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The cloud feedback may result in amplification or damping of Arctic warming. Two common techniques used to diagnose the top‐of‐the‐atmosphere cloud feedback are the Adjusted Cloud Radiative Effect (AdjCRE) method and the Cloud Radiative Kernel (CRK) method. We apply both to CMIP5 and CMIP6 model data, finding that the AdjCRE calculated Arctic shortwave cloud feedback is twice as correlated with sea ice loss in CMIP5, and four times in CMIP6, as the CRK method. We find that the CRK method produces Arctic all‐sky residual percentages exceeding 20% in 15 of 18 models. We use the CRK method to decompose the feedback in CMIP5 and CMIP6 finding that its median value changed from negative to positive driven by a less‐negative cloud optical depth feedback. Despite its lack of closure, we conclude that the CRK method is better suited for Arctic SW feedbacks as it is less impacted by surface albedo changes. Plain Language Summary: The cloud feedback is the process by which cloud property changes in a warming climate can either further enhance warming or damp it. The Arctic is warming faster than the rest of the globe, and one of the largest sources of uncertainty in its climate projections is the cloud feedback. There are two popular methods to calculate the cloud feedback: the Adjusted Cloud Radiative Effect technique, and the Cloud Radiative Kernel technique. In this paper we compare the two methods in a suite of climate models by considering the extent to which changes in Arctic sea ice impact the cloud feedbacks. From this analysis we conclude that the Cloud Radiative Kernel method is less affected by sea ice loss. We then apply the Cloud Radiative Kernel technique to data from the two most recent generations of global climate models to investigate how polar day Arctic cloud feedbacks have changed between these generations. We find that the median value of these Arctic feedbacks is slightly positive in the newest generation of models, a change from slightly negative in the previous generation that is largely fueled by a weakening of the feedback associated with changes in cloud optical depth. Key Points: The Cloud Radiative Kernel method is less sensitive to surface albedo changes than the Adjusted Cloud Radiative Effect techniqueThe Cloud Radiative Kernel method provides poor radiative closure in a suite of global climate modelsThe median shortwave Arctic cloud feedback in recent climate models is slightly positive due to a weakened cloud optical depth feedback [ABSTRACT FROM AUTHOR]

Since the early 1990s the Pacific Walker circulation shows a multi‐decadal strengthening, which contradicts future model projections. Whether this trend, evident in many climate indices especially before the 2015 El Niño, reflects the coupled ocean‐atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature trends during 1980–2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere‐Indo‐West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat‐like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central‐eastern equatorial Pacific but warming in the western Pacific and tropical Indian Ocean. Together with the PDO, this pattern drives the Walker circulation strengthening in the equatorial band. Historical simulations appear to underestimate this pattern, contributing to the models' inability to replicate the Walker cell strengthening. Plain Language Summary: This paper investigates the observed changes in the tropical Pacific during the satellite era, including the recent decadal strengthening of the atmospheric zonal circulation—the Walker cell. We aim to understand the extent to which these changes represent a forced response to rising CO2 concentrations versus natural variability. We apply an approach in which we decompose the observed sea surface temperature trends into three components—a pattern associated with the Pacific Decadal Oscillation, which is part of natural variability, a uniform warming pattern, and a residual pattern. This residual pattern shows a remarkable resemblance to a forced ocean thermostat‐like transient response generated in some of the climate models, characterized by equatorial Pacific (EP) cooling, and a broad warming of the Northern Hemisphere, and the Indian Ocean and West Pacific. These results challenge studies arguing that the recent strengthening of the Pacific Walker cell can be explained simply by multi‐decadal natural variability in the tropics. Furthermore, the inability of climate models at large to fully capture this forced pattern with historical forcing puts into focus the reliability of future projections of climate change in the tropical Pacific, specifically the timing of emergence of the eastern EP warming. Key Points: A multi‐decadal strengthening of the Pacific Walker cell is observed in a wide range of indices, especially after 1990A Northern Hemisphere ‐ Indo West Pacific warming sea surface temperature pattern, which differs from the Pacific Decadal Oscillation, is evident since 1980This pattern resembles a forced response to abrupt CO2 forcing, emerging in a subset of climate models, and contributes to the Walker circulation strenthening [ABSTRACT FROM AUTHOR]

Probabilistic projections from the UK Climate Projections 2018 are presented for four global warming levels (GWLs) at 1.5, 2, 3, and 4°C above the 1850–1900 baseline. Our results show how uncertainties associated with climate models and four representative concentration pathways (RCP) emission scenarios translate to UK regional scale changes in maximum temperature and precipitation, with data also available for minimum and mean temperatures, humidity and surface net downward shortwave radiation flux. We compare weighting the likelihood of RCPs based on (hypothetical) policy decisions, against our baseline assumption that each RCP is equally likely. Differences between weighted and unweighted GWL distributions are small, particularly in relation to the full breadth of uncertainties that are incorporated into the probabilistic projections. Finally we quantify the relative importance of scenario, model and internal variability on regional projected GWLs and show that uncertainty associated with an uncertain climate response to forcings dominates at all GWLs. Plain Language Summary: Global warming levels define the global averaged increase in temperature since the pre‐industrial period. This study looks at what the regional climate in the UK could be like in the future. We estimate how temperature and precipitation in the UK might change under four different global warming levels (GWLs) (1.5, 2, 3, and 4°C above pre‐industrial times). We account for a wide range of outcomes from both climate models and future emissions scenarios. Assuming that the range of future emissions scenarios are equally likely, the center of the ranges of years that we could reach GWLs of 1.5, 2, 3, and 4°C are 2032, 2050, 2072, and 2078 respectively. The largest influence of future climate on these range estimates comes from our understanding of how the Earth actually responds to changes in greenhouse gases. Even if we knew the exact quantity of greenhouse gas emissions we would emit in the future, we still wouldn't be able to predict the exact changes in UK climate with perfect accuracy because our understanding of the regional climate response to these emissions requires further investigation. Key Points: Probabilistic climate projections from the UK are presented for four global warming levels (GWLs) across four representative concentration pathways (RCP)We show how uncertainty associated with climate models and future emission scenarios translate to UK regional scale changesUncertainty associated with an uncertain climate response to forcings dominates at all GWLs [ABSTRACT FROM AUTHOR]

Surface, aircraft, and satellite measurements indicate pervasive early cold season (Augut–September) CO2 emissions across Arctic regions, consistent with increased ecosystem metabolism in plants and soils. A key remaining question is whether cold season sources will become large enough to permanently shift the Arctic into a net carbon source. Polar orbiting GHG satellites provide robust estimation of regional carbon budgets but lack sufficient spatial coverage and repeat frequency to track sink‐to‐source transitions in the early cold season. Mission concepts such as the Arctic Observing Mission (AOM) advocate for flying imaging spectrometers in highly elliptical orbits (HEO) over the Arctic to address sampling limitations. We perform retrieval and flux inversion simulation experiments using the AURORA mission concept, leveraging a Panchromatic imaging Fourier Transform Spectrometer (PanFTS) in HEO. Our simulations demonstrate the potential benefits of increased CO2 sampling for detecting emissions during the early cold season. Plain Language Summary: The Arctic is warming rapidly, leading to an acceleration of carbon exchange across tundra and boreal ecosystems. These changing flows of carbon can be difficult to detect using traditional observing systems. We propose that measurement systems which observe Arctic ecosystems continuously from a quasi‐GEO orbit can more accurately detect these important changes, such that unexpected and dangerous feedbacks to climate warming can be monitored and accounted for. Key Points: The Arctic Fourier Transform Spectrometer Investigation (AURORA) mission concept addresses GHG sampling limitations in the ArcticAURORA uses a highly elliptical orbit (HEO) to increase repeat frequency, and a panchromatic iFTS for SWIR O2 and CO2 bands (0.7–2.5 μm)Simulation experiments show potential benefits of increased sampling frequency for grid scale detection of early cold season CO2 emissions [ABSTRACT FROM AUTHOR]

Drought has affected much of the western United States since about the year 2000, including the Upper Colorado River Basin (UCRB). Using a time series of UCRB streamflow derived from a tree‐ring based reconstruction of UCRB streamflow for the years 1 CE through 1905 CE, together with naturalized UCRB streamflow values for 1906 CE through 2021 CE, we identify 51 drought events, including the 2000–2021 drought. Although the recent 2000–2021 drought has been relatively severe, it is not the most severe of the UCRB drought events we identified. Results also indicate that natural variability combined with projected climate warming can result in UCRB drought events that are more severe than any drought since 1 CE. Plain Language Summary: A long and severe drought has affected much of the western United States since about the year 2000 CE, including the Upper Colorado River Basin (UCRB). Comparing this drought to past UCRB droughts (during 1 CE through 2021 CE), we find that the 2000–2021 drought is not the most severe UCRB drought. The results also suggest that natural variability combined with projected climate warming could result in UCRB drought events that are more severe than any drought since 1 CE. Key Points: We identify 51 drought events for the years 1 CE through 2021 CE for the Upper Colorado River Basin (UCRB)Twelve of the 51 UCRB droughts we identify were more severe than the 2000–2021 droughtNatural variability and/or a 1°C temperature increase can trigger UCRB droughts that are more severe than the 2000–2021 drought [ABSTRACT FROM AUTHOR]

