Strong Warming Research Articles

Loop Current Eddies as a Possible Cause of the Rapid Sea Level Rise in the Gulf of Mexico

AbstractThe Gulf of Mexico (GOM), with its densely populated coastline, is one of the world's most vulnerable regions to climate change and sea level (SL) rise. Over the last three decades, various works have been conducted to assess coastal SL trends around the basin using tide gauge stations, separately from studies dealing with regional dynamical processes. Using altimetry, Argo, and eddy atlas products over the period from January 1993 to December 2020, we have analyzed the regional SL variations in the area to define their characteristics and explore their possible dynamical causes. We observe a mean GIA‐corrected SL rise rate of 4.81 ± 0.85 mm yr−1, which is 25% higher than that of the adjacent Caribbean Sea and 44% higher than that of the global ocean. This result highlights the singular SL evolution in the GOM over the 28‐year study period. Over 2010–2020, the SL trend in the GOM has even accelerated, along with a strong warming of the upper‐layer (0.58 ± 0.17°C), which explains ∼60% of the SL rise rate through the thermosteric effect. Finally, the heat input estimates emphasize the role of the Loop Current eddies as a major contributor to the recent acceleration of SL rise due to upper‐layer warming.

On the Non‐Linear Nature of Long‐Term Sea Surface Temperature Global Trends

AbstractThe year‐to‐year rate and acceleration of the changes in global sea surface temperatures (SSTs) for 1870–2022 are determined using non‐linear techniques. Our methodology identifies the non‐linear, long‐term tendencies of the warming, revealing subtle but essential features of the SST changes that are impossible to identify by linear techniques. Our analysis identifies inhomogeneous patterns of SST evolution. For instance, in most equatorial regions, including the western Pacific, the acceleration of the warming is positive, and sustained warming replaces the cooling at the beginning of the records. On the contrary, the warming acceleration in the central and eastern equatorial Pacific is negative, indicating a sustained warming at the beginning of the record that weakens over the years, resulting in a stagnated warming or even a cooling in recent years. We identify a strong warming with a negative acceleration for the North Atlantic. We hypothesize that this warming is part of the multidecadal variability of SST rather than a long‐term trend.

The Advancement in Spring Vegetation Phenology in the Northern Hemisphere Will Reverse After 2060 Under Future Moderate Warming Scenarios

AbstractGlobal warming has largely advanced spring vegetation phenology, which has subsequently affected terrestrial carbon and water cycles. However, further shifts in vegetation phenology under future climate change remain unclear. We estimated the start of the growing season (SOS) by applying multiple extraction methods based on the NDVI3g data set, and then parameterized and evaluated 11 spring vegetation phenology models that included chilling, forcing, and the photoperiod. Based on scenario data from three Shared Socioeconomic Pathways (SSP126, SSP245, and SSP585) derived from eight climate models, future vegetation phenology was predicted using the phenology models. Results showed that all the phenology models performed better than the NULL model (mean of the SOS), with the performance of one‐phase models broadly matching that of two‐phase models, although the best models varied by vegetation type. The spatial pattern of simulated SOS was similar among the models, and it explained >75% of the variation. Based on the mean predicted SOS, we found that spring vegetation phenology will continue to advance under strong warming conditions (SSP245 and SSP585), but that the trend of advance will reverse at around 2060 under the SSP126 scenario. The continued trend in SOS advance is likely related to rapid forcing fulfillment under stronger warming conditions. However, under moderate warming, chilling might be reduced and it might require longer to compensate for higher forcing, which ultimately would result in SOS delay. Our findings highlight that trends will likely change under different warming conditions, potentially causing widespread impact on species interaction, biodiversity, and ecosystem function.

