Bolin Centre for Climate Research

Palsa mires are located at the southern border of the permafrost region. Rising temperatures have led to widespread permafrost thaw and rapid palsa and peat plateau degradation. Palsa mires store significant amounts of carbon (C), which is released when permafrost thaws, potentially changing the peatlands from long-term C sinks to temporary C sources. To improve predictions of how palsa mires will respond to ongoing climate change, a better understanding of their past dynamics is vital. Here, we reconstruct the permafrost history of a peat plateau in Karlebotn, northern Norway, using a multiproxy approach. A peat profile was dated by accelerator mass spectrometry radiocarbon method, and analyzed for oribatid mite communities, peat type and peat properties. Oribatid mite analysis is a relatively novel method suggested as a proxy for permafrost initiation. Here, we compared it to more established approaches based on peat characteristics and vegetation changes. Our results suggest that gradual permafrost aggradation at the site began after ca. 1000 calibrated years Before Present (cal year BP; present = 1950 AD) with stable permafrost conditions established by ca. 400 cal year BP. Three peatland succession phases were identified: the first wet fen phase, a transitional phase with wet and dry condition taxa and the last phase dominated by species typical of dry and permafrost-associated conditions. Our results show that integrating oribatid mite analysis into paleoecological studies offers a more robust reconstruction of past permafrost dynamics, thereby enhancing our understanding of the past, present and future development of palsa mires.

Abstract Rivers are increasingly fragmented and degraded, yet the European Union Nature Restoration Regulation calls for restoring at least 25,000 kilometres of free-flowing rivers by 2030. Translating this ambition into effective implementation remains challenging because restoration priorities differ across ecological, social, economic, and governance contexts. Here, we synthesize expert knowledge from 45 countries through a structured, multi-step prioritization process to identify research priorities for restoring free-flowing rivers in Europe. We identified 27 priorities and analysed how expert background and spatial context influenced their ranking. Restoration priorities differed systematically depending on whether experts emphasized ecological integrity, community engagement, economic considerations, or governance capacity, revealing clear patterns in how disciplinary and professional perspectives shape implementation pathways. This demonstrates that restoration strategies cannot be universal but must be adapted to local and regional political, institutional, and ecological conditions. Building on these findings, we propose a structured prioritisation framework that links barrier removal, connectivity restoration, governance mechanisms, and policy instruments to context-specific needs. Together, our results provide an empirically grounded and implementation-oriented roadmap to support European Union Member States in delivering ambitious river restoration targets in a context-sensitive and socially robust manner.

Abstract. We present a fully implicit finite element method for viscous free surface Stokes problems which are common in glaciology and geodynamics. Standard fully implicit iterative solvers diverge for such problems when using large time steps. We address this by incorporating a stabilization term that vanishes upon convergence. This approach enables the use of large time steps, thereby significantly improving the efficiency of simulations. Furthermore, we leverage the fully implicit solver to develop second-order time-stepping schemes that mitigate the high errors associated with first-order methods for large time steps. The methods are tested and evaluated on model domains relevant to geodynamics and glaciology.

The climate and carbon cycle interact in multiple ways. An increase in carbon dioxide in the atmosphere warms the climate through the greenhouse effect, but also leads to uptake of CO2 by the land and ocean sink, a negative feedback. However, the warming associated with a CO 2 increase is also expected to suppress carbon uptake, a positive feedback. This study addresses the question: “under what circumstances could the climate–carbon cycle system become unstable?” It uses both a reduced form model of the climate–carbon cycle system as well as the complex land model JULES, combined with linear stability theory, to show that: (i) the key destabilising loop involves the increase in soil respiration with temperature; (ii) the climate–carbon system can become unstable if either the climate sensitivity to CO2 or the sensitivity of soil respiration to temperature is large, and (iii) the climate–carbon system is stabilized by land and ocean carbon sinks that increase with atmospheric CO2 , with CO2-fertilization of plant photosynthesis playing a key role. For central estimates of key parameters, the critical equilibrium climate sensitivity (ECS) that would lead to instability at current atmospheric CO2 lies between about 11K (for large CO2 fertilization) and 6K (for no CO2 fertilization). Given the apparent stability of the climate–carbon cycle, we can view these parameter combinations as implausible. The latter value is close to the highest ECS values amongst the latest Earth Systems Models. We find that the stability of the climate–carbon system increases with atmospheric CO2 , such that the glacial CO2 concentration of 190 ppmv would be unstable even for ECS greater than around 4.5 K in the absence of CO2 fertilization of land photosynthesis.

