layer, air‐sea heat flux, and surface air temperature in wintertime. In particular, dramatic enhancement of Arctic Ocean convection down to a few hundred meters is projected in these models but not in others. Interestingly, these models employ the same ocean model family (NEMO) while the choice of models for the atmosphere and sea ice varies. The magnitude of Arctic climate change is proportional to the strength of the increase in poleward ocean heat transport, which is considerably higher in this group of models. Establishing the plausibility of this group of models with high Arctic climate sensitivity to anthropogenic forcing is imperative given the implied ramifications. Plain Language Summary: Arctic climate is projected to change dramatically in the future based on the latest climate models. However, there is a large discrepancy between these climate models. We find that simulated Arctic climate change tends to be much larger in models using the community ocean model "NEMO" as their ocean component. Future Arctic climate change in winter strongly depends on the strength of the increase in poleward ocean heat transport, which is considerably higher in the NEMO‐family climate models. There are some indications that this group of models can better reproduce the observed winter sea ice decline and mixed layer depth in the Barents Sea in their historical simulations. We suggest that investigating the model physics in the NEMO‐family climate models in comparison with other models may ultimately help to reduce the overall uncertainty in climate change projections. Key Points: Future Arctic climate change in winter strongly depends on the strength of the increase in poleward ocean heat transport (OHT)Arctic climate change in the NEMO‐family climate models is much more pronouncedNEMO‐family climate models have larger increases in poleward OHT [ABSTRACT FROM AUTHOR]

Since the mid-1990s, there has been a marked decrease in the sea ice extent (SIE) in the Arctic Ocean. After reaching an absolute minimum in September 2012, the seasonal variations in the SIE have settled at a new level, which is almost one-quarter lower than the average climatic norm of 1979–2022. Increased melting and accelerated ice export from marginal seas ensure an increase in the open water area, which affects the lower atmosphere and the surface layer of the ocean. Scientists are cautiously predicting a transition to a seasonally ice-free Arctic Ocean as early as the middle of this century, which is about 50 years earlier than was predicted just a few years ago. Such predictions are based on the fact that the decrease in sea ice extent and ice thinning that occurred at the beginning of this century, initially caused by an increase in air temperature, triggered an increase in the thermal and dynamic contribution of the ocean to the further reduction in the ice cover. This paper reviews published evidence of such changes and discusses possible mechanisms behind the observed regional anomalies of the Arctic Sea ice cover parameters in the last decade. [ABSTRACT FROM AUTHOR]

Climate change is occurring rapidly in the Arctic, and an improved understanding of the response of aquatic biota and ecosystems will be important for this data‐limited region. Here, we applied biochronology techniques and mixed‐effects modelling to assess relationships among growth increments found on lake trout (Salvelinus namaycush) otoliths (N = 49) captured from 13 lakes on the Arctic Coastal Plain of northern Alaska, observed and modelled climate patterns, and individual‐level fish and lake characteristics. We found that annual growth varied by year, fish growth slowed significantly as individuals aged, and females grew faster than males. Lake trout had higher growth in flow‐through lakes relative to lakes that were perennially or seasonally connected. Annual growth was positively correlated with observed air temperature measurements from a local weather station for the period 1998–2014, but no clear warming trend was evident for this period. Modelled August air temperatures from 1978–2014 predicted lake trout annual growth (root mean squared error = 0.045 mm) and indicated increasing temperatures and annual lake trout growth over the period 1950–2014. This study demonstrated that biochronology techniques can reconstruct recent climate patterns and provide a better understanding of trends in Arctic lake ecosystems under a changing climate. [ABSTRACT FROM AUTHOR]

It is now well established that the Arctic is warming at a faster rate than the global average. This warming, which has been accompanied by a dramatic decline in sea ice, has been linked to cooling over the Eurasian subcontinent over recent decades, most dramatically during the period 1998–2012. This is a counter-intuitive impact under global warming given that land regions should warm more than ocean (and the global average). Some studies have proposed a causal teleconnection from Arctic sea-ice retreat to Eurasian wintertime cooling; other studies argue that Eurasian cooling is mainly driven by internal variability. Overall, there is an impression of strong disagreement between those holding the "ice-driven" versus "internal variability" viewpoints. Here, we offer an alternative framing showing that the sea ice and internal variability views can be compatible. Key to this is viewing Eurasian cooling through the lens of dynamics (linked primarily to internal variability with some potential contribution from sea ice; cools Eurasia) and thermodynamics (linked to sea-ice retreat; warms Eurasia). This approach, combined with recognition that there is uncertainty in the hypothesized mechanisms themselves, allows both viewpoints (and others) to co-exist and contribute to our understanding of Eurasian cooling. A simple autoregressive model shows that Eurasian cooling of this magnitude is consistent with internal variability, with some periods exhibiting stronger cooling than others, either by chance or by forced changes. Rather than posit a "yes-or-no" causal relationship between sea ice and Eurasian cooling, a more constructive way forward is to consider whether the cooling trend was more likely given the observed sea-ice loss, as well as other sources of low-frequency variability. Taken in this way both sea ice and internal variability are factors that affect the likelihood of strong regional cooling in the presence of ongoing global warming. [ABSTRACT FROM AUTHOR]