Preparing for climate change requires an understanding of the degree to which global warming has regional implications. Here we document a strong relationship between the magnitude and extent of warming and explain its origin using a simple model based on binomial statistics. Applied to HadCRUT5 instrumental observations, the model shows that 96% of interannual variability in the proportion of regions experiencing anomalous warmth over the last century can be explained on the basis of the magnitude of global mean surface temperature (GMST) anomalies. The model performs similarly well when applied to a variety of unforced and forced model simulations and represents a general thermodynamic link between global and local warming on annual timescales. Our model predicts that, independent of the baseline that is chosen, 95% of the globe is expected to experience above‐average annual temperatures at 0.7°C of GMST warming, and 99% at 1.0°C of warming. Plain Language Summary: Whereas many studies focus on global mean surface temperature (GMST), it is also critical to understand the spatial patterns of this warming in preparing for a warmer world. Using a simple statistical model, we describe a strong relationship between GMST and the spatial extent of above‐average temperature anomalies each year. This relationship is shown to hold across various model simulations and historical temperature data, capturing both annual and decadal trends. We can use our model alongside projections to gain insight into the extent of warming variations. For instance, 1°C GMST warming from a given baseline climate generally corresponds to a world in which 99% of places across the globe will be anomalously warm every year. Key Points: A simple statistical model relates the proportion of Earth's area that is anomalously warm to global mean surface temperature (GMST)The model predicts 96% of the interannual variance in the proportion of anomalous warmth between 1900 and 2021 on the basis of GMST1.0°C of GMST warming corresponds to 99% of the globe being anomalously warm, independent of baseline [ABSTRACT FROM AUTHOR]

This study presents a methodological advancement in the field of clumped‐isotope (∆47) thermometry, specifically tailored for application to freshwater ostracods. The novel ostracod clumped isotope approach enables quantitative temperature and hydrological reconstruction in lacustrine records. The relationship between ∆47 and the temperature at which ostracod shell mineralized is determined by measuring ∆47 on different species grown under controlled temperatures, ranging from 4 ± 0.8 to 23 ± 0.5ºC. The excellent agreement between the presented ∆47 ostracod data and the monitored temperatures confirms that ∆47 can be applied to ostracod shells and that a vital effect is absent outside the uncertainty of measurements. Results are consistent with the carbonate clumped‐isotope unified calibration (Anderson et al., 2021, https://doi.org/10.1029/2020gl092069), therefore, an ostracod‐specific calibration is not needed. The ostracod clumped‐isotope thermometer represents a powerful tool for terrestrial paleoclimate studies all around the world, as lakes and ostracods are found in all climatic belts. Plain Language Summary: In the framework of global warming, the reconstruction of past climatic conditions is important to understand the future evolution of climate and its impact. Lake sediments can be used as archives to quantify these effects. This study presents a novel paleo‐thermometer based on the application of clumped‐isotope technique (i.e., measurement of the number of 13C–18O bonds in carbonate minerals that depends on the temperature of carbonate precipitation) on carbonatic microcrustacea, named ostracods that commonly live in lakes. By using ostracods that formed their shells at known temperatures, we demonstrate that they can be easily used to reconstruct water temperature and hydrological conditions (precipitation/evaporation). The ostracod clumped‐isotope thermometer represents a powerful tool for terrestrial paleoclimate studies around the world, as lakes and ostracods are located in all climatic belts. Key Points: ∆47—ostracod signal accurately records the shell calcification temperature∆47—ostracod signal is not affected by the so called "vital effect"The unified calibration of Anderson et al. (2021) can be used to convert the ∆47—ostracod signal into accurate temperatures [ABSTRACT FROM AUTHOR]

Accurately representing the ocean carbon cycle in Earth System Models (ESMs) is essential to understanding the oceanic CO2 sink evolution under CO2 emissions and global warming. A key uncertainty arises from the ESM's inability to explicitly represent mesoscale eddies. To address this limitation, we conduct eddy‐resolving experiments of CO2 uptake under global warming in an idealized mid‐latitude ocean model. In comparison with similar experiments at coarser resolution, we show that the CO2 sink is 34% larger in the eddy‐resolving experiments. 80% of the increase stems from a more efficient anthropogenic CO2 uptake due to a stronger Meridional Overturning circulation (MOC). The remainder results from a weaker reduction in CO2 uptake associated to a weaker MOC decline under global warming. Although being only a fraction of the overall response to climate change, these results emphasize the importance of an accurate representation of small‐scale ocean processes to better constrain the CO2 sink. Plain Language Summary: Today, the ocean absorbs ∼25% of the CO2 emissions caused by human activities. This CO2 sink is primarily driven by the increase of CO2 in the atmosphere, but it is also influenced by physical changes in the ocean's properties. Earth System Models are used to project the future of the ocean CO2 sink. Due to limited computational capacity, ESMs need to parameterize flows occurring at scales smaller than ∼100 km, their typical horizontal grid resolution. To overcome the computational limitations, we use an ocean biogeochemical model representing an idealized North Atlantic ocean of reduced dimensions. We conduct simulations of global warming using increasingly finer horizontal resolutions (from ∼100 km to ∼4 km). Our findings demonstrate that the ocean CO2 uptake is highly influenced by resolution. This sensitivity primarily stems from how the overturning circulation's mean state depends on resolution, as well as how it responds to global warming. Although our results capture only a fraction of the overall oceanic response to climate change, they emphasize the significance of accurately representing the role of small‐scale ocean processes to better constrain the future evolution of ocean carbon uptake. Key Points: We conducted idealized ocean simulations under global warming and rising atmospheric CO2 at coarse and eddy‐resolving resolutionsCO2 sink is larger by 34% at eddy resolution, due to larger anthropogenic CO2 uptake combined with weaker climate feedbackThis ensues from the model's overturning circulation sensitivity to resolution in both historical and future state [ABSTRACT FROM AUTHOR]

The influence of climate feedbacks on regional hydrological changes under warming is poorly understood. Here, a moist energy balance model (MEBM) with a Hadley Cell parameterization is used to isolate the influence of climate feedbacks on changes in zonal‐mean precipitation‐minus‐evaporation (P − E) under greenhouse‐gas forcing. It is shown that cloud feedbacks act to narrow bands of tropical P − E and increase P − E in the deep tropics. The surface‐albedo feedback shifts the location of maximum tropical P − E and increases P − E in the polar regions. The intermodel spread in the P − E changes associated with feedbacks arises mainly from cloud feedbacks, with the lapse‐rate and surface‐albedo feedbacks playing important roles in the polar regions. The P − E change associated with cloud feedback locking in the MEBM is similar to that of a climate model with inactive cloud feedbacks. This work highlights the unique role that climate feedbacks play in causing deviations from the "wet‐gets‐wetter, dry‐gets‐drier" paradigm. Plain Language Summary: Climate feedbacks, which act to amplify or dampen global warming, play an important role in shaping how the climate system responds to changes in greenhouse‐gas concentrations. Here, we use an idealized climate model, which makes a simplified assumption about how energy is transported in the atmosphere, to examine how climate feedbacks influence the patterns of precipitation and evaporation change under global warming. We find that the cloud feedback acts to narrow the band of rainfall on the equator known as the Intertropical Convergence Zone and that the surface‐albedo feedback acts to shift the location of maximum rainfall. We also find that the cloud feedback accounts for most of the uncertainty associated with feedbacks in regional hydrological change under warming. The idealized model with a locked cloud feedback also simulates a change in precipitation and evaporation that is similar to a comprehensive climate model with an inactive cloud feedback. Key Points: A moist energy balance model (MEBM) is used to investigate the influence of climate feedbacks on regional hydrological changes under warmingCloud feedbacks act to narrow and increase tropical P − E and are the dominant source of feedback uncertainty in regional hydrological changesThe MEBM with a locked cloud feedback largely replicates a climate model with an inactive cloud feedback [ABSTRACT FROM AUTHOR]