Sea level variability in Gulf of Guinea from satellite altimetry

Coastal zones with dense populations, low elevations and/or inadequate adaptive capacity are on the frontline of unprecedented impacts from climate change. The Gulf of Guinea (GoG), stretching from Liberia to Gabon, is in particular vulnerable to coastal flooding caused by local and/or climate-induced sea level rise. In this region, interannual to decadal coastal sea level changes remain poorly understood, mainly due to a lack of tide gauge stations. Here we use nearly three decades (1993–2021) of satellite altimetry data to study the link between the Equatorial Atlantic and coastal GoG sea level variability. The rate of mean sea level rise increased from 3.47 to 3.89 ± 0.10 mm/yr from the Equatorial oceanic domain to the GoG coastal area, with an acceleration of 0.094 ± 0.050 mm/yr2. This corresponds to a mean sea level rise of about 8.9 cm over the entire altimetry period, 1993–2021. We focus on the (extreme) warm/cold events that occur in both the GoG during Atlantic Niños, and along the Angola-Namibia coast during Benguela Niños. Both events are driven by remote forcing via equatorial Kelvin waves and local forcing by local winds, freshwater fluxes and currents intensifications. Analysis of altimetry-based sea level, sea surface temperature anomalies, 20 °C isotherm based PIRATA moorings, and the Argo-based steric and thermometric sea level allows us to follow the coastal trapped waves (CTWs) along the GoG, and its link with major events observed along the strong Equatorial Atlantic warmings in 2010, 2012, 2019 and 2021. Both 2019 and 2021 warming have been identified as the warmest event ever reported in this region during the last 40 years. A lag of 1 month is observed between equatorial and West African coastal trapped wave propagation. This observation may help to better anticipate and manage the effects of extreme events on local ecosystems, fisheries, and socio-economic activities along the affected coastlines. In order to enable informed decision-making and guarantee the resilience of coastal communities in the face of climate change, it emphasises the significance of ongoing study in this field.

The effect of forest cover changes on the regional climate conditions in Europe during the period 1986–2015

Abstract. Afforestation affects the earth's climate system by changing the biogeochemical and biogeophysical characteristics of the land surface. While the regional effects of afforestation are well understood in the tropics and the high latitudes, its climate impact on the midlatitudes is still the subject of scientific discussions. The general impact of afforestation on the regional climate conditions in Europe during the last decades is investigated in this study. For this purpose, regional climate simulations are performed with different forest cover fractions over Europe. In a first simulation, afforestation in Europe is considered, while this is not the case for a second simulation. We focus on the years 1986–2015, a period in which the forest cover in Europe increased comparatively strongly, accompanied by a strong general warming over the continent. Results show that afforestation has both local and non-local effects on the regional climate system in Europe. Due to an increased transport of turbulent heat (latent + sensible) into the atmosphere, afforestation leads to a significant reduction of the mean local surface temperatures in summer. In northern Europe, mean local surface temperatures were reduced about −0.3 K with afforestation, in central Europe about −0.5 K, and in southern Europe about −0.8 K. During heat periods, this local cooling effect can reach −1.9 K. In winter, afforestation results in a slight local warming in both northern and southern Europe because of the albedo effect of forests. However, this effect is rather small and the mean temperature changes are not significant. In the downwind direction, locally increased evapotranspiration rates with afforestation increase the general cloud cover, which results in a slight non-local warming in winter in several regions of Europe, particularly during cold spells. Thus, afforestation had a discernible impact on the climate change signal in Europe during the period 1986–2015, which may have mitigated the general warming trend in Europe, especially on the local scale in summer.