Energy balance models (EBMs) have been widely used in a range of climate problems, but the assumption of constant diffusivity in the parameterization of the moist static energy (MSE) flux can hardly be justified. We demonstrate in this study that the diffusive MSE flux can be derived from the basic energy balance equation with a few tolerable assumptions. The estimated diffusivity is both spatially and seasonally dependent, and its midlatitude average is then tested against several scaling theories for the midlatitude eddy diffusivity. The result supports the diffusivity theory of Held and Larichev modified for the moist atmosphere, affording a dynamics-based parameterization of MSE diffusivity. The implementation of the parameterization in an EBM leads to an interactive MSE diffusivity that accounts for the midlatitude eddy response to climate forcing perturbations. Under a uniform radiative forcing, the EBM with a diffusivity so parameterized produces a weakening of the midlatitude diffusivity and a modestly polar-amplified surface temperature response as an inevitable outcome under the dual constraints of the nonlinear Clausius–Clapeyron relation and the temperature gradient-dependent diffusivity, even in the absence of any poleward-amplifying radiative feedbacks. As the consequence of more isothermal temperature and reduced diffusivity, the variance of the midlatitude surface temperature also decreases with warming. [ABSTRACT FROM AUTHOR]

Trends in moist static energy (MSE) transport are investigated for the years 1980 through 2018 using four different reanalysis data sets. The reanalysis data sets show agreement in the eddy MSE transport trends and the latitudinal structure of the MSE trends, but vary widely in the trend of the flux of the climatological zonal mean MSE by the anomalous zonal mean meridional wind. The latter dominates the total MSE transport trends in all four data sets. Therefore, none of the four total MSE flux trends is downgradient of the corresponding MSE trend. Further analysis of the MSE trends reveals that dry static energy increases strongly dominate MSE trends at all latitudes, including in the tropics where climate models and theory predict latent energy increases to dominate. As changes in MSE transport are routinely assumed to be downgradient when interpreting changes in climate, including Arctic amplification, further investigation of reanalysis MSE transport is warranted. Plain Language Summary: The Earth receives more energy than it loses over lower latitudes and loses more energy than it receives over high latitudes. Consistently, the atmospheric circulation fluxes much of the excess energy from low latitudes toward high latitudes where there is an energy deficit. This poleward movement of energy in the atmosphere plays a crucial role in shaping the climate, but it remains unclear how this energy transport will change as the climate warms. In this study, we investigate how energy transport has changed in recent decades using four different reanalysis data sets, data sets that obtain global coverage by combining atmospheric measurements with numerical models. We find that data sets do not agree on the trends in the energy transports which limits the extent to which they can be used to investigate energy transports. Nonetheless, reanalysis data sets do agree on certain aspects that contrast notably with climate models. First, energy transport trends in the reanalysis data sets do not flux energy from locations with high energy to locations with low energy. Second, reanalysis temperature increases drive greater increases in energy than that from water vapor, whereas in climate models water vapor increase is the most important contributor to the energy over the tropics. Key Points: Reanalysis data sets disagree on interannual variability and trends in moist static energy (MSE) transportReanalysis MSE transport trends do not show a clear flux‐gradient response to the MSE gradient trendReanalysis MSE transport trends are dominated by fluxes of time‐mean dry static energy by anomalous zonal mean meridional winds [ABSTRACT FROM AUTHOR]

CO2-forced surface warming in general circulation models (GCMs) is initially polar amplified in the Arctic but not in the Antarctic—a largely hemispherically antisymmetric signal. Nevertheless, we show in CESM1 and 11 LongRunMIP GCMs that the hemispherically symmetric component of global-mean-normalized, zonal-mean warming ( T sym * ) under 4 × CO2 changes weakly or becomes modestly more polar amplified from the first decade to near-equilibrium. Conversely, the antisymmetric warming component ( T asym * ) weakens with time in all models, modestly in some including FAMOUS, but effectively vanishing in others including CESM1. We explore mechanisms underlying the robust T sym * behavior with a diffusive moist energy balance model (MEBM), which given radiative feedback parameter (λ) and ocean heat uptake (O) fields diagnosed from CESM1 adequately reproduces the CESM1 T sym * and T asym * fields. In further MEBM simulations perturbing λ and O , T sym * is sensitive to their symmetric components only, and more to that of λ. A three-box, two-time-scale model fitted to FAMOUS and CESM1 reveals a curiously short Antarctic fast-response time scale in FAMOUS. In additional CESM1 simulations spanning a broader range of forcings, T sym * changes modestly across 2–16 × CO2, and T sym * in a Pliocene-like simulation is more polar amplified but likewise approximately time invariant. Determining the real-world relevance of these behaviors—which imply that a surprising amount of information about near-equilibrium polar amplification emerges within decades—merits further study. [ABSTRACT FROM AUTHOR]

Millions of lakes from around the world freeze during winter. These frozen surfaces provide essential ecosystem services that are vital to many northern communities. However, the availability of safe lake ice that is oftentimes required to support these services is under threat from climate change. Here we use a 100‐member ensemble of climate model simulations to investigate changes in the presence of safe lake ice for different recreation and provisioning activities across the Northern Hemisphere. Our projections suggest that the duration of safe ice for recreational purposes will shorten, on average, by 13, 17, and 24 days within a 1.5°C, 2°C, and 3°C warmer world (relative to 1900–1929), respectively. The projected change in the duration of safe ice will be greater in higher latitudes, but with the most densely populated lower‐latitude regions experiencing the greatest percent change. The use of lake ice to support critical transportation infrastructure will also be influenced by future warming through the loss of thicker ice this century. However, our projections suggest that the magnitude of change in the duration of safe ice will differ depending on the ice thickness requirements. For transportation that requires the thickest ice cover, the number of days with safe ice will decline by 90%, 95%, and 99% with 1.5°C, 2°C, and 3°C global warming, respectively. We highlight the need for the development and implementation of adaptation plans to address the imminent loss of critical wintertime transportation infrastructure across the Northern Hemisphere. Key Points: The availability of safe lake ice for recreation will decrease, on average, by 13 and 24 days in response to a global warming of 1.5°C and 3°CThe most densely populated regions across the Northern Hemisphere are projected to experience the greatest percent loss of safe lake iceWithout human intervention, the duration of safe ice for winter ice roads is projected to decline by up to 99% within a 3°C warmer world [ABSTRACT FROM AUTHOR]

