Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung

During Arctic summer, meltwater inputs and a fragmented ice cover impede quantifying the role of boundary stress for turbulent mixing in the ice–ocean boundary layer. Here, we show that less than two-thirds of the turbulent kinetic energy (TKE) generated from mean flow shear under drifting sea ice is dissipated, and the remainder can be attributed to balancing stabilizing buoyancy fluxes. We deployed a high-resolution acoustic Doppler current profiler under an ice floe to estimate Reynolds stress, shear production, and dissipation rate of TKE. At 0.75 m below the interface, dissipation rates from 1.5 × 10⁻⁹ to 4.2 × 10⁻⁷ m² s⁻³ and shear production from 6.9 × 10⁻¹⁰ to 7.7 × 10⁻⁷ m² s⁻³ were measured (5%–95% percentiles), with shear production exceeding dissipation on average. The turbulent stress was largest during an event with ∼9.2-h-period oscillations in the upper ocean, consistent with tidally forced lee waves generated near steep topography. An overall estimate of the quadratic skin drag coefficient representative of the ice floe is CD0=7.0×10−4. We further identified three qualitative regimes of atmosphere–ice–ocean coupling in our observations: a high-frequency range [>4 cycles per day (cpd)] in which the ice acts like a rigid lid atop the ocean, an intermediate range, and a low-frequency range (<0.8 cpd), where wind-driven ice drift determines the under-ice current. As the latter only contained half of the variance of the ice-relative flow, we emphasize that resolving subdaily time scales is crucial in observing and modeling atmosphere–ice–ocean coupling.

Limited progress with mitigation makes it almost inevitable that global warming of 1.5°C will be exceeded. This realization confronts Parties to the United Nations Framework Convention on Climate Change (UNFCCC) with a choice either to stabilize warming above but as close as possible to 1.5°C or to reverse global warming back to this level. We review core concepts and current knowledge relating to overshoot: an exceedance and subsequent decline back below a specified global warming level. We clarify the concept and origins of overshoot in science and climate policy, discuss the key drivers of climate-related risks and how they might evolve under overshoot trajectories to foster more systematic research into those risks, and consider the role of adaptation. We then consider the feasibility of overshoot in terms of mitigation across the six feasibility dimensions introduced by the Intergovernmental Panel on Climate Change (IPCC) in its sixth Assessment Report. We conclude by discussing critical barriers, challenges, and knowledge gaps related to overshoot.

Cellular solids are appealing for load‐bearing engineering components due to their remarkable global mechanical properties. However, their complexity makes utilizing them challenging and computationally intensive. Homogenization, a common method for simplifying these structures, replaces heterogeneous media with a material possessing equivalent effective properties. Despite its utility, homogenization introduces challenges, particularly the significant influence of lattice geometry on the method's accuracy and the performance of final optimized designs, which is often overlooked. This study evaluates the efficacy of biologically inspired sheet‐based triply‐periodic minimal surface (TPMS) lattices in homogenization‐based stiffness optimization and benchmarks them against other lattice types. Using tailored probe‐based metrics introduced in this study, which measure key relevant attributes such as subtopological homogeneity, load path alignment, resilience to edge effects, and achievable channel clearance, TPMS lattices like gyroids outperform strut‐based lattices across all criteria. This results in significantly enhanced end‐properties of 3D models optimized through numerical homogenization workflows. The findings emphasize the importance of lattice geometry in homogenization‐based optimization and highlight the benefits of TPMS structures in delivering predictable performance with minimal design constraints. Additionally, the metrics developed provide a robust framework for evaluating cellular solid designs, enabling engineers to make more informed lattice design choices in comparable optimization scenarios.

Antarctic basal melt is crucial for the future evolution of the Antarctic ice sheet and ocean circulation. However, few Earth system models explicitly simulate ice-shelf cavities. Here, using an Earth system model with interactive Antarctic ice-shelf cavities, we show that regional hydrography and topography determine a cavity tipping point. The Filchner–Ronne ice-shelf cavity will encounter such a tipping point with abrupt warm-water intrusion, rapid basal melt increase and massive freshwater release in response to increasing CO2 levels within this century. Conversely, the Ross Ice Shelf shows a more gradual response. Our results also suggest that previous ice-sheet modelling overestimated future ice-shelf melt, highlighting the need for comprehensive Earth system models with interactive ice-sheet dynamics and cavities for better climate projections.