Future climate projections suggest a poleward shift of the maximum intensity of tropical cyclones (TCs) over the western North Pacific. However, the global nature of the latitudinal change in TC genesis under global warming remains poorly understood. We show, using large‐ensemble high‐resolution atmospheric model simulations (d4PDF) with four warming scenarios, that the poleward shift is a robust change over the globe, attributable to the weakening of the Hadley circulation. The weakened ascent driven by the upper‐tropospheric warming suppresses the TC genesis within 5°–20° latitudes, whereas the weakened descent enhances the TC genesis in the poleward latitudes. We further estimate the poleward shift of TC genesis to emerge at the 2 K global warming over the Arabian Sea, South Atlantic and Pacific Oceans and at the 4 K warming over the North Pacific. The present results underscore the potential for increasing social and economic risks associated with TCs at higher latitudes. Plain Language Summary: Climate models have projected a decrease in TC genesis frequency in future warming. However, the global nature of the latitudinal change in TC genesis under global warming remains uncertain partly due to insufficient resolution as well as the ensemble size of climate model simulations. We show a global feature of the robust poleward shift of the TC genesis during the active seasons of both hemispheres scaled with the global warming level, which can be attributed to the weakening of the Hadley circulation. The weakened ascending branch of the Hadley circulation, driven by the increased upper tropospheric warming, potentially hinders TC genesis within 5°–20° latitudes. Conversely, the weakened descending branch of the Hadley circulation enhances the likelihood of TC genesis within 20°–35° latitudes. We further estimate that the signal of TC genesis is expected to emerge over high latitudes of the Arabian Sea, South Atlantic and South Pacific Oceans at the 2 K warming and at the 4 K warming over the North Pacific. The present analyses have significant implications not only for assessing the reliability of future TC‐related changes in climate models but also for estimating the increased TC‐related hazards at higher latitudes under global warming. Key Points: We project a global feature of the robust poleward shift of tropical cyclone (TC) genesis during active seasons of both hemispheresMore TC genesis at high latitudes can be attributed to the weakening of the Hadley circulationPoleward shift of TC genesis emerges at 2 K warming over Arabian Sea, South Atlantic and Pacific Oceans and at 4 K warming over North Pacific [ABSTRACT FROM AUTHOR]

Based on outputs of 28 coupled models from the Phase 6 of the Coupled Model Intercomparison Project (CMIP6), we show that the response of the Southeast Asian summer monsoon to the El Niño‐Southern Oscillation (ENSO) during post‐ENSO summer will likely strengthen in a warmer climate, which can be attributed to concurrently weakened sea‐surface temperature anomalies (SSTAs) in the western equatorial Pacific (WEP). The weakened WEP SSTAs are primarily caused by enhanced latent heat damping due to increased surface wind speed anomalies, which are associated with the eastward shift of the El Niño‐induced anomalous Walker circulation due to El Niño‐like sea surface temperature change in the tropical Pacific under global warming. Besides, the climatological zonal ocean currents will slow down due to the weakening of climatological Walker circulation, which also acts to weaken the WEP SSTAs via reducing the advection of anomalous temperature by the mean current. Plain Language Summary: As an important component of the Asian monsoon system, the Southeast Asian summer monsoon (SEASM) is crucial for the livelihoods of billions of people in East Asia, and is closely connected to a climate phenomenon called El Niño‐Southern Oscillation (ENSO). Understanding how the relationship between SEASM and ENSO will change in the future is important for enhancing our knowledge of climate change in East Asia. Outputs of 28 climate models from the Phase 6 of the Coupled Model Intercomparison Project show that ENSO will exert enhanced impacts on the SEASM in a warmer climate. The enhanced influences of ENSO on the monsoon will exacerbate the reduction of rainfall over the western North Pacific during the post‐El Niño summer. We find that such projected changes are mainly caused by weakened warm sea surface temperature (SST) anomalies (SSTAs) in the western equatorial Pacific (WEP). Further analyses indicate that the change in WEP SSTAs can be linked to the El Niño‐like change in climatological SSTs in the tropical Pacific. This study depicts detailed physical processes responsible for the projected changes in ENSO's impacts on the SEASM. Key Points: The effects of the El Niño‐Southern Oscillation on the Southeast Asian summer monsoon will strengthen under global warmingThe enhanced El Niño's impacts result from the weakened warm sea‐surface temperature (SST) anomalies in the western equatorial Pacific (WEP)The weakened WEP SST anomalies are related to the eastward shift of anomalous Walker circulation and the slackened mean zonal ocean currents [ABSTRACT FROM AUTHOR]

The impact of rising atmospheric CO2 (pCO2atm) from anthropogenic activities on pH, dissolved inorganic carbon, carbonate mineral saturation, and aluminum (Al) speciation is evaluated for 18 northeastern USA lakes using polythermal, sliding activity reaction path models. pCO2atm was forced using two scenarios from the IPCC's Sixth Assessment Report in which pCO2atm attains either 600 or 1,100 ppm in 2,100. Results suggest pH will decrease 0.15 and 0.32 pH units, aCO32− ${a}_{\mathrm{C}{\mathrm{O}}_{3}^{2-}}$ will decrease 24% and 49%, and Ωaragonite will decrease 21% and 45%, respectively. These changes are of the same magnitude as those expected for the oceans. The effects of rising pCO2atm on sub‐lethal 20% effect concentrations (i.e., EC20) of Al for brook trout (Salvelinus fontinalis) are evaluated with the biotic ligand model, which indicates Al toxicity effects will increase as pH decreases. These changes could reverse gains in water quality and fisheries health achieved since implementation of the Clean Air Act. Plain Language Summary: Fossil fuel consumption, deforestation, and changing land use are all contributing to rising CO2 concentrations in the atmosphere. In addition to atmospheric warming and climate impacts, rising atmospheric CO2 will also promote acidification of ocean surface waters, which is expected to have detrimental effects on marine ecosystems. Much less is known about how rising atmospheric CO2 will impact terrestrial freshwaters such as lakes and streams. We developed reaction path models for 18 lakes from northeastern USA to investigate possible impacts of rising atmospheric CO2 and temperature on these systems. The models indicate that acidification of these lakes could be of similar magnitude to ocean acidification. The saturation state of these lakes with respect to the mineral aragonite, which is important for calcifying organisms, will also decrease. Finally, acidification will alter aluminum (Al) speciation, which could augment toxic effects from this metal on important sport fish like brook trout, possibly reversing gains made in water quality since implementation of the Clean Air Act decreased the input of strong acids via acid rain. Key Points: Rising atmospheric CO2 from anthropogenic activities will lead to acidification of lakes in the northeastern USA during the 21st centuryAcidification will alter Al speciation and lower concentrations required to cause a sub‐lethal response in 20% of the resident brook troutAnthropogenic‐sourced CO2 could reverse gains in lake water quality realized since implementation of the Clean Air Act [ABSTRACT FROM AUTHOR]

Despite multiple advances in the understanding of the water cycle intensification in a warmer climate, climate models still diverge in their hydrological projections. Here we constrain annual runoff projections over individual and aggregated Arctic river basins. For this purpose, we use two ensembles of global climate models and two statistical methods: a regression scheme assuming similar runoff sensitivities at interannual versus climate change timescales, and a Bayesian method where models are used to derive a posterior runoff response conditioned on historical observations. While both techniques are shown to narrow model uncertainties, more or less substantially depending on rivers, the Bayesian method is less sensitive to the choice of the model ensemble and is more skillful when tested with synthetic observations. It has also been applied over the whole Arctic watershed, showing so far a limited narrowing of the inter‐model spread, but its skill will further improve with increasing climate change. Plain Language Summary: Despite considerable progress in understanding the intensification of the water cycle in response to global warming, projections of changes in the various components of the continental water balance remain highly model‐dependent. This is particularly the case for the evolution of runoff, which remains very sensitive to the way atmospheric and continental processes are represented. Here we evaluate and compare two statistical methods to constrain climate projections of annual runoff at high latitudes via available or synthetic observations. Our Bayesian method, based on historical observations, is found to be more robust than a previous method based on observed interannual climate variability. It underlines the importance of having reliable flow reconstructions to constrain runoff projections and of using empirical observational constraints with caution, as they are likely to lead to over‐confident projections. Key Points: Runoff sensitivity to temperature depends on the considered timescaleObservational constraints based on interannual variability can thus lead to overconfident projectionsObservational constraints based on historical trends can be effective if such trends are driven by human emissions of greenhouse gasses [ABSTRACT FROM AUTHOR]