One of the most striking manifestations of ongoing climate change is a rapid shrinking of the Arctic sea ice area (SIA). An important feature of the observed SIA loss is a nonlinear rate of a decline with an accelerated decrease in the 2000–2019 period relative to a more gradual decline in 1979–1999. In this study, we perform a quantitative assessment and comparison of the spatial-temporal SIA changes during these two periods. It was found that winter Arctic SIA loss is primarily associated with changes in the Barents Sea, where the SIA decline in 2000–2019 has accelerated more than three-fold in comparison with 1979–1999. In summer and autumn, rates of SIA decline in 2000–2019 increased most strongly in the Kara, Beaufort Seas, the Northwestern Passage, and inner Arctic Ocean. The amplitude of the SIA seasonal cycle has also increased in 2000–2019 in comparison with the earlier period, with the largest changes in the inner Arctic Ocean, the Kara, Laptev, East Siberian and Beaufort Seas in summer and in the Barents Sea in winter. The results may reflect a transition to a new dynamic state in the recent two decades with the triggering of positive feedbacks in the Arctic climate system. [ABSTRACT FROM AUTHOR]

Sea ice loss and accelerated warming in the Barents Sea have recently been one of the main concerns of climate research. In this study, we investigated the trends and possible relationships between sea surface temperature (SST), sea ice concentration (SIC), and local and large-scale atmospheric parameters over the last 39 years (1982 to 2020). We examined the interannual and long-term spatiotemporal variability of SST and SIC by performing an empirical orthogonal function (EOF) analysis. The SST warming rate from 1982 through 2020 was 0.35 ± 0.04 °C/decade and 0.40 ± 0.04 °C/decade in the ice-covered and ice-free regions, respectively. This climate warming had a significant impact on sea-ice conditions in the Barents Sea, such as a strong decline in the SIC (−6.52 ± 0.78%/decade) and a shortening of the sea-ice season by about −26.1 ± 7.5 days/decade, resulting in a 3.4-month longer summer ice-free period over the last 39 years. On the interannual and longer-term scales, the Barents Sea has shown strong coherent spatiotemporal variability in both SST and SIC. The temporal evolution of SST and SIC are strongly correlated, whereas the Atlantic Multidecadal Oscillation (AMO) influences the spatiotemporal variability of SST and SIC. The highest spatial variability (i.e., the center of action of the first EOF mode) of SST was observed over the region bounded by the northern and southern polar fronts, which are influenced by both warm Atlantic and cold Arctic waters. The largest SIC variability was found over the northeastern Barents Sea and over the Storbanken and Olga Basin. The second EOF mode revealed a dipole structure with out-of-phase variability between the ice-covered and ice-free regions for the SST and between the Svalbard and Novaya Zemlya regions for SIC. In order to investigate the processes that generate these patterns, a correlation analysis was applied to a set of oceanic (SST) and atmospheric parameters (air temperature, zonal, and meridional wind components) and climate indices. This analysis showed that SST and SIC are highly correlated with air temperature and meridional winds and with two climate indices (AMO and East Atlantic Pattern (EAP)) on an interannual time scale. The North Atlantic Oscillation (NAO) only correlated with the second EOF mode of SST on a decadal time scale. [ABSTRACT FROM AUTHOR]

Climate change impacts the electric power system by affecting both the load and generation. It is paramount to understand this impact in the context of renewable energy as their market share has increased and will continue to grow. This study investigates the impact of climate change on the supply of renewable energy through applying novel metrics of intermittency, power production and storage required by the renewable energy plants as a function of historical climate data variability. Here we focus on and compare two disparate locations, Palma de Mallorca in the Balearic Islands and Cordova, Alaska. The main results of this analysis of wind, solar radiation and precipitation over the 1950–2020 period show that climate change impacts both the total supply available and its variability. Importantly, this impact is found to vary significantly with location. This analysis demonstrates the feasibility of a process to evaluate the local optimal mix of renewables, the changing needs for energy storage as well as the ability to evaluate the impact on grid reliability regarding both penetration of the increasing renewable resources and changes in the variability of the resource. This framework can be used to quantify the impact on both transmission grids and microgrids and can guide possible mitigation paths. [ABSTRACT FROM AUTHOR]