Understanding the effects of artificial structures in marine landscapes is required for ecosystem-based management. Global demand for oil and gas and accelerated commitments to renewable energy development has led to the proliferation of marine artificial structures. Investigating the cumulative effects of these structures on marine ecosystems requires data on the benthic community over large geographical and long-time scales. It is imperative to share the data collected by many stakeholders in an integrated information system to benefit science, industry and policy. BISAR is the first data product containing harmonised and quality-checked international data on benthos from artificial structures in the North Sea. BISAR was compiled from environmental impact assessment studies and scientific projects (3864 samples, 890 taxa). Data derive from 34 artificial structures and surrounding soft sediments (years: 2003 to 2019). Structures include offshore wind turbines, oil and gas platforms and a research platform. Data from a geogenic reef, allow comparison of natural and artificial reef communities. We aim to host future BISAR data dynamically in the CRITTERBASE web portal.

Biological nitrogen fixation is an important source of new nitrogen, influencing ocean fertility and carbon uptake. While recently documented in Arctic waters, its role in the Southern Ocean remains uncertain. We measured nitrogen fixation along the Western Antarctic Peninsula and at Palmer Station over two austral summer months. Rates from ¹⁵N2 assay were below conservative detection limits but detectable under less stringent detection thresholds. Continuous acetylene reduction assay provided further support. nifH gene sequencing identified Gammaproteobacteria as the dominating identified diazotrophs, while Epsilonproteobacteria contributed disproportionally to nifH expression when putative nitrogen fixation was highest. Combined with environmental observations, we hypothesize that vertical water mixing resuspended sediments into the water column and contributed to the limited nitrogen fixation. Given the sporadic and low rates, further research is needed to determine whether nitrogen fixation plays a minor role or represents an overlooked process with biogeochemical significance in the Southern Ocean.

The impact of the vertical distribution of tropospheric water vapor on the cloud‐free downward, broadband thermal‐infrared irradiance FTIR $\left({F}_{\text{TIR}}\right)$ was quantified using observations in the Central Arctic, north of 85°N, collected during the Arctic winter. The water vapor profiles were measured with a temporal resolution of 30s $30\,\mathrm{s}$ using a Raman lidar. The observations revealed maximum values of integrated water vapor (IWV) contents of 3.6kgm−2 $3.6\,\mathrm{k}\mathrm{g}\ {\mathrm{m}}^{-\mathrm{2}}$. Seven measurement cases of several‐hour durations of slowly changing air masses were examined. Furthermore, 53 rather short‐term (10 min) measurement cases were studied. The temporal evolution of the slowly changing air masses revealed a linear relationship between FTIR ${F}_{\text{TIR}}$ and IWV with slopes between 7.17 and 12.95Wkg−1 $12.95\,\mathrm{W}\ \mathrm{k}{\mathrm{g}}^{-\mathrm{1}}$ and a coefficient of determination larger than 0.95 for most of the selected cases. The slopes and the ordinate intercepts showed a dependence on the water vapor‐weighted mean temperature (representative temperature of the water vapor distribution). The temperature determined with the Stefan‐Boltzmann law from FTIR ${F}_{\text{TIR}}$ correlated with the representative temperature with a coefficient of determination of 0.92. The analysis of 53 independent short‐term observations of different air masses confirmed the linear relationship between FTIR ${F}_{\text{TIR}}$ and IWV at wintertime cloud‐free conditions in the Arctic (coefficient of determination of 0.75, slope of 19.95Wkg−1 $19.95\,\mathrm{W}\ \mathrm{k}{\mathrm{g}}^{-\mathrm{1}}$, and ordinate intercept of 107.22Wm−2 $107.22\,\mathrm{W}\ {\mathrm{m}}^{-\mathrm{2}}$).