Multispecies population-scale emergence of climate change signals in an ocean warming hotspot

Abstract Ocean waters of the Northeast US continental shelf have warmed rapidly in recent years, with sea surface temperatures rising 2.5 times faster than those of the global oceans. With this strong warming trend, the frequency and duration of marine heatwaves have increased. These temperature changes stood out as a distinct warm temperature regime during the 2010s. During this decade, fish population characteristics also differed from the past. Species distribution shifts were detected for many species, demonstrating one way species could adapt to warming conditions. However, for most species, distribution shifts were insufficient to avoid warmer surface or bottom temperatures. As species occupied warmer habitats, growth patterns aligned with expectations for warming temperatures. Consistent with the temperature-size rule, some species exhibited faster growth at early life stages but plateaued at smaller body sizes; other species, however, experienced reduced growth across all ages, indicating thermal stress. Finally, population productivity indexed by the recruit-to-spawner ratio declined significantly during the 2010s for some populations. Changes in these three processes—distribution, growth, and productivity—indicate the emergence of climate change signals across multiple Northeast US fish populations. These effects create new challenges for fishery managers and industry participants operating in the context of non-stationarity and uncertainty.

Effects of Stratospheric Warming on Ural blocking Events in Winter

AbstractUtilizing the Open Integrated Forecasting System, the responses of Ural blocking (UB) to different stratospheric warming scenarios are investigated. Numerical results show that stratospheric warming with moderate strength in minor patterns prolongs the UB duration and enhances its intensity, while strong stratospheric warming in minor patterns tends to shorten its duration and weaken its intensity, even leading to the collapse of the UB events. Further diagnosis reveals that the planetary wave activity flux propagates downward from the stratosphere to the troposphere after stratospheric warming. Moreover, the convergence of planetary wave activity flux is a key factor for UB enhancement and maintenance. In addition, the weakened meridional temperature gradients, decelerated zonal westerly winds, and a reduced meridional potential vorticity gradient (PVy) result in UB enhancement in response to stratospheric warming with moderate strength. As stratospheric warming strengthens, planetary wave activity flux diverges, westerly winds in the tropospheric mid‐latitudes accelerate and the PVy in the Ural sector enlarges, which further weakens UB. Regarding the stratospheric perturbations in major patterns, they have similar influences on UB events, that is, UB enhances with moderate stratospheric warming and weakens with strong warming. However, the strengthened warming would trigger UB re‐enhancement, which is closely associated with anomalous activities of tropospheric synoptic‐scale waves induced by stratospheric perturbations. These results reveal UB events respond differently to stratospheric warming with various intensities and patterns in the short term, which makes a contribution to understanding stratosphere‐troposphere coupling.

Contrasts of Large-Scale Moisture and Heat Budgets between Different Sea Areas of the South China Sea and the Adjacent Land

Abstract This study investigates and compares large-scale moisture and heat budgets over the eastern rainy sea area around Dongsha, the western rainless sea area around Xisha, and the northern coastland of the South China Sea. Ten-year (2011–20) surface, balloon-sounding, satellite measurements, and ERA5 reanalysis are merged into the physically consistent data to study annual and vertical variations of the budgets. It shows that the surface and column-integrated heat and moisture budgets have the smallest annual evolution over the coastland. The latent heat as a key heat contributor in summer is mainly offset by total cold advection and partially offset by net radiative cooling. The horizontal moisture advection below 700 hPa presents moistening over the sea whereas drying over the coastland during rainy months, in which the vertical moisture advection presents moistening up to 250 hPa for all three subregions. The horizontal temperature advection is weak throughout the year over the sea but displays strong top warming and bottom cooling in summer and nearly the opposite in winter over the coastland. The diabatic cooling with a peak at ∼700 hPa in winter is largely due to the enhanced radiative cooling and latent cooling. While the diabatic heating with a peak at ∼500 hPa in summer is largely due to the enhanced latent heating. The earliest atmospheric heating and moistening occur in spring over the coastland, inducing the earliest precipitation increase. The enhanced heating and moistening over Xisha have a 1-month lag relative to Dongsha, resulting in lagging precipitation.

Divergent gametic thermal performance and floral warming across an elevation gradient.