The Atlantic Water inflow to the Arctic Ocean is transformed and modified in the area north of Svalbard, which influences the Arctic Ocean heat and salt budget. Year‐round observations are relatively sparse in this region partially covered by sea ice. We took advantage of one‐year‐long records of ocean currents and hydrography from seven moorings north of Svalbard. The moorings are organized in two arrays separated by 94 km along the path of the Atlantic Water inflow to investigate the properties, transport and heat loss of the Atlantic Water in 2018/2019. The Atlantic Water volume transport varies from 0.5 Sv (1 Sv = 106 m3s−1) in spring to 2 Sv in fall. The first mode of variation of the Atlantic Water inflow temperature is a warm/cold mode with a seasonal cycle. The second mode corresponds to a shorter time scale (6–7 days) variability in the onshore/offshore displacement of the temperature core linked to the mesoscale variability. Heat loss from the Atlantic Water in this region is estimated, for the first time using two mooring arrays and conserving the volume transport. The heat loss varies between 302 W m−2 in winter to 60 W m−2 in spring. The onshore moorings show a westward countercurrent driven by Ekman setup in spring, carrying transformed‐Atlantic Water. The offshore moorings show a bottom‐intensified current that covaries with the wind stress curl. These two mooring arrays allowed for a better comprehension of the structure and transformation of the slope currents north of Svalbard. Plain Language Summary: Atlantic Water enters the Arctic Ocean along two pathways, through the Barents Sea and through Fram Strait west of Svalbard, and is the main heat and salt source for the Arctic Ocean. We use temperature, salinity and current observations from seven lines of oceanic instruments moored to the seafloor and organized in two arrays, deployed in fall 2018 for one year north of Svalbard to analyze the properties and variations of the Atlantic Water inflow and its rate of cooling between the two arrays on its path to the Arctic Ocean. We report a westward flow on the upper slope, carrying diluted Atlantic Water mixed with colder shelf waters. At about 1,000 m depth a deep‐intensified current carries deep waters. Both currents are influenced by the winds. The heat loss of the Atlantic Water while it travels eastward varies between 302 W m−2 in winter to 60 W m−2 in spring. These new observations from two mooring arrays offer new insight on the structure and variations of the slope currents north of Svalbard, as the circulation pathways and the heat transported by the Atlantic Water influence the seasonal variations of the sea ice and the marine ecosystems. Key Points: The heat loss from the Atlantic Water inflow North of Svalbard varies between 302 W m−2 in winter and 60 W m−2 in springA surface countercurrent on the upper slope carrying transformed Atlantic Water is modulated by the Atlantic Water inflow and Ekman setupAn offshore bottom‐intensified current is detected on the lower slope, partly correlated with the wind stress curl [ABSTRACT FROM AUTHOR]

Arctic Ocean waters sourced from the Atlantic contain a vast amount of heat. In the Arctic's Beaufort Gyre, diffusive convection is the primary mechanism by which this heat is transported vertically. This mixing process is characterized by a "staircase" where convective layers are separated by interfaces in temperature and salinity. It is not well‐understood what governs layer thickness, which is an important parameter in heat transport. Here we relate staircase properties to the background water‐mass structure of the Beaufort Gyre via analysis of Ice‐Tethered Profiler observations. We find that staircase layer thicknesses vary with intrusive features below the staircase and the stratification overlying the staircase. We relate these features to the pathway of anomalously warm Atlantic Water in the Beaufort Gyre. Results suggest that intrusive features in context with the Gyre's large‐scale geostrophic flow may be key to understanding layer thicknesses and the propagation of warm waters into the Gyre. Plain Language Summary: Heat from a warm Arctic Ocean water layer, known as the Atlantic Water Layer, is vertically transported by double‐diffusive convection. This process frequently is evidenced by a staircase structure. We relate changes in staircase properties to both the background oceanographic setting as well as to an anomalously warm water pulse which entered the Arctic Ocean in the 1990s–2000s. We show that staircase properties are likely related to the propagation of warm water intrusions and subsequent dissipation of their property gradients. This work sheds light on what the western Arctic Ocean may look like under continued climate change. Key Points: Atlantic Water layer diffusive‐convective staircases in the Beaufort Gyre exhibit a west‐east gradient in layer thicknessLayer thicknesses are correlated with the stratification overlying, and intrusive features surrounding, the Atlantic Water coreThis layer thickness gradient may be due to lateral propagation of a warm Atlantic water pulse that entered the central Arctic in the 2000s [ABSTRACT FROM AUTHOR]

Analytical conditions for estimating the contribution of various internal and external factors to the formation of Arctic (polar) amplification under warming of the Earth's climate system with strongest changes in the surface temperature at high latitudes compared to lower latitudes have been obtained. [ABSTRACT FROM AUTHOR]

Climate change is generating sufficient risk for nation-states and citizens throughout the Arctic to warrant potentially radical geoengineering solutions. Currently, geoengineering solutions such as surface albedo modification or aerosol deployment are in the early stages of testing and development. Due to the scale of deployments necessary to enact change, and their preliminary nature, these methods are likely to result in unforeseen consequences. These consequences may range in severity from local ecosystem impacts to large scale changes in available solar energy. The Arctic is an area that is experiencing rapid change, increased development, and exploratory interest, and proposed solutions have the potential to produce new risks to both natural and human systems. This article examines potential security and ethical considerations of geoengineering solutions in the Arctic from the perspectives of securitization, consequentialism, and risk governance approaches, and argues that proactive and preemptive frameworks at the international level, and especially the application of risk governance approaches, will be needed to prevent or limit negative consequences resulting from geoengineering efforts. Utilizing the unique structures already present in Arctic governance provides novel options for addressing these concerns from both the perspective of inclusive governance and through advancing the understanding of uncertainty analysis and precautionary principles. [ABSTRACT FROM AUTHOR]

The processes that contribute to the Arctic amplification of global surface warming are often described in the context of climate feedbacks. Previous studies have used a traditional feedback analysis framework to partition the regional surface warming into contributions from each feedback process. However, this partitioning can be complicated by interactions in the climate system. Here we focus instead on the physically intuitive approach of inactivating individual feedback processes during forced warming and evaluating the resulting change in the surface temperature field. We investigate this using a moist energy balance model with spatially varying feedbacks that are specified from comprehensive climate model results. We find that when warming is attributed to each feedback process by comparing how the climate would change if the process were not active, the water vapor feedback is the primary reason that the Arctic region warms more than the tropics, and the lapse rate feedback has a neutral effect on Arctic amplification by cooling the Arctic and the tropics by approximately equivalent amounts. These results are strikingly different from previous feedback analyses, which identified the lapse rate feedback as the largest contributor to Arctic amplification, with the water vapor feedback being the main opposing factor by warming the tropics more than the Arctic region. This highlights the importance of comparing different approaches of analyzing how feedbacks contribute to warming in order to build a better understanding of how feedbacks influence climate changes. [ABSTRACT FROM AUTHOR]