The fate of hydrophobic organic pollutants in the marine environment is largely controlled by organic carbon (OC) cycling processes. The Arctic is warming three times as fast as the global average, resulting in a profound alteration in OC fluxes to the Arctic Ocean. Consequently, remote Arctic shelf sediments serve as an ideal receptor for assessments of the impact of OC quantity and quality on pollutant fate.Here, we compiled a database of congener-specific polychlorinated biphenyl (PCB) concentrations (510 entries, 84 sites) in surface sediment of four Eurasian Arctic shelves via integration of new measurements and literature data. Total organic carbon content and isotopic data were retrevied from CASCADE (The Circum-Arctic Sediment CArbon DatabasE) to examine the PCB storage in sediment as a function of OC source (marine versus terrestrial). In order to reduce the impact of variability in water-phase concentrations caused by region, latitude and depositional year, we controlled for these factors. The adjusted concentrations (in ng-PCB/g-OC) in sediment with high fractions of marine OC are 0.82-1.22 log units higher in Barents and Kara Seas and 0.092-1.49 log units higher in Laptev and East Siberian Seas, compared to those with high fractions of terrestrial OC. Albeit uncertainties in current estimations due to wide geographical coverage and correction assumptions, these values are comparable to previously reported differences of partition coefficients between marine and terrestrial OC in other regions (e.g., 0.2-1 log units higher in marine OC sites in Baltic Sea, compared to terrestral OC sites).Based on these results, PCB accumulation in Arctic shelf sediment was predicted for future climate change scenarios using an observational dataset of terrestrial inputs and marine OC fluxes derived from the global state-of-the-art ocean- and sea ice biogeochemistry model FESOM2.1-REcoM3. The accumulated amount of PCBs in marine OC in Arctic shelf sediments from 2000-2100 is estimated to be about 26 tonnes, which is more than 8 times higher than the accumulated amount in terrestral OC from both coastal erosion and riverine inputs. These results demonstrate that shifts in OC fluxes as a consequence of climate change can impact storage capacity of hydrophobic organic pollutants in aquatic systems.

The Arctic is warming nearly four times faster than the global average, with particularly profound impacts along Arctic coastlines. Coastal erosion is accelerating due to longer open-water seasons, stronger winds, and rising permafrost temperatures. These changes have far-reaching consequences for Arctic communities, whose livelihoods and cultural practices are closely tied to local ecosystems and the services they provide. Approximately 1162 seasonal and year-round settlements are located directly along Arctic coasts.This study focuses on an Arctic summer settlement on Qikiqtaruk (Herschel Island) in northwestern Canada, a site where coastal environmental change has been documented by natural science research for several decades. Building on this long-term record, we conducted ecosystem services (ES) mapping to integrate social and natural science perspectives on these changes. Questionnaire-based interviews with a diverse range of stakeholder groups - including Yukon Territorial Park Rangers, Indigenous community members, a Yukon Parks Conservation Biologist and scientific groups from Canada and Europe - were combined with participatory mapping methods. The resulting maps identify a wide range of ES across this Territorial Park and reveal spatial patterns and hotspot areas of ES provision and change. These outputs provide a valuable foundation for future management and planning by linking observed environmental change with human use, values, and dependencies.By bridging the natural and social sciences, this study provides a more comprehensive understanding of the consequences of a rapidly changing and highly sensitive Arctic coastal environment. As one of the first ES assessments conducted in an Arctic community, this work demonstrates the potential of an expanded ES approach to capture the complex socio-ecological impacts of climate change along Arctic coasts.