We present a probabilistic approach to climate indices to derive high exposure zones across the European continent, utilizing high-resolution observed data over the last 70 years. 56 distinct climate indices related to drought, global radiation, precipitation, relative humidity, sea level pressure, and temperature are identified, shedding light on the complexity and multifaceted nature of risks encountered by European regions during co-occurrences of the different climatic events. Our findings suggest that precipitation and temperature-based indices are particularly useful in identifying high-risk regions in southern and southeastern Europe, whereas precipitation-based indices are for Northern and Western Europe. Temperature indices and potential evapotranspiration account for most risk exposure to Europe’s dominant land use type. The highest exposure percentage of the population occurs with differences in days above and below the maximum temperature of 17 °C. About 17 climate indices with high-risk magnitudes are present regionally and in specific months, emphasizing diverse risk exposure. Russia, Norway, Iceland, and Sweden experience diverse high-risk co-occurrences, with multiple climate indices related to precipitation and temperature. The findings expand the range of climate indices and demarcate hotspots and risk zones, allowing for more effective climate monitoring and risk mitigation strategies.

Eddies are considered important for the dynamics within the Antarctic Slope Current but are difficult to observe due to the often year‐round ice cover in the southern Weddell Sea. Here we present novel findings from acoustically tracked profiling floats, which observed the spin‐down of an eddy under an ice shelf. Two profiling floats were deployed at 8°W in the Antarctic Slope Current and drifted westward along the slope at 800 m depth. One of the floats was captured by an anticyclonic eddy in the wake of the Riiser‐Larsen Ice shelf. We postulate the eddy was generated by baroclinic instability due to the interaction of the Antarctic Slope Current with floating ice shelves. Float trajectories show the eddy propagated westward along the slope and ultimately became trapped under the Stancomb‐Wills Ice Tongue, where the eddy spun down because of ocean‐ice shelf stresses. Simple bulk mixing calculations were fitted to observations to explore the role of eddies under the ice shelf, in relation to an “Eddy‐Ice‐Pumping” mechanism, and revealed that significant basal melting occurred. Subsequent vertical mixing of the eddy's cold and fresh core, due to the Eddy‐Ice‐Pumping mechanism, resulted in a cold and fresh subsurface signal which was still evident downstream at the Filchner Trough 3 months later. Estimates of eddy contributions to basal melt and freshwater transport show eddies could have a significant impact on the stratification and thermocline depth downstream, potentially affecting the inflow of modified Warm Deep Water into the Filchner Trough.

Anthropogenic sound caused by ship traffic as well as the construction and operation of offshore windfarms have increased exponentially in the last decades. While its impact on marine life is relatively well studied for mammals and fish, the implications of anthropogenic sound on benthic invertebrates are poorly understood. Here, we tested for potential stress responses of common marine invertebrates using two widespread mesograzing crustaceans: the isopod Idotea balthica and the amphipod Gammarus locusta. All experimental animals were gathered from laboratory cultures in the facilities of the Alfred Wegener Institute in Bremerhaven, Germany, in spring 2023. Oxygen consumption rates and the activities of four key mitochondrial enzymes (cytochrome c oxidase, electron transport system complex I and III, citrate synthase and lactate dehydrogenase) were examined under the influence of added low-frequency sound (+ 25 dB SPLRMS re 1 µPa at 90 Hz, above background soundscape) to assess how basal energy demands and supplies were affected. The isopod I. balthica seemed to be robust against added sound exposure over 72 h as neither oxygen consumption rates nor enzyme activities were significantly altered. The amphipod G. locusta, however, displayed significantly lower oxygen consumption rates in response to both short-term (1–4 h; 39% reduction) and longer-term (68–72 h; 35% reduction) added sound exposure, although enzymatic activities were not significantly affected. This study underlines the need to address the potential impact of sound on the energy available for the growth and reproduction of small invertebrates. Overlooked vulnerabilities to noise pollution in key taxa could have far reaching implications for marine food webs, nutrient cycles and ecosystem functioning.