We show that the seasonal cycles of clouds over the mid‐latitude oceans in the Northern Hemisphere are predictors of the responses of clouds to increasing sea‐surface temperatures globally. These regions are therefore "natural laboratories" in which the processes responsible for low‐cloud feedbacks on global scales are observed as seasonal changes in local cloud properties. We use an ensemble of configurations of a global‐climate model to show that the sensitivities of cloud‐radiative anomalies to surface temperature and lower‐tropospheric stability in the "laboratory" regions predict the models' global cloud‐radiative feedbacks. Models with greater changes in low‐clouds between seasons are shown to have stronger negative feedbacks in the mid‐latitudes, and stronger positive feedbacks from the subtropical stratocumulus. The biases in the simulated seasonal cycles, compared to observations, imply that both feedbacks are too weak in the model. The consequences of this for configuring our model to have a lower climate sensitivity are discussed. Plain Language Summary: As the Earth's climate warms, feedbacks from changes in clouds are crucial for determining the rate of warming. In some climate models, positive cloud feedbacks are known to contribute to projected temperature increases which fall outside the range that is considered plausible given our current understanding of the Earth's climate. Methods are needed which directly relate present‐day model errors to projected cloud feedbacks so that we can understand how to improve models and simultaneously obtain plausible feedback strengths. We show that in the Met Office Hadley Centres' climate model, the seasonal cycles of clouds in a few key regions, specifically the poleward ocean basins in the Northern Hemisphere, can be used to predict cloud feedbacks, globally, across a wide range of configurations of the model. These regions, particularly the North‐west Pacific, are "natural laboratories" in which the same processes that are responsible for global, cloud‐feedbacks onto global‐warming rates, occur annually as part of the local seasonal cycle. Studying clouds in these regions and model biases in predictions of these clouds will allow us to directly improve estimates of global climate change. Key Points: Seasonal cycles of clouds constrain cloud feedbacksThe northern ocean basins are natural laboratories for low‐cloud feedbacksThe hot‐model problem in HadGEM3 will not easily be solved by tuning [ABSTRACT FROM AUTHOR]

Persistent precipitation deficits are among the most impactful consequences of global warming. Here we focus on changes in the annual number of dry days (NDD) and in the annual maximum length of dry spells due to a quadrupling of atmospheric CO2. We use atmosphere‐only simulations to decompose the projected changes into additive contributions. A fast adjustment leads to a global increase in NDD despite notable regional exceptions (e.g., South Asia and Sahel). The effect of the uniform component of the surface ocean warming is model‐dependent but shapes the regional distribution of the NDD response in each model. Finally, the ocean warming pattern also contributes to large uncertainties, likely through contrasting changes in large‐scale circulation. Our results thus highlight the complexity of the NDD response, with policy‐relevant practical implications for mitigation and adaptation strategies. Plain Language Summary: Global warming is expected to intensify the global water cycle, including the intensity and frequency of precipitation extremes. Yet, the response of the dry side of the daily precipitation distribution to increased atmospheric CO2 has received so far less attention. Here we show that this response remains highly model‐dependent across the latest generation of global climate models. Furthermore, we use atmosphere‐only simulations to isolate different drivers of the precipitation response. A fast radiative and vegetation adjustment to increased CO2 leads to an overall increase in the mean annual number of dry days (NDD) despite some regional exceptions. The effect of the ocean surface warming is highly model‐dependent, only partly due to the diversity of the simulated patterns of sea surface temperature anomalies. The regional response of NDD and of the maximum length of dry spells thus does not simply scale with global warming across the different models. Key Points: AGCM simulations are used to split the dry day response to increased CO2 into fast adjustment versus slower uniform and patterned SST effectsNone of the three components consistently dominates the global land response of dry days across the selected CMIP6 modelsThe uniform SST warming dominates the regional anomalies, which however do not scale with global warming across the multi‐model ensemble [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]

The current study extends earlier work that demonstrated future extratropical transition (ET) events will feature greater intensity and heavier precipitation to specifically consider potential changes in the impacts of landfalling ET events in a warming climate. A quasi‐idealized modeling framework allows comparison of highly similar present‐day and future event simulations; the model initial conditions are based on observational composites, increasing representativeness of the results. The future composite ET event features substantially more impactful weather conditions in coastal areas, with heavier precipitation and greater storm intensity. Specifically, a Category 2 present‐day storm attained Category 4 Saffir‐Simpson intensity in the future simulation and maintained greater intensity throughout the entire life cycle, although the storm undergoes less reintensification during the post‐ET process, a result of reduced baroclinic conversion. These findings suggest increased potential for coastal hazards due to stronger tropical cyclone winds and heavier rainfall, leading to more severe coastal flooding and storm surge. Plain Language Summary: Earlier studies demonstrate that climate change can amplify the strong winds and heavy rainfall accompanying tropical cyclones (TCs) that are undergoing transformation into midlatitude cyclones. The current study extends earlier work, which focused on oceanic TCs, to specifically consider potential changes in the impacts of landfalling transformation events in a warming climate. The future events feature substantially more impactful weather conditions in coastal areas with heavier rainfall and greater storm intensity. In particular, we find that a Saffir‐Simpson Category 2 tropical storm in present‐day conditions becomes a Category 4 storm in the future, resulting in more extreme weather conditions along high‐latitude coastlines throughout its entire life cycle, even though the storm undergoes less reintensification after it completes the transformation, due to reduced future temperature contrasts in the lower atmosphere The possibility of compound coastal hazards, including stronger winds, lower central pressure, and heavier rainfall, also indicates a heightened risk of coastal flooding and storm surges. Key Points: Substantially stronger winds attributed to the intensified transitioning storm are evident, impacting the U.S. East Coast in the futurePrecipitation analysis features substantial future increases in total rainfall associated with the transitioning storm along the East CoastIn the innermost storm core region, rainfall increased at a rate more than double that predicted by Clausius‐Clapeyron scaling [ABSTRACT FROM AUTHOR]

Rapid retreat of Arctic sea ice extends the area of open ocean for new trans‐Arctic shipping routes. However, the projected routes may be too optimistic in terms of savings in shipping costs from shortened trans‐Arctic distances as they do not consider the increased sea fog frequency (SFF) over areas of the retreating sea ice. We show that delays due to sea fog can be 1–4 days, about 23%–27% along the Northwest Passage and 4%–11% along the Northern Sea Route than previous estimated. We design a route based on the projected sea‐ice extent and SFF. The new route can reduce the sailing time by 0.3–1 day by detouring the routes with lighter impacts of sea fog. More importantly the new route will lower the risk of catastrophic accidents compared to the shortest route and saves the additional costs due to unscheduled port calls. Plain Language Summary: Rapid loss of sea ice under global warming opens up the Arctic Ocean to shipping. The available trans‐Arctic shipping in the near future will strongly decrease sailing time and financial costs from the Far East to Northwest Europe. Previous designs of trans‐Arctic shipping routes mainly considered sea ice distribution, but ignored the important effect of frequent and high‐risk polar sea fog. Here, we project Arctic sea fog frequency in 21st century and evaluate its influence on trans‐Arctic shipping routes. The results highlight the necessity to redesign the routes by including sea fog effect for safer ship navigation. Key Points: About 20%–30% of Arctic shipping routes experience frequent sea fog frequency more than 20%Sea fog increases the shipping time by 23%–27% along the Northwest Passage and 4%–11% along the Northern Sea Route than previous estimationsShipping routes by minimizing the impacts of sea fog can save 0.3–1 days sailing time by detouring dense‐sea‐fog region [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) are long, narrow synoptic scale weather features important for Earth's hydrological cycle typically transporting water vapor poleward, delivering precipitation important for local climates. Understanding ARs in a warming climate is problematic because the AR response to climate change is tied to how the feature is defined. The Atmospheric River Tracking Method Intercomparison Project (ARTMIP) provides insights into this problem by comparing 16 atmospheric river detection tools (ARDTs) to a common data set consisting of high resolution climate change simulations from a global atmospheric general circulation model. ARDTs mostly show increases in frequency and intensity, but the scale of the response is largely dependent on algorithmic criteria. Across ARDTs, bulk characteristics suggest intensity and spatial footprint are inversely correlated, and most focus regions experience increases in precipitation volume coming from extreme ARs. The spread of the AR precipitation response under climate change is large and dependent on ARDT selection. Plain Language Summary: Atmospheric rivers (ARs) are long and narrow weather features often referred to as "rivers in the sky." They often transport water from lower latitudes to higher latitudes typically across climate zones and produce precipitation necessary for local climates. Understanding ARs in a warming climate is challenging because of the variety of ways an AR can be defined on gridded data sets. Unlike weather features such as tropical cyclones where identification methodologies are similar, algorithms that determine the characteristics of ARs vary depending on the science question posed. Because there is no real consensus on AR identification methodology, we aim to quantify the algorithmic uncertainty in AR metrics and precipitation. We compare 16 different ways of defining an AR on gridded data sets and present the range of possibilities in which an AR could change under global warming. Generally, ARs are projected to increase but the amount of that increase is a function of the algorithm. Across all algorithms and focus regions, AR precipitation is projected to become more extreme. Key Points: High‐resolution historical and future simulations are used to evaluate atmospheric river detection tools (ARDT) uncertaintyARDTs mostly show increases in frequency and intensity of future atmospheric rivers (ARs) but the scale of response is dependent on algorithmic restrictivenessMost regions experience an increase in precipitation volume coming from extreme ARs [ABSTRACT FROM AUTHOR]