Thermal environments vary widely across species ranges, establishing the potential for local adaptation of thermal performance optima and tolerance. In the absence of local adaptation, selection should favor mechanisms to meet thermal optima. Floral temperature is a major determinant of reproductive success in angiosperms, yet whether gametic thermal performance shows signatures of local adaptation across temperature gradients, and how variation in gametic thermal performance influences floral evolution, is unknown. We characterized flowering season temperatures for the forb, Argentina anserina, at extremes of a 1000 m elevation gradient and generated thermal performance curves for pollen and ovule performance in populations at each extreme. Thermal optima fell between mean and maximum intrafloral temperatures. However, cooler high-elevation populations had ~4 °C greater pollen thermal optima than warmer low-elevation populations, while tolerance breadths did not differ. We then tested whether plants at elevational extremes differentially warmed the floral microenvironment. High-elevation flowers warmed significantly more than low, bringing intrafloral temperatures nearer the pollen optima. A manipulative experiment demonstrated that stronger warming in high elevation was conferred by floral tissues. Elevational divergence in floral warming may be driven, in part, by selection on flowers to meet different thermal demands of the gametophytes.

Effects of Ground Subsidence on Permafrost Simulation Related to Climate Warming

We develop a moving-mesh permafrost model that contains a ground subsidence computation module to estimate the effects of ground subsidence on permafrost simulation under different warming scenarios. Including the ground subsidence process in the permafrost simulation produces only a relatively small improvement in the simulation performance of the ground temperature field, as validated by observations from two sites on the Qinghai–Tibetan Plateau (QTP). It is shown that ignoring ground subsidence tends to achieve a larger active layer thickness (ALT) but a smaller original thickness of permafrost that has thawed when simulating permafrost changes in a warming climate. The heat consumed by permafrost changes will be underestimated in simulations that do not consider ground subsidence. The effects that ground subsidence exerts within the permafrost simulation are clearly demonstrated under a strong warming scenario, which will influence the global energy budget. Projections indicate that the permafrost in the continuous permafrost area of the QTP may be close to the phase transition temperature to become zero thermal gradients in 2030–2040 under the SSP5-8.5 scenario, and there will be a great risk of ground subsidence by that stage. For permafrost regions with rich ground ice, the downward propagating temperature signals caused by ground subsidence are more attenuated. However, the heat calculation error will be larger in a simulation that does not consider ground subsidence there. This study quantifies the effects of ground subsidence, which can provide a better understanding of the permafrost thaw and energy budget of the QTP.

The impact of ENSO and NAO initial conditions and anomalies on the modeled response to Pinatubo-sized volcanic forcing

Abstract. Strong, strato-volcanic eruptions are a substantial, intermittent source of natural climate variability. Initial atmospheric and oceanic conditions, such as El Niño Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO), also naturally impact climate on interannual timescales. We examine how initial conditions of ENSO and NAO contribute to the evolution of climate in the period following a Pinatubo-type eruption using a large (81-member) ensemble of model simulations in GISS model E2.1-G. Simulations are initialized from sampled conditions of ENSO and NAO using the protocol of the coordinated CMIP6 Volcanic Model Intercomparison Project (VolMIP) – where aerosols are forced with respect to time, latitude, and height. We analyze paired anomalous variations (perturbed – control) to understand changes in global and regional climate responses under positive, negative, and neutral ENSO and NAO conditions. In particular, we find that for paired anomalies there is a high probability of strong (∼1.5 ∘C) warming of northern Eurasia surface air temperature in the first winter after the volcanic eruption for negative NAO ensembles coincident with decreased lower stratospheric temperature at the poles, decreased geopotential height, and strengthening of the stratospheric polar vortex. Climate anomalies (relative to average conditions across the control period), however, show no mean warming and suggest that the strength of this response is impacted by conditions present in the selected period of the control run. Again using paired anomalies, we also observe that under both +ENSO and −ENSO ensembles sea surface temperature decreases in the first post-eruptive boreal winter coinciding with surface cooling from volcanic aerosols. Neutral ENSO ensembles, on the other hand, show variability in their response with no clear trend in post-eruptive warming or cooling. In general, paired anomalies from unperturbed simulations give insight into the evolution of the climate response to volcanic forcing; however, when compared with anomalies from climatological conditions, it is clear that paired anomalies are significantly affected by sampled initial conditions occurring at the time of the volcanic eruption.