Climate change continues to increase the intensity, frequency and impacts of weather and climate extremes. This work uses bias-adjusted Coupled Model Intercomparison Project Phase six (CMIP6) model datasets to investigate the future changes in temperature extremes over Mediterranean (MED) and Sahara (SAH) regions. The mid- (2041–2070) and far-future (2071–2100) are studied under two Shared Socioeconomic Pathways: SSP2-4.5 and SSP5-8.5 scenarios. Quantile mapping function greatly improved the performance of CMIP6 by reducing the notable biases to match the distribution of observation data, the Climate Prediction Center (CPC). Results show persistent significant warming throughout the 21st century, increasing with the increase in radiative forcing. The MED will record a higher increase in temperature extremes as compared to SAH. The warming is supported by the projected reduction in cold days (TX10p) and cold nights (TN10p), with the reduction in the number of cold nights exceeding cold days. Notably, warm spell duration index (WSDI) and summer days (SU) have a positive trend in both timelines over the entire study area. There is a need to simulate how climate sensitive sectors, such as water and agriculture, are likely to be affected by projected changes under different scenarios for informed decision making in the choice and implementation of adaptation and mitigation effective measures. [ABSTRACT FROM AUTHOR]

This study investigates the formation mechanism of the ocean surface warming pattern in response to a doubling CO2 with a focus on the role of ocean heat uptake (or ocean surface heat flux change, ΔQnet). We demonstrate that the transient patterns of surface warming and rainfall change simulated by the dynamic ocean–atmosphere coupled model (DOM) can be reproduced by the equilibrium solutions of the slab ocean–atmosphere coupled model (SOM) simulations when forced with the DOM ΔQnet distribution. The SOM is then used as a diagnostic inverse modeling tool to decompose the CO2-induced thermodynamic warming effect and the ΔQnet (ocean heat uptake)–induced cooling effect. As ΔQnet is largely positive (i.e., downward into the ocean) in the subpolar oceans and weakly negative at the equator, its cooling effect is strongly polar amplified and opposes the CO2 warming, reducing the net warming response especially over Antarctica. For the same reason, the ΔQnet-induced cooling effect contributes significantly to the equatorially enhanced warming in all three ocean basins, while the CO2 warming effect plays a role in the equatorial warming of the eastern Pacific. The spatially varying component of ΔQnet, although globally averaged to zero, can effectively rectify and lead to decreased global mean surface temperature of a comparable magnitude as the global mean ΔQnet effect under transient climate change. Our study highlights the importance of air–sea interaction in the surface warming pattern formation and the key role of ocean heat uptake pattern. [ABSTRACT FROM AUTHOR]

Spin‐up is essential to provide initial conditions for land surface models (LSM) when they cannot be given reliably as in the application to regional permafrost change studies. In this study, the impacts of spin‐up strategy including total spin‐up length and cycling scheme on modeling of permafrost dynamics on the Qinghai‐Tibet Plateau (QTP) were evaluated through two groups of experiments using a modified Noah LSM. The first group aims to test different total spin‐up lengths and the second group for different cycling schemes. The results show that the presence of permafrost prolongs the convergence of the model. Vertically, the slowest convergence is observed at the permafrost table. The insufficiency of total spin‐up length is prone to underestimate permafrost area and overestimate the degradation rate. Different cycling schemes considerably affect the resulting initial thermal fields and result in degradation rates with a difference of 3.37 × 103 km2/a on the QTP, which exceeds the difference (2.92 × 103 km2/a) in the degradation rates reported in existing studies. The multi‐year cycling scheme is generally preferred, but overlong cycle length should be avoided to prevent the introduction of climate change trends in the spin‐up period. We recommend a spin‐up strategy of a 500‐year cycling with the first 5‐ to 10‐year of forcing for modeling permafrost on the QTP with the Noah LSM. Our findings highlight the importance of the spin‐up strategy, which is usually neglected in present LSM‐based permafrost modeling studies. Plain Language Summary: Recently, land surface models (LSM) have been applied to model regional permafrost changes in the context of climate change. In these applications, spin‐up is typically used to provide initial conditions for the model. However, the way we generate the spin‐up forcing strongly affects the resulting initial fields at the end of the spin‐up simulation and the accuracy of the subsequent simulation of permafrost changes. This study aims to investigate these impacts through two groups of experiments using the Qinghai‐Tibet Plateau (QTP) as the study area where the Noah LSM is used to simulate permafrost changes in response to decadal climate changes. It is found that convergence is slower in permafrost regions than non‐permafrost regions. Different spin‐up strategies affect the initial thermal fields to varying degrees. However, they can cause large differences in permafrost area change and degradation rate in follow‐up simulations driven by real climate forcing, exceeding the uncertainties associated with choices of different methods and data in existing studies. In the end, we recommend a strategy of 500‐year cycling with the first 5‐ to 10‐year of available meteorological data for the QTP LSM applications. Key Points: Convergence is slower in permafrost regions and different spin‐up strategies have varying degrees of influence on the initial fieldSpin‐up strategies can lead to a difference in permafrost degradation rate comparable to the differences of the rates in existing studiesA spin‐up strategy of a 500‐year cycling with the first 5‐ to 10‐year of forcing is recommended for permafrost simulation on the Qinghai‐Tibet Plateau [ABSTRACT FROM AUTHOR]