An integrated perspective of permafrost dynamics is a key bridge between permafrost and global socioeconomic assessments. This study investigated the air temperature changes (1976–2020) among permafrost zones in the Northern Hemisphere and their potential impacts on permafrost. We found that continuous permafrost zones experienced faster warming than other regions. The freezing index declined 724°C‐day while the thawing index increased only 166°C‐day over continuous permafrost zones. This may explain why the temperature of cold permafrost increased rapidly but the active layer thickness changed only slightly. Assuming permafrost carbon emissions arise only from thaw processes may miss a significant source of the emissions. An often‐neglected factor is that cold‐season snow amplifies permafrost warming caused by summertime air temperature changes. Due to seasonal effects, using mean‐annual air temperature to depict permafrost evolution under integrated assessment frameworks may lead to significant errors. Plain Language Summary: Permafrost dynamics remains a core aspect of what people are concerned about. Some researchers are making efforts to couple permafrost to socioeconomic processes. Challenges include mismatched spatiotemporal scales and developing mathematical descriptions that are not too complex. To address these challenges, we attempt to capture the fundamental features of permafrost dynamics from an integral perspective in this study. We found that permafrost temperature changes during 1976–2020 were largely controlled by rapid changes in the air temperature during the cold seasons. We also realized that similar warming magnitudes during cold season and warm season may play an equivalent role in permafrost temperature evolution because seasonal snow cover can amplify the warming that occurred during the previous summer. The asymmetrical responses of cold and warm permafrost are critical for assessing permafrost carbon cycles and feedbacks. This is particularly true for cold continuous permafrost because about half of permafrost carbon is stored in the upper 1 m of soils and the carbon density in continuous permafrost is generally higher than other permafrost zones. Key Points: Rapid changes in cold permafrost during 1976–2020 may be explained by a strong air temperature increase during the cold seasonUnchanging snow cover is still able to enhance the effect of climate warming on permafrostThe asymmetric seasonal responses in cold and warm permafrost are critical for assessing permafrost carbon cycles and feedbacks [ABSTRACT FROM AUTHOR]

This work uses reanalysis and NOAA Geophysical Fluid Dynamics Laboratory's Coupled Model Intercomparison Project 6 model, CM4, to investigate the colocation of heat extremes and atmospheric blocking in the current climate and an end of 21st century, extreme‐emissions projection. In the present day, the colocation of heat events and blocking is greatest for the strongest heat events. Block‐heat extreme colocation is found to be less prevalent over ocean than land, exhibiting regional variation throughout the Northern Hemisphere. Over North America, colocation is greatest near the northwestern and northeastern coasts, minimizing near the center; over Eurasia, colocation is most prevalent in northern regions. In an RCP 8.5 projection, the historical 90th percentile temperature decreases to 0–70th percentile, depending on the region. This is primarily driven by mean state warming. Blocking is found to decrease along with the colocation of blocking and heat extremes, suggesting that in some regions, the mechanisms driving heat extremes will change in future climates. Plain Language Summary: This work investigates extreme heat events and atmospheric blocks (persistent high‐pressure systems), which are known to be strongly linked. The analysis herein shows that the strongest extreme heat events coincide with atmospheric blocks, however, the likelihood of the events being colocated varies from region to region. In a high emissions warming scenario, temperatures exceeding the warmest 10% in the current climate, occur 30%–100% of the time depending on the region. This is mostly driven by an increase in average temperature everywhere—rather than an increase in the variability of the temperature. At the same time, the model predicts that blocking, and the colocation of extremes heat events with blocking, decreases, suggesting the physical drivers of heat extremes will change in a warmer climate. Key Points: Blocking is more colocated with heat extremes over land than ocean. Stronger heat extremes are more colocated with blockingEnd of 20th century 90th percentile temperatures become 0–70th percentile by the end of the 21st century in a high emissions scenarioLand heat extreme duration is unchanged in RCP 8.5, but colocated blocking decreases, suggesting changes in dynamics driving heat extremes [ABSTRACT FROM AUTHOR]

Changes in ocean heat content are a critical element of climate change, with the oceans containing about 90% of the excess heat stored in the climate system and 60% in the upper 700 db. Estimates of these changes are sensitive to horizontal mapping of the sparse historical data and errors in eXpendable BathyThermograph data. Here we show that they are also sensitive to the vertical interpolation of sparsely sampled data through the water column. We estimate, using carefully constructed vertical interpolation methods with high‐quality hydrographic (bottle and CTD) data, the observationally based upper ocean heat content increase (thermosteric sea level rise) from 1956 to 2020 is 285 Zeta Joules (0.55 mm yr−1), 14% (14%) larger than estimates relying on simple but biased linear interpolation schemes. The underestimates have a clear spatial pattern with their maximum near 15°N and 12°S, around the maxima in the curvature of the temperature‐depth profile. Plain Language Summary: Change in ocean heat content is a critical element of anthropogenic climate change, with the oceans containing about 90% of the excess heat stored in the climates system. We show that linear vertical interpolation of sparse ocean temperature observations results in warm biases in upper ocean heat storage because of the curvature of the depth‐temperature profile. Because the vertical sampling has increased in recent years, these biases result in underestimates of heat content trends. Our results show that more sophisticated vertical interpolation methods (rather than linear interpolation) result in 14% larger trends in upper ocean heat content increase and thermosteric sea level rise from 1956 to 2020. The underestimates have a clear spatial pattern with their maximum near 15°N and 12°S around the location with the maxima in the curvature of the temperature depth profile. Key Points: Estimates of upper ocean warming with an improved vertical interpolation method are 14% larger than linear interpolation (LIN) schemesThe corresponding upper ocean thermal expansion is 14% larger than LINThe larger values occur most strongly near 15°N and 12°S, near the maxima in the curvature of the temperature depth profile [ABSTRACT FROM AUTHOR]

The IPCC's scientific assessment of the timing of net‐zero emissions and 2030 emission reduction targets consistent with limiting warming to 1.5°C or 2°C rests on large scenario databases. Updates to this assessment, such as between the IPCC's Special Report on Global Warming of 1.5°C (SR1.5) of warming and the Sixth Assessment Report (AR6), are the result of intertwined, sometimes opaque, factors. Here we isolate one factor: the Earth System Model emulators used to estimate the global warming implications of scenarios. We show that warming projections using AR6‐calibrated emulators are consistent, to within around 0.1°C, with projections made by the emulators used in SR1.5. The consistency is due to two almost compensating changes: the increase in assessed historical warming between SR1.5 (based on AR5) and AR6, and a reduction in projected warming due to improved agreement between the emulators' response to emissions and the assessment to which it is calibrated. Plain Language Summary: The IPCC's latest physical science report, the Working Group 1 Contribution to the Sixth Assessment Report (AR6), was released in August 2021. That report includes an update to the tools used to project the climate outcome of emission scenarios. Here we apply these newly calibrated tools, called earth system model emulators, to the set of scenarios assessed in the IPCC's Special Report on warming of 1.5°C (SR1.5). We find that two compensating changes lead to a remarkable consistency (peak warming projections within 0.1°C) between the projections made by the emulators used in SR1.5 and their descendants used in AR6. First, updates to the historical warming assessment since the SR1.5 (which was based on the IPCC's 2013 physical science report (AR5)) increase future warming projections. However, improved consistency between the emulators and the assessment of the underlying physics, particularly the short‐term warming response to emissions, lowers warming projections by an approximately equivalent amount. Our work reinforces the key messages from the IPCC: limiting warming to around 1.5°C is a great and urgent challenge, and it is up to us to decide whether we pull out all the stops to hold temperatures around 1.5°C or whether we sail on by. Key Points: Emulators used in IPCC Special Report on warming of 1.5°C and Sixth Assessment Report are remarkably consistent, despite their entirely new calibrationsThe consistency is due to two compensating factors: change in assessed historical warming and improvements to emulator calibration methods [ABSTRACT FROM AUTHOR]