A potential collapse of the Atlantic Meridional Overturning Circulation may stabilise eastern Amazonian rainforests

Observations and models suggest that the Amazon rainforest might transition to a savanna-like state in response to anthropogenic climate and land use change. Here, we combine observations of precipitation, temperature and tree cover with high-resolution comprehensive climate model simulations to investigate the combined effect of global warming and a potential Atlantic Meridional Overturning Circulation collapse on the Amazon. Our results show that, while strong warming lead to forest dieback, an Atlantic Meridional Overturning Circulation collapse would stabilize the Amazon by increasing rainfall and decreasing temperature in most parts. Although an Atlantic Meridional Overturning Circulation collapse would have devastating impacts globally, our results suggest that it may delay or even prevent parts of the Amazon rainforest from dieback. Besides the many negative consequences of its collapse, the interactions we identify here make a tipping cascade, i.e., that an Atlantic Meridional Overturning Circulation collapse would trigger Amazon dieback, appear less plausible.

Spatial differentiation and mechanism of carbon source/sink of forest swamps in riverside of Changbai Mountains, China

To quantify the carbon source/sink function of riparian zone swamps and explore the feedback relationship with climate change, we measured the annual fluxes of soil greenhouse gas, soil carbon emission, net vegetation carbon sequestration and related environmental factors (temperature, water level, etc.) by static chamber-gas chromatography and relative growth equation methods in three kinds of forest swamps (Alnus sibirica swamp, Betula platyphylla swamp, and Larix olgensis swamp) distributed along the water reduction gradient of lowland to highland in the stream riparian zone of Changbai Mountains. The results showed that the annual fluxes of CH4(0.19-0.85 mg·m-2·h-1), CO2(60.81-228.63 mg·m-2·h-1), and N2O (-0.02-0.05 mg·m-2·h-1) showed spatial variations along the water gradient of lowland to highland, with a trend of first constant then decreasing, decreasing, and first absorption then emission, respectively. The spatial variations of annual fluxes of these greenhouse gases were controlled by water levels. The annual net carbon sequestration of vegetation (2.61-3.45 t C·hm-2·a-1) was constant along the water gradient, which was mainly promoted by nitrate nitrogen content. The carbon source/sink and global warming potential (GWP) undergo regular changes along water gradients. The A. sibirica swamp was a carbon sink (1.93 t C·hm-2·a-1), the B. platyphylla swamp was a weak carbon source (-0.18 t C·hm-2·a-1), and the L. olgensis swamp was a strong carbon source (-2.51 t C·hm-2·a-1). The spatial variation of carbon source/sink in forest swamps was jointly promoted by water level and nitrate nitrogen content. A. sibirica swamp exhibited a strong cooling effect with a strong negative feedback effect on climate change (-5.88 t CO2·hm-2·a-1). L. olgensis swamp exhibited a strong warming effect with a strong positive feedback effect (10.97 t CO2·hm-2·a-1). B. platyphylla swamp exhibited a weak warming effect, approximately neutral (2.95 t CO2·hm-2·a-1). The spatial variation of GWP in forest swamps was mainly inhibited by water level.