The phytoplankton present on the surface and up to a certain depth of the ocean modifies the ocean’s light absorption by attenuating the incoming irradiance. This light attenuation varies with region and season depending on the phytoplankton concentration and levels. In the majority of climate models, this attenuation is parameterized by a constant coefficient. This incomplete representation can have a cascading effect on the model physics. This study assesses how the phytoplankton-light feedback may affect the Marine heatwaves (MHWs) over the Tropical Indian Ocean (TIO). A fully coupled Regional Earth System Model, namely ROM, is employed. Two experiments over the CORDEX-South Asia domain from historical to future period using RCP8.5 scenarios at 0.22°x0.22° horizontal resolution were performed. In the first experiment (INDJ), a constant light attenuation coefficient (equal to 0.06 m− 1) was applied in the model, and in the second experiment (INDB), a space and time-varying light attenuation coefficient depending upon the phytoplankton concentration was introduced in the model to fully incorporate the phytoplankton-light feedback into the system. Hence, the difference between these two experiments is solely due to the effect of the biochemistry module on how shortwave solar radiation penetrates into the ocean. The results revealed a noticeable difference in the MHWs intensity and duration between experiments. Notably, in the INDB experiment, the First Permanent MHWs and Absolute Permanent MHWs get delayed over approximately 55% and 64% of the area of TIO, respectively. Considerable delays (two decades) in the INDB experiment were found over approximately 29% and 26% of the TIO for the First Permanent MHWs and Absolute Permanent MHWs, respectively. This study suggests that phytoplankton-light feedback can be another physical player for simulating and understanding MHWs in the climate models.

The elemental content of life is a key trait shaping ecology and evolution, yet organismal stoichiometry has largely been studied on a case-by-case basis. This limitation has hindered our ability to identify broad patterns and mechanisms across taxa and ecosystems. To address this, we present StoichLife, a global dataset of 28,049 records from 5,876 species spanning terrestrial, freshwater, and marine realms. Compiled from published and unpublished sources, StoichLife documents elemental content and stoichiometric ratios (%C, %N, %P, C:N, C:P, and N:P) for individual plants and animals. The dataset is standardized and, where available, includes information on taxonomy, habitat, body mass (for animals), geography, and environmental conditions such as temperature, solar radiation, and nutrient availability. By providing an unprecedented breadth of organismal stoichiometry, StoichLife enables the exploration of global patterns, ecological and evolutionary drivers, and context-dependent variations. This resource advances our understanding of the chemical makeup of life and its responses to environmental change, supporting progress in ecological stoichiometry and related fields.

Millennial-scale variations in the strength and position of the Antarctic Circumpolar Current exert considerable influence on the global meridional overturning circulation and the ocean carbon cycle. The mechanistic understanding of these variations is still incomplete, partly due to the scarcity of sediment records covering multiple glacial-interglacial cycles with millennial-scale resolution. Here, we present high-resolution current strength and sea surface temperature records covering the past 790,000 years from the Cape Horn Current as part of the subantarctic Antarctic Circumpolar Current system, flowing along the Chilean margin. Both temperature and current velocity data document persistent millennial-scale climate variability throughout the last eight glacial periods with stronger current flow and warmer sea surface temperatures coinciding with Antarctic warm intervals. These Southern Hemisphere changes are linked to North Atlantic millennial-scale climate fluctuations, plausibly involving changes in the Atlantic thermohaline circulation. The variations in the Antarctic Circumpolar Current system are associated with atmospheric CO2 changes, suggesting a mechanistic link through the Southern Ocean carbon cycle.

The Beaufort Gyre (BG) is an important feature of the Arctic Ocean. By accumulating or releasing freshwater, it influences ocean properties both within the Arctic and as far as the North Atlantic. Yet, its future remains uncertain: the gyre could strengthen as sea ice declines and allows increased wind stress on the ocean, or weaken along with the Beaufort High (BH) pressure system. Here, we provide a first evaluation of the BG in historical and climate‐change simulations from 27 available global climate models. We find that the vast majority of models overestimate the gyre area, strength, and northward extent. After discarding the models with too inaccurate a gyre and its drivers—namely, the sea ice cover and BH—we quantify changes in the BG under two emission scenarios: the intermediate SSP2‐4.5 and the high‐warming SSP5‐8.5. By the end of the 21st century, most models simulate a significant decline or even disappearance of the BG, especially under SSP5–8.5. We show that this decline is mainly driven by a simulated future weakening of the BH, whose influence on the BG variations is enhanced by the transition to a thin‐ice Arctic. The simulated gyre decline is associated with an expected decrease in freshwater storage, with reduced salinity contrasts between the gyre and both Arctic subsurface waters and freshwater outflow regions. While model biases and unresolved processes remain, such possible stratification changes could shift the Atlantic‐Arctic meridional overturning circulation northward.