Africa's central Sahel region has experienced prolonged drought conditions in the past, while rainfall has recovered more recently. Global climate models project anything from no change to a strong wetting trend under unabated climate change; and they have difficulty reproducing the complex historical record. Here we show that when a period of dominant aerosol forcing is excluded, a consistent wetting response to greenhouse‐gas induced warming emerges in observed rainfall. Using the observed response coefficient estimate as a constraint, we find that Coupled Model Intercomparison Project Phase 6 climate models with a realistic past rainfall response show a smaller spread, and higher median, of projected future rainfall change, compared to the full ensemble. In particular, very small or negative rainfall trends are absent from the constrained ensemble. Our results provide further evidence for a robust Sahel rainfall increase in response to greenhouse‐gas forcing, consistent with recent observations, and including the possibility of a very strong increase. Plain Language Summary: Rainfall is a critical resource for a large population in the African Sahel region, but rainfall levels have strongly varied in the past. It is unclear what will happen in the next decades, because some climate models suggest little to no change in average rainfall, while other models project a strong increase due to climate change. Our study aims to narrow down this uncertainty. In the past, both greenhouse gases and aerosols influenced Sahel rainfall, to different degrees during different periods; and models may not capture both mechanisms equally well. However, future rainfall changes will likely be driven mainly by greenhouse gas levels. We identify models that closely match the rainfall change that was observed in a recent period with dominant greenhouse gas forcing. These models also tend to project stronger rainfall increases than other models. Indeed, their multi‐model mean projection is an increase by almost 50% by 2040; while none of them projects stable rainfall levels or even a reduction. Thus, it appears more likely now that rainfall in the Sahel will increase substantially over the next few decades. Key Points: We identify the response of Sahel rainfall to greenhouse gas‐induced global warming in the historical recordMany global climate models of the latest generation (Coupled Model Intercomparison Project Phase 6) do not reproduce the observed rainfall responseObservational constraint reduces model spread and implies stronger rainfall increase under further warming than in full ensemble [ABSTRACT FROM AUTHOR]

Compound wind and precipitation (CWP) extreme events can cause a significant increase in socio‐economic loss in coastal regions. This study investigated the potential impact of climate change on CWP events using Coupled Model Intercomparison Project model outputs for the coastal areas impacted by tropical cyclones on a global scale. We identified global hotspots of higher dependence between extreme wind and precipitation events. Under climate change, the results show a substantial increase in precipitation extremes compared to individual wind extreme events. The likelihood of CWP events under climate change indicates an increase (about 40%–50%) in most coastal regions in North Atlantic, East, and South Asia. The results of this study can help to identify hotspot regions under climate change and further assist in minimizing the impact of future disasters in vulnerable coastal areas. Plain Language Summary: Coastal regions are more densely populated than the hinterland and continue to focus on economic activity and development. Coastal areas are often exposed to compound wind and precipitation extreme events in hurricane‐prone regions, causing significant socio‐economic losses compared to individual events. Increased compound events will bring higher risks to populated areas along the coast. Our research focused on identifying the critical hotspots that witnessed such compound hazards and how they are impacted by global warming. The results identify the critical areas based on the higher dependence between extreme wind and precipitation events and the increased frequency of occurrences. Robust and proactive measures are needed for these hotspot regions to minimize the risk from compound events under climate change. Key Points: Precipitation extremes showed a significant increase in magnitude compared to wind extremes in coastal regions globallyHigher dependence between extreme wind and precipitation was observed on the Southeast USA, Philippines, China, and North Australia coastsA higher percentage of coastal areas in the North Atlantic, East, and South Asia are likely to see more compound events in the future [ABSTRACT FROM AUTHOR]

The daily variation of ground‐level ozone (O3), a harmful pollutant, is positively correlated with air temperature (T) in many midlatitude land regions in the summer. The observed temporal regression slope between O3 and T (dO3/dT) is referred to as the "ozone‐climate change penalty" and has been proposed as a way to predict the impact of future climate warming on O3 from observations. Here, we use two chemical transport models to show that the spatial variation of dO3/dT is primarily determined by simultaneous meridional advection of O3 and T. Furthermore, the sign and magnitude of dO3/dT can be approximated by their climatological meridional gradient ratio (O3 gradient divided by T gradient). Consideration of expected changes in the meridional gradients of T and O3 due to climate change indicates that dO3/dT will likely change. Caution is needed when using the observed climate penalty to predict O3 changes. Plain Language Summary: At Earth's surface, ozone is a harmful pollutant. In the summer, we observe higher ozone concentrations on hotter days over land in the midlatitudes. This leads researchers to expect higher ozone concentrations as a result of global warming, based on chemistry that associates higher ozone concentrations with higher temperatures. Here, we show that the relationship between ozone and temperature is largely controlled by the north‐south movement of air which simultaneously transports ozone and heat. Therefore, the background spatial distributions of ozone and temperature determine how ozone and temperature covary. The ozone‐temperature relationship in the future may look different from today, because global warming is not spatially uniform. We advise caution in using observed ozone‐temperature relationship to estimate future ozone changes. Key Points: Transport by meridional advection of O3 and T explains the spatial variation in the daily O3‐T relationshipDaily regression slope dO3/dT can be approximated by the ratio of O3 and T mean meridional gradients when temperature gradients are strongGradient ratio suggests dO3/dT will change with warming, making it questionable to use observed dO3/dT in O3 projections [ABSTRACT FROM AUTHOR]

While the annual mean Arctic Amplification (AA) index varied between two and three during the 1970–2000 period, it reached values exceeding four during the first two decades of the 21st century. The AA did not change in a continuous fashion but rather in two sharp increases around 1986 and 1999. During those steps the mean global surface air temperature trend remained almost constant, while the Arctic trend increased. Although the "best" CMIP6 models reproduce the increasing trend of the AA in 1980s they do not capture the sharply increasing trend of the AA after 1999 including its rapid step‐like increase. We propose that the first sharp AA increase around 1986 is due to external forcing, while the second step close to 1999 is due to internal climate variability, which models cannot reproduce in the observed time. Plain Language Summary: The rate of Arctic warming varied in time, while the rate of global warming was nearly constant over the period 1960–2020. This led to a variable Arctic Amplification (AA) defined as a ratio of the Arctic warming trend to mean global warming trend. In addition, the Arctic experienced sudden changes in the rate of warming with step‐like changes in annual mean Arctic Amplification around the years 1986 and 1999. Climate models do not capture the second observed sharp increase of the AA. We suggest that the first AA increase around 1986 is primarily due to external forcing (increasing concentration of carbon dioxide) while the second sharp increase around 1999 is dominated by internal climate variability which current models cannot capture accurately within the time scale of the physical phenomena. Key Points: Annual mean Arctic Amplification (AA) within the period 1970–2020 changed in steep steps around 1986 and 1999. It reached values over 4.0Even those CMIP6 models best at reproducing the AA did not reproduce the second sudden increase near 1999We conjecture that the first AA sharp increase is due to external forcing, while the second is due to internal climate variability [ABSTRACT FROM AUTHOR]

Flash droughts are characterized by rapid development and intensification, generating a new risk for drought impacts on natural and socio‐economic systems. In the current climate change scenario, the meteorological drivers involved in triggering flash droughts are uncertain. We analyzed the role of meteorological drivers underlying the development of flash droughts in Spain over the last six decades, evidencing that the effect of atmospheric evaporative demand (AED) on flash drought is mainly restricted to water‐limited regions and the warm season. However, the contribution of the AED has increased notably in recent years and particularly in summer (∼3.5% per decade), thus becoming a decisive driver in explaining the occurrence of the latest flash droughts in some regions of Spain. Our findings have strong implications for proper understanding of the recent spatiotemporal behavior of flash droughts in Spain and illustrate how this type of event can be related to global warming processes. Plain Language Summary: Flash drought is a complex phenomenon characterized by rapid development and intensification, which increases potential impacts on natural and socio‐economic systems. Nowadays, little is known about the role played by the meteorological drivers involved in triggering this type of events. In this study, we analyze the influence of these drivers on the development and intensification of flash droughts in Spain over the last six decades. We show that atmospheric evaporative demand (AED) plays a minor role compared to precipitation deficits. However, the contribution of the AED to flash drought development has increased notably in recent years. Our findings highlight the importance of AED role in explaining the occurrence of the latest flash droughts in Spain and how this type of event can be more and more related to global warming. Key Points: The role of the atmospheric evaporative demand on the development of flash droughts exhibits a notable contrast between regions and seasonsThe contribution of the atmospheric evaporative demand on the development of flash droughts has increased notably in Spain over last yearsAtmospheric evaporative demand has become a decisive driver in explaining the occurrence of the latest flash droughts in Spain [ABSTRACT FROM AUTHOR]