Many areas in the Arctic are vulnerable to the impacts of climate change. We observe large-scale effects on physical, biological, economic and social parameters, including ice cover, species distributions, economic activity and regional governance frameworks. Arctic living marine resources are affected in various ways. A holistic understanding of these effects requires a multidisciplinary enterprise. We synthesize relevant research, from oceanography and ecology, via economics, to political science and international law. We find that multidisciplinary research can enhance our understanding and promote new questions and issues relating to impacts and outcomes of climate change in the Arctic. Such issues include recent insights on changing spawning migrations of the North-east Arctic cod stock that necessitates revisions of socioeconomic estimates of ecosystem wealth in the Barents Sea, better integrated prediction systems that require increased cooperation between experts on climate prediction and ecosystem modelling, and institutional complexities of Arctic governance that require enhanced coordination. [ABSTRACT FROM AUTHOR]

Within Arctic deltas, surficial hydrologic connectivity of lakes to nearby river channels influences physical processes like sediment transport and ice phenology as well as biogeochemical processes such as photochemistry. As the Arctic hydrologic cycle is impacted by climate change, it is important to quantify temporal variability in connectivity. However, current connectivity detection methods are either spatially limited due to data availability constraints or have been applied at only a single time. Additionally, the relationship between connectivity and lake ice is still poorly quantified. In this study, we present a multitemporal classification of functional lake connectivity in the Colville River Delta, Alaska. We introduce a connectivity detection algorithm based on remote sensing of high sediment river water recharge that is expandable to other high sediment Arctic deltas. We compare results to three existing data sets, producing 64.4%, 75.8%, and 85.2% lake classification accuracy. Mismatches between the data sets are often due to inconsistencies in methodology or definition of connectivity. Connectivity varies temporally in about 10% of studied lakes and correlates strongly with discharge and lake elevation, supporting the idea that future changes in discharge will be a driver of changes in connectivity. Highly connected lakes start and end ice break up an average of 26 and 17 days earlier, respectively, compared to lakes that are poorly connected. Because spring and summer ice conditions drive Arctic lake photochemistry processes, our research suggests that surface connectivity is an important parameter to consider when studying biogeochemistry of Arctic delta lakes. Plain Language Summary: Arctic deltas contain a complex tapestry of channels and lakes. Lakes can be isolated from channels or connected to channels—either by feeder channels, other lakes, or by water movement over land during flooding. This connectivity is important because it impacts the movement of sediment, light penetration through water, and when lake ice forms and disappears. While we anticipate future changes in connectivity due to climate change, our ability to monitor connectivity is limited to individual deltas or single instances in time. Additionally, the relationship between lake ice and connectivity is poorly understood. In this study, we used a method based on satellite observations of sediment in water to examine lake connectivity variations within the Colville River Delta, Alaska. We found that most lakes stay connected or disconnected, but that about 10% of lakes have variable connectivity. These lakes are more likely to be connected when river discharge is high. Additionally, lake ice tends to break up earlier and last for a shorter period of time in connected lakes. Because lake ice conditions drive chemical interaction between sunlight and organic compounds in the water, connectivity is important to consider when studying the biology and chemistry of Arctic delta lakes. Key Points: We have developed an algorithm using optical properties of lakes and channels to detect lake‐to‐channel connectivity in an Arctic deltaWe show that this algorithm can be used to detect lakes where functional connectivity varies based on maximum summer dischargeDifferences in functional connectivity drive lake ice phenology, which is likely important for photochemical processes [ABSTRACT FROM AUTHOR]

Arctic precipitation (PG) that occurs as rainfall (Prain) or snowfall (Psnow) depending on the prevailing climatic conditions results in seasonally specific hydrological events. Climate change can affect the PG‐ and permafrost‐originated water (Pice) regimes, resulting in change to ecohydrological processes. However, the relative influences of source waters (i.e., Prain, Psnow, and Pice) on terrestrial hydrological processes have not yet been fully established. Here, we report the development and implementation of a numerical water tracer model designed to quantify changes in the storages and fluxes of the source waters and the hydrogen and oxygen isotopic tracers associated with hydrometeorological events. The presented tracer model was used to illustrate the spatiotemporal variability of the tracers in the surface–subsurface system of a deciduous needleleaf boreal forest and to separate the contribution rates of the tracer waters to evapotranspiration (ET). Although Psnow accounted for 22%–57% of ET and the subcomponents, the contribution rates to soil evaporation and transpiration were significant only during spring. The major source water for soil moisture was Prain, which accounted for 69.2% of ET and showed an increasing trend during 1980–2016. Additionally, Prain also accounted for 77.2% of transpiration. Under the present conditions of warming permafrost, Pice demonstrated negligibly low impact on ET. The tracer model was shown capable of quantifying the contribution rates of tracer waters to ET, highlighting the advantages of the tracer model for a similar quantitative separation regarding future climate change. Plain Language Summary: Although snowfall and rainfall are seasonally different processes, their influence on hydrological processes (i.e., evapotranspiration and river discharge) occurs mainly during the growing season. Quantification of their contributions to hydrological processes is an ongoing scientific problem in the field of Arctic hydrology. Thus, we developed a tracer model, which includes isotopic processes, to separate the contributions of snowfall, rainfall, and permafrost‐originated water to evapotranspiration and applied it to a boreal forest. The model simulation suggested that summer rainfall has the greatest impact on evapotranspiration, while water from snowfall is strongly implicated in springtime evaporation. Permafrost‐originated water is not connected directly to evapotranspiration at the study site. However, future warming climate could potentially increase the role of permafrost‐related water in ecohydrological processes. Thus, the presented water tracer model could potentially provide a quantitative assessment of the changes in the physics of water dynamics caused by climate change. Key Points: Tracer model separates contributions of precipitation‐ and permafrost‐originated waters to evapotranspiration in deciduous boreal forestRainfall is the primary contributor to seasonal and interannual variability of evapotranspirationPermafrost‐originated water has negligibly low impact on evapotranspiration [ABSTRACT FROM AUTHOR]