Plain Language Summary Temperature rise in the High Arctic intensifies permafrost thaw. When ice‐rich permafrost thaws and large amounts of ground ice are lost within years, the landscape lowers and erosion and meltwater produce mudflows that transport sediment, nutrients, and soil organic matter downstream into lakes and the ocean. Yet, little is known about growth rates of mudslides formed by thawing permafrost, so‐called retrogressive thaw slumps, in the Russian High Arctic. We used very high‐resolution satellite images to map the development of thaw slumps at six study sites in the Russian High Arctic between 2011 and 2020. We measured their growth in area and number over time. Climate data including trends of regional precipitation, air temperatures as well as sea surface temperatures and length of ice‐free periods of the nearby Arctic Ocean in these regions clearly indicate a warming trend in recent years. Our results show a rapid increase in recent years in area and in number of retrogressive thaw slumps in several regions in the Russian High Arctic. In previous studies conducted in the Canadian High Arctic, climate warming has shown to be a driver of increasing thaw slump dynamics, and our study suggests similar strong changes in the Russian High Arctic.

Interactions of ice and ocean around Antarctica determine the stability of ice shelves and the onshore ice masses they buttress. These interactions and the exchange of water between the open ocean and ice shelf cavities depend heavily on seabed morphology and depth. However, vast areas of the Antarctic margin remain bathymetrically undersampled, and some, including the cavity beneath the ~44,000 km2 Riiser-Larsen Ice Shelf, Antarctica’s fourth largest, were never yet visited. Here, we use new aerogeophysical and existing oceanographical data to infer bathymetry under the cavity and assess the interactions of ice, ocean and bed. We find a bed shaped by its setting at an extended continental margin, and modified by glacial processes. A 520 m-deep bathymetric gateway at the continental shelf break permits seasonal intrusion of Warm Deep Water into a subglacial trough that allows the warm water to reach the 1000 m deep grounding zone at the ice shelf’s main inlet. The existence of analogous gateways at several ice shelves of Dronning Maud Land indicates that the buttressing effect on a large marine-based portion of the East Antarctic Ice Sheet, albeit whilst currently stable, may be approaching a serious tipping point with minimal changes to the oceanographic regime.

Boreal forests, covering more than half of the world's permafrost, are essential for maintaining permafrost stability. However, climate change and forest shifts are threatening the delicate balance in the thermal equilibrium between the atmosphere, vegetation, and permafrost. We focus on Central Yakutia's ice‐rich boreal regions, specifically two sites located in Spasskaya‐Pad and Churapcha, to investigate the interplay of hydrothermal and climatic conditions that induce thermokarst. We employ a numerical permafrost model (CryoGrid), with a canopy model, and features for excess ground ice, lateral water flow and lake formation, to simulate the underlying physical processes under two forcing scenarios until 2060. The results reveal that forest delays the onset of thermokarst and ground ice melting by 3–18 years, depending on ice depth, climate forcing, and local conditions. Our simulations additionally reveal that a canopy slows excess ice melt by up to 7 years compared to bare ground simulations. Furthermore, in exceptionally warm and wet years, thermokarst initiation occurred rapidly in the bare ground simulations. In contrast, the canopy buffered against these conditions, suggesting that canopies might mitigate the impacts of small temperature and precipitation anomalies. This research highlights the critical role of forests in shaping the trajectory of thermokarst‐related landscape transformations in ice‐rich boreal permafrost regions. With the study region warming faster than average, forest cover transformations could significantly alter the hydrological balance. By integrating thermodynamics, hydrology, and ecology, our findings underscore the importance of forests in delaying thermokarst initiation and slowing ground ice melt, ultimately stabilizing permafrost ecosystems.