There is low confidence in projections of Antarctic sea ice area (SIA), due to deficiencies in climate model sea ice processes. Ensemble regression techniques can help to reduce this uncertainty. We investigate relationships between SIA climatology and 21st century change in the Coupled Model Intercomparison Project, phase 6 (CMIP6) multi‐model ensemble. In summer, under a strong forcing scenario, each model loses the majority of its sea ice. Therefore, models with greater historical SIA exhibit greater reductions, so the observed climatology of SIA strongly constrains projections. Ensemble spread in historical summer SIA is smaller than in CMIP phase 5 (CMIP5), and CMIP6 gives a more robust constraint on future SIA. In winter, by 2100 under a strong forcing scenario, 40% of SIA disappears on average, and ensemble spread in historical mean SIA explains approximately half the spread in projected change. A greater winter ice loss in CMIP6 than CMIP5 is explained by the higher climate sensitivities of some CMIP6 models. Plain Language Summary: Antarctic sea ice area is important for ecosystems, human activity, and for the dynamics of the atmosphere and ocean. However, major climate change assessments such as the Intergovernmental Panel on Climate Change Sixth Assessment Report have placed little confidence in projections of Antarctic sea ice area. This is because climate models struggle to simulate many aspects of sea ice area as observed by satellites. However, not all models should be treated equally. Models with relatively more sea ice area in the recent past project greater losses in the future, especially in summer. This suggests that models with historical sea ice area closest to what we have observed may be more reliable in their future simulations. We use this information in a simple statistical model to show that, firstly, the newest climate models largely show near‐total sea ice loss in summer by the end of the 21st century, which was not universally true for older climate models. Secondly, newer climate models lose more winter sea ice than their predecessors in the same time period, which is related to the known greater global warming response to given greenhouse gas concentrations in the newer models. Key Points: We compare Coupled Model Intercomparison Project (CMIP), phase 5 and the CMIP, phase 6 (CMIP6) projections of Antarctic sea ice loss, assessing drivers of uncertainty and the robustness of projectionsUnder strong forcing, CMIP6 models lose most Antarctic summer sea ice by 2100, so projections are constrained by historical climatologyIncreased winter sea ice loss in CMIP6 is related to increased equilibrium climate sensitivity [ABSTRACT FROM AUTHOR]

Understanding the evolution of heatwaves is important for their prediction, mitigation, and adaptation. While most studies focused on either their temporal variability at individual station (or grid point) or the spatial variation over fixed durations, their daily joint evolution over space and time remains largely unexplored. Here, we investigate multi‐dimensional characteristics of spatiotemporally contiguous heatwaves across China during 1961–2018 using observational data from a 3D perspective (i.e., latitude × longitude × time). Results obtained by the 3D identification show that large contiguous heatwaves in China exhibit distinct characteristics in different geographical locations and time periods. Heatwaves in northern areas prefer to move from west to east at a faster speed, persist longer, and have greater extent and intensity than southern heatwaves, which are generally originated from the east and move westward. It is also found that contiguous heatwaves are growing in frequency, magnitude, areal extent, duration, and traveling distance across China. Plain Language Summary: Known as episodes with temperature exceeding a certain threshold, heatwaves exert severe detrimental impacts on human health. Understanding their joint evolution at both temporal and spatial dimensions is beneficial for us to mitigate the adverse effects and adapt ourselves to future heatwave increases under global warming. However, previous studies mainly focused on either the temporal or spatial changes of heatwaves rather than their co‐variability over both time and space. Here we introduce a new method, the 3D connected component algorithm, to track the 3D evolution of spatiotemporally contiguous heatwaves over China during 1961–2018 using observational data. We find that contiguous heatwaves over northern China prefer to move eastward at a faster speed, persist longer, and have greater extent and intensity than southern heatwaves, which are generally originated from the east and move westward. Contiguous heatwaves show significant increases in frequency, magnitude, coverage, duration, and traveling distance. Key Points: True 3D structure and spatiotemporal evolution of large contiguous heatwave over China are tracked by the 3D connected component labeling algorithmContiguous heatwaves in China exhibit distinct characteristics and evolution patterns in different geographical locations and time periodsSubstantial increases are observed in the frequency, area, magnitude, duration, and moving distance of contiguous heatwaves since the 1960s [ABSTRACT FROM AUTHOR]

Understanding past changes in precipitation extremes could help us predict their future dynamics. We present a novel approach for analyzing trends in extremes and attributing them to changes in the local precipitation regime. The approach relies on the separation between intensity and occurrence of storms. We examine the relevant case of the Eastern Italian Alps, where significant trends in extreme precipitation were reported. The model is able to reproduce the observed trends at all durations between 15 min and 24 hr, and allows us to quantify trends in extreme return levels. Despite the significant increase in storm occurrence and typical intensity, the observed trends can be only explained considering changes in the tail heaviness of the intensity distribution, that is the proportion between heavy and mild events. Our results suggest that the observed changes are caused by an increased proportion of summer convective storms. Plain Language Summary: Quantifying past trends in extreme rainfall is important because it can help us understand future changes caused by global warming. Climate scientists and hydrologists use specific statistical models to do so, but interpreting the results is complicated because extremes are rare and the structure of the models is not linked to the local meteorology. We use a new statistical model that allows to better understand the mechanisms behind the trends we detect. We find that rainfall extremes in the Eastern Italian Alps increased over the past decades and we associate this change to an increased proportion of summer thunderstorms. Key Points: We present a method for analyzing extreme precipitation trends based on the separation of storm intensity and occurrence frequencyOur approach reproduces observed trends in annual maxima and allows to quantify trends on rare return levelsObserved trends in the Eastern Italian Alps are explained by an increased proportion of heavy convective storms in the summer [ABSTRACT FROM AUTHOR]

Previous studies documented possible connections between low frequency climate modes in the Northern Hemisphere ocean basins. We use observed sea surface temperatures and 270 large ensemble climate model simulations, which allows for improved methods of separating external and internal variability, such as removing the ensemble mean from each simulation. Detrending methods for observations have also improved since some of these previous studies were conducted. We also devise a modified statistical test using bootstrapping that is tuned specifically to this analysis. With these tools, we reexamine relationships among these modes. While previous studies have argued for the existence of an inter‐basin link, our results suggest that any internal connections between these modes are indistinguishable from random noise. Further, we show that external forcing affects each region in similar ways. This suggests that anthropogenic warming can cause an indirect link between the two basins, confounding the interpretation of a potential relationship. Plain Language Summary: We reexamine possible connections between climate patterns in the North Atlantic and North Pacific oceans. Improved climate model simulations and appropriate statistical methods allow us to build on previous research of these linkages. In contrast to previous studies, no natural connections are detected. However, global warming is shown to affect each region in similar ways, suggesting that climate change could cause an indirect link between the two basins. Key Points: A multi model large ensemble archive and modified statistical assumptions provide a framework for studying climatic relationshipsAn analysis of relationships between leading Northern Hemisphere climate modes does not show any internal connectionsExternal forcing such as global warming is shown to be a possible confounding factor in climate relationships [ABSTRACT FROM AUTHOR]

Palaeogene environmental evolution in East Asia remains ambiguous. Here we present integrative work including magnetostratigraphy, grain‐size, geochemistry, and clay mineralalogy from a 1609 m‐thick fluviolacustrine sequence in eastern China. The results reveal two periods of tectonic control alternating with three periods of climatic control on the sedimentary evolution. Tectonic activity in the study area, as revealed by particle coarsening and reduced weathering, occurred during 65.6–59 Ma and strengthened in Asia during 55–54 Ma in response to the India‐Eurasia collision. Weathering gradually enhanced in East Asia during 59–55 Ma, probably caused by global warming. Continuous global warming during 54–50.5 Ma is responsible for enhanced aridification in East Asia. From 50.5 to 37.6 Ma, global cooling weakened evapotranspiration and increased westerlies‐derived moisture. Both aspects increased effective moisture and chemical weathering in East Asia. These results shed light on how alternating tectonism and climate change impacted environmental evolution in Asia during the Palaeogene. Plain Language Summary: Most investigations in sedimentary basins generally focus exclusively on climatic signals at the expense of tectonic inputs. In this study, we extract both climatic and tectonic signals from long fluviolacustrine sediment records in eastern China comprehensively and objectively. We find that environmental evolution during the Paleogene of East Asia was dominated by tectonism during 65.6–59 and 55–54 Ma, and by climatic changes during 59–55 and 54–37.6 Ma. This work not only constrains the India‐Asia collision to a short 55–54 Ma interval for the first time, but also offers a sound explanation for one of the most important but disputed issues of eolian sediments in the North Pacific Ocean. Key Points: Tectonic activity occurred during 65.6–59 Ma in East Asia and strengthened during 55–54 Ma in response to the India‐Eurasia collisionIn East Asia, global warming led to enhanced weathering during 59–55 Ma and aridification during 54–50.5 MaGlobal cooling between 50.5 and 37.6 Ma increased westerlies‐derived moisture and enhanced chemical weathering in East Asia [ABSTRACT FROM AUTHOR]