Stream temperature responses to forest harvesting with different riparian buffer prescriptions in northern California, USA

Forested riparian zones adjacent to headwater streams provide unique habitat attributes and influence water quantity and quality. Forest harvesting adjacent to and within these zones has led to changes in stream temperature (Ts) and impacts on aquatic ecosystems. As such, policies and regulations have sought to limit forest management activities in riparian zones; however, water temperature responses have been variable, necessitating additional research to improve our understanding of the effectiveness of contemporary riparian harvesting prescriptions in limiting water temperature increases after harvest. We sought to quantify the effects of three different riparian buffer prescriptions with increasing intensity of basal area removal (2–43% area harvested) on Ts in forested headwaters in northern California, USA. We measured Ts in 18 headwater catchments (12 harvested, 6 six unharvested references) during a pre-harvest year and two post-harvest years using 12 thermistors distributed longitudinally down each stream (i.e., 216 total sensors). We analyzed the Ts data to assess (1) the seasonal variability of seven-day moving average of daily maximum stream temperature (T7-day-max) and (2) the relative importance of the different harvesting prescriptions and catchment physiographic characteristics in influencing the T7-day-max. Our analysis indicated substantial changes in basal area and shade in the riparian areas with the most intensive harvesting treatment (i.e., 50% reduction in canopy cover). However, these changes in riparian canopy were poorly related to stream temperature. Results indicated a median T7-day-max increase of ∼ 2 °C in the sites with the most intensive harvesting treatment. The greatest changes in seasonal T7-day-max occurred during the summer and fall but only during the first year after harvesting. There was no evidence of increases in T7-day-max during the second post-harvest year. While air temperatures in the riparian areas increased by 1–5 °C after harvesting, this warming did not directly transfer to strong warming in stream temperatures. Rather, random forest models revealed that T7-day-max was more strongly related to topography (i.e., elevation) and climatic variability (i.e., changes in precipitation and stream stage) than to the riparian harvesting prescription or the harvesting period. Our study further highlights the challenges in understanding the thermal regimes of headwater streams and their responses to forest disturbances. Predictions of stream temperature responses to forest disturbances are complicated by the heterogeneity of the factors that influence this important physical water quality parameter. However, with global climate change and increasing pressures on water resources and aquatic ecosystems it is increasingly important to continue to provide insights into the relationships between forest management activities and the thermal regimes of headwater streams.

Interdecadal change in the response of winter North Atlantic Oscillation to the preceding autumn sea ice in the Barents-Kara Seas around the early 1990s

Based on the reanalysis data for the period 1980–2019, this study reveals that the linkage of autumn sea ice concentration in the Barents-Kara Seas (BKSIC) to the subsequent winter North Atlantic Oscillation (NAO) underwent an interdecadal weakening in the early 1990s. Such a weakening is attributed to the discrepancy in the tropospheric warming resulting from the decrease in the interannual variability of the BKSIC from the former period to the latter period. During the former period (1980–1993), the decrease in autumn BKSIC can induce strong warming in the troposphere through enhancing the in-situ diabatic heating, significantly weakening the circumpolar westerly. The weakening of autumn circumpolar westerly then promotes the upward propagation of the Eliassen-Palm (EP) flux from the troposphere to the stratosphere. Till winter, the EP flux propagates downward from the stratosphere to the troposphere. It leads to significant positive stream function anomalies over the Greenland-Barents Seas region, accompanied with negative stream function anomalies over Western Europe. They are linked by a strong southeastward-propagating wave activity flux and result in a negative NAO phase with its southern center located eastward. During the latter period (1994–2019), due to smaller interannual variability of the BKSIC, its resultant warming of the tropospheric temperature is much weaker, not conducive to the aforementioned stratosphere-troposphere interaction and thus diluting the relationship between autumn BKSIC and the subsequent winter NAO.

Seasonally distinct contributions of greenhouse gases and anthropogenic aerosols to historical changes in Arctic moisture budget