Snowpack and snowmelt‐driven extreme events (e.g., floods) have large societal consequences including infrastructure failures. However, it is not well understood how projected changes in the snow‐related extremes differ across North America. Using dynamically downscaled regional climate model (RCM) simulations, we found that the magnitudes of extreme snow water equivalent, snowmelt, and runoff potential (RP; snowmelt plus precipitation) decrease by 72%, 73%, and 45%, respectively, over the continental United States and southern Canada but increase by up to 8%, 53%, and 41% in Alaska and northern Canada by the late 21st century. In California and the Pacific Northwest, there is a notable increase in extreme RP by 21% contrary to a decrease in snowmelt by 31% by the late century. These regions could be vulnerable to larger rain‐on‐snow floods in a warmer climate. Regions with a large variability among RCM ensembles are identified, which require further investigation to reduce the regional uncertainties. Plain Language Summary: Even though snow‐driven extreme events (e.g., snowmelt floods) have large societal impacts including infrastructure failures, how much future changes in the magnitude of snow‐driven extremes differ across North America is not well understood. Here, we found that the magnitudes of future extreme snow water equivalent (SWE) and snowmelt decrease over the continental United States and southern Canada but increase in Alaska and northern Canada by the late 21st century. In California and the Pacific Northwest, there is a notable increase in runoff potential (snowmelt plus precipitation) contrary to a decrease in snowmelt itself, suggesting that these regions may be vulnerable to larger rain‐on‐snow events in a warmer climate. Also, regions with a large variability among this study's regional climate models are identified. The large variabilities in extreme SWE and snowmelt in the western mountain regions and northern Canada as well as runoff potential in the southeastern United States require further investigation to reduce the regional uncertainties. Key Points: Extreme snow water equivalent (SWE), snowmelt, and runoff potential maps were developed using a regional climate model ensemble for historical and future periodsThe magnitudes of projected extreme SWE and snowmelt decrease over the CONUS and southern Canada but increase in Alaska and northern CanadaThere is a projected decrease in snowmelt but an increase in runoff potential (e.g., rain‐on‐snow) in California and the Pacific Northwest [ABSTRACT FROM AUTHOR]

The mobilization and land‐to‐ocean transfer of dissolved organic carbon (DOC) in Arctic watersheds is intricately linked with the region's climate and water cycle, and furthermore at risk of changes from climate warming and associated impacts. This study quantifies model‐simulated estimates of runoff, surface and active layer leachate DOC concentrations and loadings to western Arctic rivers, specifically for basins that drain into coastal waters between and including the Yukon and Mackenzie Rivers. Model validation leverages data from other field measurements, synthesis studies, and modeling efforts. The simulations effectively quantify DOC leaching in surface and subsurface runoff and broadly capture the seasonal cycle in DOC concentration and mass loadings reported from other studies that use river‐based measurements. A marked east‐west gradient in simulated spring and summer DOC concentrations of 24 drainage basins on the North Slope of Alaska is captured by the modeling, consistent with independent data derived from river sampling. Simulated loadings for the Mackenzie and Yukon show reasonable agreement with estimates of DOC export for annual totals and four of the six seasonal comparisons. Nearly equivalent loading occurs to rivers which drain north to the Beaufort Sea and west to the Bering and Chukchi Seas. The modeling framework provides a basis for understanding carbon export to coastal waters and for assessing impacts of hydrological cycle intensification and permafrost thaw with ongoing warming in the Arctic. Plain Language Summary: Arctic rivers transfer a relatively large amount of freshwater to the Arctic Ocean compared to other oceans. The rivers contain organic carbon dissolved in the water, with the bulk arriving during the high flow in spring that follows snowmelt. There is evidence that climate warming is thawing permafrost and resulting in more carbon traveling through rivers to the Arctic Ocean, so it is important to understand how much enters river networks from soils. We used a computer model designed to capture the seasonal thawing and freezing of Arctic soils and seasonal snowpack accumulation to estimate how much dissolved organic carbon is loaded to rivers in the western Arctic over the period 1981 to 2010. For northern Alaska rivers the simulation shows a gradient in dissolved organic carbon concentration from the east side to the west, similar to the pattern in independent data derived from river measurements. Our new estimates suggest that roughly equivalent amounts of dissolved organic carbon are loaded to rivers which empty north to the Beaufort Sea, and west to the Bering and Chukchi Seas. The modeling allows us to better understand how climate change is impacting flows of water and carbon to the Arctic Ocean. Key Points: Modeling quantifies daily DOC loadings in surface and subsurface runoff into river networks across the western ArcticSimulation captures a gradient in seasonal DOC concentrations for North Slope Alaska rivers consistent with field measurementsSimilar loadings are estimated for basins draining to the north and to the west [ABSTRACT FROM AUTHOR]