Ocean deoxygenation is an emerging hazard for marine ecosystems and a fingerprint of anthropogenic change. Interior ocean oxygen concentrations respond to changes in ventilation that supply oxygen and the demand of biogeochemical processes that consume oxygen. A better understanding of their regional importance would improve confidence in Earth System Model projections, which underpin ecosystem risk and vulnerability assessments. Using a hindcast reanalysis simulation, we find that oxygen trends between 1975 and 2014 in low‐oxygen zones along eastern margins are strongly affected by biogeochemical processes that alter oxygen demand, while oxygen‐rich regions of the open ocean are driven by ventilation changes. A similar regional distinction emerges among CMIP6 Earth System Models. Therefore, while biogeochemical functioning is an important source of uncertainty in low‐latitude, low‐oxygen regions, uncertainty in global trends is due to insufficient physical supply changes in Earth System Models, which is confirmed using repeat hydrographic section data in the southern mid‐latitudes. Plain Language Summary: Measurements of dissolved oxygen (O2) in the ocean since the 1960s have revealed a multidecadal decrease in ocean O2 levels that is a major concern. It represents a fingerprint of human‐induced climate change and is also an ecological hazard in the highly productive upwelling systems of the lower latitudes that host present‐day low‐O2 zones. Unfortunately, Earth System Models that are used to predict future changes fail to reproduce the magnitude of recent deoxygenation and are inconsistent in low‐O2 regions. This undermines our confidence in both reconstructions and future projections that form the basis of decision making. In this work, we use a suite of model simulations and observations over the 1975–2014 period to demonstrate that the biogeochemical response is important in the low‐O2 waters of the tropics and highly productive eastern margins, while physical supply is dominant elsewhere. Reducing our uncertainty in the low‐O2 regions will therefore rely partly on constraining the contribution of biogeochemical processes, while large‐scale trends are dependent on how models represent physical processes, which are chronically underestimated by models. Key Points: Observed and modeled mesopelagic oxygen trends are driven (85%) by apparent oxygen utilization that integrates oxygen supply and demandBiogeochemical processes shaping oxygen demand are important along eastern margins and some tropical areasLarge‐scale oxygen trends are driven by changing supply, and weak model trends are linked to misrepresented physical processes [ABSTRACT FROM AUTHOR]

When the climate system is forced, for example, by the emission of greenhouse gases, it responds on multiple time scales. As temperatures rise, feedback processes might intensify or weaken. Such state dependencies cannot be fully captured with common linear regression techniques that relate feedback strengths linearly to changes in the global mean temperature. Hence, transient changes are difficult to track and it becomes easy to underestimate future warming this way. Here, we present a multivariate and spatial framework that facilitates the dissection of climate feedbacks over time scales. Using this framework, information on the composition of projected transient future climates and feedback strengths can be obtained. The new framework is illustrated using the Community Earth System Model version 2. Plain Language Summary: When the Earth warms, the internal processes of the climate system change. This can lead to additional warming, forming a feedback loop. For instance, as the ice melts due to increased temperatures, less solar radiation gets reflected back to outer space, causing temperatures to rise even more. To properly understand and assess (future) climatic changes, it is, therefore, necessary to quantify these so‐called climate feedbacks and track how they change over time. However, with current techniques, it is not easy to explicitly track these temporal changes, which hampers the interpretation of long‐term projections of the future climate state and easily leads to systematic underestimation of future (committed) warming. Here, we present a new feedback framework that can capture the temporal evolution of climate feedbacks. Consequently, a better insight into the development of a (projected) future climate is obtained, because not only global mean temperatures can be tracked, but also the temporal change in individual climate feedbacks, including their spatial distribution. Key Points: A multivariate climate feedback framework is introduced that takes into account the transient state dependency of climate feedbacksUsing the new framework, changes in feedback processes can be analyzed per time scale and temporal evolution can be trackedWithin the framework, it is possible to create transient and equilibrium projections of (the spatial patterns of) climate feedbacks [ABSTRACT FROM AUTHOR]

The Interdecadal Pacific Oscillation (IPO) features a basin‐scale horseshoe‐like sea surface temperature (SST) anomaly pattern. Its cold‐phase shift around 1999, implicated as a driver for the early‐2000s global warming slowdown, has been potentially linked to the Atlantic warming during the positive‐phase Atlantic Multidecadal Variability (AMV). However, the key mechanism for the trans‐basin Atlantic‐Pacific teleconnection remains debatable. Here, we show that an AMV‐SST cooling can initiate a pan‐Pacific response. The North Atlantic cooling induces westerly wind anomalies over the central‐western equatorial Pacific as Kelvin‐wave responses and easterly wind anomalies over the far‐eastern equatorial Pacific as Rossby‐wave responses. Additionally, anomalous lows are generated over the extratropical North and South Pacific through the midlatitude Rossby wave propagation. The tropical and midlatitude teleconnections act together to develop into a warm‐phase IPO‐like pattern through the wind‐induced latent heat. Our results suggest a potential of advancing the predictability of IPO through a skillful simulation of AMV. Plain Language Summary: The Interdecadal Pacific Oscillation (IPO) is the most prominent mode of decadal climate variability over the whole Pacific basin, characterized by a horseshoe‐shaped sea surface temperature (SST) anomaly pattern. Its Atlantic counterpart, the Atlantic Multidecadal Variability (AMV) is characterized by a basin‐wide SST warming or cooling pattern across the North Atlantic. In recent decades, the Atlantic warming strongly tied to the positive phase of AMV has significantly strengthened the tropical Pacific cooling, contributing to the recent global warming hiatus. Yet, the root cause of the trans‐basin Atlantic‐Pacific teleconnection on decadal timescales remains unclear. In this study, using the idealized pacemaker simulations with an imposed AMV‐SST anomaly pattern in a fully coupled model, we rigorously investigate how an AMV‐SST cooling impacts the Pacific‐basin decadal‐scale climate change. We found that an AMV cooling initiates a tropical Gill‐type response and midlatitude Rossby wave trains. The tropical and midlatitude teleconnections work together to form a warm‐phase IPO‐like SST anomaly pattern. Our results highlight that realistic simulation of the AMV may improve the predictability of the IPO state and predictions of global warming rates. Key Points: The Atlantic Multidecadal Variability cooling can drive an Interdecadal Pacific Oscillation‐like warm‐phase patternThe North Atlantic cooling excites a tropical Gill‐type response and midlatitude Rossby wave train responsesBoth the tropical and midlatitude dynamical pathways are central to the decadal‐scale trans‐basin Atlantic‐Pacific connection [ABSTRACT FROM AUTHOR]

Regional climate variability is strongly related to large‐scale circulation modes. However, little is known about changes in their spectral characteristics under climate change. Here, we introduce piecewise evolutionary spectra to quantify time‐varying variability and co‐variability of climate variables, and use ensemble periodograms to estimate these spectra. By employing a large ensemble of climate change simulations, we show that changes in the variability and relationships of the North Atlantic Oscillation (NAO) and regional surface temperatures are disparate on individual timescales. The relation between NAO and surface temperature over high‐latitude lands weakens the most on 20‐year timescales compared to shorter timescales, whereas the relation between NAO and temperature over subtropical North Africa strengthens more on shorter timescales than on 20‐year timescales. These projected evolution and timescale‐dependent changes shed new light on the controlling factors of circulation‐induced regional changes. Accounting for them can lead to the improvement of future regional climate predictions. Plain Language Summary: Large‐scale atmospheric circulation modes influence regional climate variability. For example, the North Atlantic Oscillation (NAO) is a circulation mode closely linked to surface temperatures variations over Europe, Africa, and North America. However, under global warming, changes in regional climate variability and their relation to circulation modes (co‐variability) can evolve differently and disparately depending on timescales. Here, we use the theory of evolutionary spectra to quantify these nonstationary changes and present a novel approach to estimate such changes on various timescales. The estimation approach is based on a large ensemble of climate change simulations. We show that changes in the NAO and regional surface temperature variability and their relationships evolve differently on individual timescales. On 20‐year timescales, co‐variability between NAO and surface temperature weakens over high‐latitude lands surrounding the northern North Atlantic, whereas the corresponding co‐variability on shorter timescales strengthens over subtropical North Africa. These differing evolution and timescale‐dependent changes shed new light on the controlling factors of circulation‐induced regional changes. Taking them into account can lead to the improvement of future regional climate predictions. Key Points: We define piecewise evolutionary spectra (special case of evolutionary spectra) to quantify time‐varying second moments in a warming climateWe introduce ensemble periodograms derived from a large ensemble as consistent estimators of piecewise evolutionary spectraWe find time‐dependent and timescale‐dependent changes in relations between NAO and surface temperature [ABSTRACT FROM AUTHOR]