Arctic hydrological cycle has important global implications through modifying deep ocean circulation via fresh water fluxes. However, attribution of the observed changes in Arctic moisture budget remains challenging due to the lack of reliable observations. Here, as a first step, past changes in hydrological cycle over the Arctic were evaluated using CMIP6 historical simulations. To examine possible influences of individual external forcings, CMIP6 multi-model simulations performed under natural-plus-anthropogenic (ALL), greenhouse gas (GHG), natural (NAT), and aerosol (AER) forcings were compared. Results indicate that Arctic precipitation and evaporation increase in ALL and GHG but decrease in AER. In ALL and GHG, Arctic precipitation increases in summer mainly due to enhanced poleward moisture transport whereas Arctic moistening during winter is affected more by increased surface evaporation over sea-ice retreat areas. In AER, Arctic precipitation tends to decrease due to reduced evaporation over sea-ice advance areas during cold months. Poleward meridional moisture flux (MMF) across 70°N is the strongest in summer due to the highest moisture. GHG has stronger MMF than other forcing simulations due to larger increase in moisture in line with stronger warming. The MMF response is found to be largely determined by variations in transient eddies with distinct seasonal and regional contributions.

Aerosol and dynamical contributions to cloud droplet formation in Arctic low-level clouds

Abstract. The Arctic is one of the most rapidly warming regions of the globe. Low-level clouds and fog modify the energy transfer from and to space and play a key role in the observed strong Arctic surface warming, a phenomenon commonly termed “Arctic amplification”. The response of low-level clouds to changing aerosol characteristics throughout the year is therefore an important driver of Arctic change that currently lacks sufficient constraints. As such, during the NASCENT campaign (Ny-Ålesund AeroSol Cloud ExperimeNT) extending over a full year from October 2019 to October 2020, microphysical properties of aerosols and clouds were studied at the Zeppelin station (475 m a.s.l.), Ny-Ålesund, Svalbard, Norway. Particle number size distributions obtained from differential mobility particle sizers as well as chemical composition derived from filter samples and an aerosol chemical speciation monitor were analyzed together with meteorological data, in particular vertical wind velocity. The results were used as input to a state-of-the-art cloud droplet formation parameterization to investigate the particle sizes that can activate to cloud droplets, the levels of supersaturation that can develop, the droplet susceptibility to aerosol and the role of vertical velocity. We evaluate the parameterization and the droplet numbers calculated through a droplet closure with in-cloud in situ measurements taken during nine flights over 4 d. A remarkable finding is that, for the clouds sampled in situ, closure is successful in mixed-phase cloud conditions regardless of the cloud glaciation fraction. This suggests that ice production through ice–ice collisions or droplet shattering may have explained the high ice fraction, as opposed to rime splintering that would have significantly reduced the cloud droplet number below levels predicted by warm-cloud activation theory. We also show that pristine-like conditions during fall led to clouds that formed over an aerosol-limited regime, with high levels of supersaturation (generally around 1 %, although highly variable) that activate particles smaller than 20 nm in diameter. Clouds formed in the same regime in late spring and summer, but aerosol activation diameters were much larger due to lower cloud supersaturations (ca. 0.5 %) that develop because of higher aerosol concentrations and lower vertical velocities. The contribution of new particle formation to cloud formation was therefore strongly limited, at least until these newly formed particles started growing. However, clouds forming during the Arctic haze period (winter and early spring) can be limited by updraft velocity, although rarely, with supersaturation levels dropping below 0.1 % and generally activating larger particles (20 to 200 nm), including pollution transported over a long range. The relationship between updraft velocity and the limiting cloud droplet number agrees with previous observations of various types of clouds worldwide, which supports the universality of this relationship.

Heat content and temperature trends in the Mediterranean Sea as derived from Argo float data