Arctic Amplification is robustly seen in climate model simulations of future warming and in the paleoclimate record. Here, we focus on the past century of observations. We show that Arctic Amplification is only a recent phenomenon, and that for much of this period the Arctic cooled while the global‐mean temperature rose. To investigate why this occurred, we analyze large ensembles of comprehensive climate model simulations under different forcing scenarios. Our results suggest that the global warming from greenhouse gases was largely offset in the Arctic by regional cooling due to aerosols, with internal climate variability also contributing to Arctic cooling and global warming trends during this period. This suggests that the disruption of Arctic Amplification was due to a combination of factors unique to the 20th century, and that enhanced Arctic warming should be expected to be a consistent feature of climate change over the coming century. Plain Language Summary: Arctic Amplification is the phenomenon by which the Arctic warms at a faster rate than the global average. Evidence for the occurrence of Arctic Amplification is widely found in climate model simulations as well as in paleo proxy reconstructions of past climate changes. In this study, we investigate the extent to which Arctic Amplification has occurred in observations from the past century. We show that Arctic Amplification is only a recent phenomenon, and that for much of the 20th century, the Arctic cooled while the global‐mean temperature rose. We investigate why this happened using a range of climate model simulations, and we find that there were two main causes for these opposing trends. The first is that regional cooling from aerosols counteracted the warming from greenhouse gases in the Arctic. However, this cannot fully explain the observed trends. The second is that natural fluctuations of the climate system manifested in a pattern of Arctic cooling under global warming. This suggests that the disruption of Arctic Amplification was due to a combination of factors unique to the 20th century, implying that enhanced Arctic warming should be expected to be a consistent feature of climate change over the coming century. Key Points: For much of the past century the Arctic cooled while the globe warmed, with Arctic Amplification of warming only emerging more recentlyWithout increases in both aerosols and greenhouse gas emissions, Arctic Amplification would have occurred throughout the past centuryInternal variability also played an important role in setting the observed Arctic cooling and global warming trends [ABSTRACT FROM AUTHOR]

Extreme heat occurred over Alaska in June–July 2019, posing risks to infrastructure, ecosystem, and human health. It is vital to improve our understanding of the causes of such events and the extent to which anthropogenic forcing may alter their likelihood and magnitude. Here, we use multiple large ensembles of climate models, comprising thousands of simulated years, to investigate these issues. Our results suggest that the presence of anthropogenic radiative forcing increased the likelihood of the 2019 extreme heat event by as much as 6%. Further we show the rate of occurrence of such an extreme heat event is likely to substantially increase in the future with increasing levels of atmospheric greenhouse gases. While uncertainty in projected climate risk from model choice leads to a broad range of future extreme heat event probabilities, some models project that with rapidly increasing levels of greenhouse gases the likelihood of such events would exceed 75% by 2090. Plain Language Summary: An extended period of high temperatures across the state of Alaska in June and July, 2019 set multiple temperature records. Here, we examine the extent to which human‐driven climate change played a role in increasing the likelihood of experiencing such an extreme event. Using global climate models, we determine that human‐driven climate change increased the probability of experiencing such high temperatures in 2019, and that the likelihood of similar or more extreme events will increase into the coming century. Key Points: The presence of anthropogenic radiative forcing increased the likelihood of the 2019 extreme heat event by as much as 6%The rate of occurrence of such an extreme heat event is likely to substantially increase in the coming decadesBy the 2060s simulations suggest that such an extreme heat event could occur almost every other year, moving from a rare to a common event [ABSTRACT FROM AUTHOR]

The recent summer surface air temperature (SAT) changes over densely populated Eurasia exhibit a non‐uniform pattern with amplified warming over Europe and East Asia (EA) but weak warming over Central Asia (CA), forming a wave train‐like structure. However, the key factors that determine this non‐uniform warming pattern remain unclear. By analyzing observations and model simulations, here, we show that more than half of the SAT multidecadal variations from 1950 to 2014 over Europe‐west Asia and EA may have resulted from external forcing, rather than from internal variability in the Atlantic as previously thought. In contrast, the recent SAT over CA is influenced mainly by internal variations in the Atlantic and Pacific oceans. Large ensemble model simulations suggest that the forced SAT multidecadal variations over Eurasia are mainly caused by changes in greenhouse gases and aerosols. Our findings provide strong evidence for major impacts of external forcing on multidecadal climate variations over Eurasia. Plain Language Summary: Previous studies have documented non‐uniform decadal warming rates in summer since the mid‐1990s over densely populated Eurasia, with amplified warming over Europe and East Asia (EA) but weak warming over Central Asia. This unique tripole warming pattern and its cause have attracted considerable attention. It may arise from internal variability (IV; e.g., Atlantic Multidecadal Oscillation [AMO]) and/or external forcing. Furthermore, it was found that the recent AMO resulted from both IV and decadal changes in aerosols. Thus, the relative contributions of IV and external forcing to the recent Eurasian warming pattern remain unclear. Here we show that the surface air temperature multidecadal variations since 1950 over Eurasia are caused by both IV and external forcing, with more than half of the variations over Europe‐west Asia and EA resulted from external forcing (i.e., greenhouse gases [GHGs] and aerosols), in contrast to the previous notion that the non‐uniform warming is mainly originated from the Atlantic Ocean. Looking ahead into the upcoming decades, aerosols will continue to decrease, whereas the AMO will likely be in its positive phase. These changes, together with continued increases in GHGs, will accelerate warming trends over Eurasia and further exacerbate summer heat waves. Key Points: More than half of the multidecadal surface air temperature (SAT) variations from 1950 to 2014 over Europe‐west Asia and East Asia resulted from external forcingThe forced SAT multidecadal variations over Eurasia are mainly caused by changes in greenhouse gases and aerosolsOur study helps reconcile human and natural drivers of the tripole pattern of multidecadal summer temperature variations over Eurasia [ABSTRACT FROM AUTHOR]