The Mediterranean Sea is very sensitive to climatic changes due to its semi-enclosed nature and is therefore defined as one of the hotspots in future climate change projections. In this study, we use Argo float data to assess climatologies and trends in temperature and Ocean Heat Content (OHC) throughout the Mediterranean Sea and for specific sub-basins (e.g. Western and Eastern Mediterranean, Gulf of Lion, South Adriatic). The amount of the OHC, spatially averaged in bins of 1°x1° over the period from 2001 to 2020, increases from west to east in the Mediterranean Sea. Time series of temperature and OHC from 2005 to 2020, estimated in the surface and intermediate layers (5-700 m) and deeper layer (700-2000 m), reveal significant warming trends and an increase of OHC. The upper 700 m of the Mediterranean Sea show a temperature trend of 0.041 ± 0.012°C·yr-1, corresponding to an annual increase in OHC of 3.59 ± 1.02 W·m-2. The Western Mediterranean Sea (5-700 m) is warming fastest with an increase in temperature at a rate of 0.070 ± 0.015°C·yr-1, corresponding to a yearly increase in OHC of 5.72 ± 1.28 W·m-2. Mixing and convection events within convection sites and along boundary currents transport and disperse the temperature and OHC changes. Significant warming trends are evident in the deeper layers (700-2000 m) of the two deep convection sites in the Mediterranean Sea (Gulf of Lion, South Adriatic), with an exceptionally strong warming trend in the South Adriatic from 2013 to 2020 of 0.058 ± 0.005°C·yr-1, corresponding to a yearly increase in OHC of 9.43 ± 0.85 W·m-2. The warming of the different water masses will show its feedback on ocean dynamics and air-sea fluxes in the next years, decades, and even centuries as these warming waters spread or re-emerge. This will provide more energy to the atmosphere, resulting in more extreme weather events and will also stress ecosystems and accelerate the extinction of several marine species. This study contributes to a better understanding of climate change in the Mediterranean region, and should act as another wake-up call for policy makers and society.

Amazon deforestation causes strong regional warming.

Tropical deforestation impacts the climate through complex land-atmosphere interactions causing local and regional warming. However, whilst the impacts of deforestation on local temperature are well understood, the regional (nonlocal) response is poorly quantified. Here, we used remote-sensed observations of forest loss and dry season land-surface temperature during the period 2001 to 2020 to demonstrate that deforestation of the Amazon caused strong warming at distances up to 100 km away from the forest loss. We apply a machine learning approach to show nonlocal warming due to forest loss at 2-100 km length scales increases the warming due to deforestation by more than a factor 4, from 0.16 K to 0.71 K for each 10-percentage points of forest loss. We estimate that rapid future deforestation under a strong inequality scenario could cause dry season warming of 0.96 K across Mato Grosso state in southern Brazil over the period 2020 to 2050. Reducing deforestation could reduce future warming caused by forest loss to 0.4 K. Our results demonstrate the contribution of tropical deforestation to regional climate warming and the potential for reduced deforestation to deliver regional climate adaptation and resilience with important implications for sustainable management of the Amazon.

Subsurface warming in the tropical Atlantic for the last 3 deglaciations: Insights from organic molecular proxies

During glacial terminations, the role of cross-equatorial currents in the northward spreading of warm and salty waters is essential to the reach of the following interglacial state. In the western tropical Atlantic, where the interhemispheric North Brazil Current develops, most of the existing temperature records focused on the last deglaciation. Here, using a marine sediment core covering the last 305 kyr before present (BP), we present two records of the surface (mixed layer) and the subsurface (upper thermocline) temperatures based on organic proxies UK’37 and TEXH86, respectively. During the last three glacial terminations, these records exhibit large temperature increases especially in the subsurface layer. We suggest the combined influences of the thermal bipolar seesaw and the Agulhas Leakage, acting together to warm the interior South Atlantic, to explain the strong warming of the tropical western Atlantic thermocline (+5 °C) during these periods of reduced Atlantic meridional overturning circulation (AMOC). In addition, we propose an enhanced cross-equatorial gyre to elucidate the synchronous deglacial warming of the western and eastern sides of the tropical Atlantic. The resumption of the AMOC at the onset of the following interglacial leads to the release of heat stored in thermocline waters mainly in the whole South Atlantic, as highlighted by the sharp upper ocean cooling recorded at the end of the deglacial interval. The combination of this temperature record and previously published ones in the western tropical Atlantic confirms the strong sensitivity of the upper ocean to AMOC changes in this region.