Arctic Amplification is not well understood. It is the result of a complicated interplay between remote and local forcing and feedback processes. Therefore, it is crucial to enhance our understanding of the transport of energy and moisture from lower latitudes. The amount of aerosol in the Arctic is also an important quantity as their role in Arctic Amplification, via direct radiative forcing and aerosol-cloud interactions, remain poorly quantified.In this work, we aim to better quantify how aerosols, energy, and moisture are transported to and distributed within the Arctic. We investigate observations at Arctic stations, including, Villum and Zeppelin, and perform backward-in-time simulations with the Lagrangian atmospheric transport model FLEXPART (Pisso et al., 2019; Bakels et al., 2024) to derive so-called emission sensitivities and use these sensitivities to better quantify source regions of aerosols, energy, and moisture.In general, we aim to better describe the spatial and temporal atmospheric transport characteristics into the Arctic and how these characteristics have changed in recent years. We focus on the transport during warm-air intrusions, since almost 30% of the total poleward transport of moisture (during winter) occurs during such events (Woods et al., 2013). Warm-air intrusions are often associated with large-scale atmospheric blocking patterns forcing a change in transport direction from east to more poleward, bringing warm, moist, and cloudy air into the Arctic. Warm-air intrusions can also be favourable for an enhanced transport of aerosols (e.g., Dada et al., 2022).Since climate models show large biases in moisture flux during these events (Woods et al., 2017), there is clearly a need to better quantify the transport of moisture, energy, and aerosols during these events. This will also help to provide better forcing for climate simulations.Bakels et al. (2024): 10.5194/gmd-17-7595-2024; Dada et al. (2022): 10.1038/s41467-022-32872-2; Lapere et al. (2024): 10.1029/2023JD039606; Pisso et al. (2019): 10.5194/gmd-12-4955-2019; Woods et al. (2013): 10.1002/grl.50912; Woods et al. (2017): 10.1175/JCLI-D-16-0710.

Soil microorganisms decompose a wide range of organic sources to meet their carbon (C) and energy needs. They further require nutrients such as nitrogen (N) in appropriate stoichiometric ratios to C. Organic sources are often N-poor (high C/N) compared to microbial biomass (low C/N). The extent of this stoichiometric imbalance influences organic matter decomposability, microbial C and N turnover, and ultimately C and N stabilization in soil.Here, we investigate how organic source C/N and system-specific conditions impact the fate of C and N across diverse microbe-plant-soil systems. We synthesized data from 14 published isotope-tracing studies that applied 13C- and 15N-enriched organic sources and quantified the recovery of C and N from these sources in microbial biomass and bulk soil. The applied organic sources included microbial necromass and various plant residues spanning C/N ratios from 4 to 42. Similarly, the soils used in the studies were diverse, with bulk soil C/N ranging from 8 to 35 and pH values from 3 to 13.The relative recovery of source N generally exceeds that of source C in microbial biomass and bulk soil, following the expected greater losses of C through microbial respiration. Moreover, low source C/N resulted in higher relative recoveries of source C and N in microbial biomass and bulk soil, likely reflecting more efficient microbial processing of sources with a stoichiometry that closely matches microbial needs. In addition, system-specific conditions, such as bulk soil C/N, influence the fate of C and N.In our contribution, we aim to provide insights into the joint microbial use of C and N related to organic source stoichiometry and discuss how system-specific conditions and experimental design shape the observed patterns across diverse microbe-plant-soil systems.

Abstract. The Earth's energy imbalance is the difference between incoming solar radiation and outgoing reflected and emitted radiation from the Earth, and quantifies the current ongoing accumulation of energy in the Earth's climate system. There are indications that the imbalance is growing, and it is important to be able to measure and monitor this quantity to better constrain future changes. The reflected shortwave component of the outgoing radiation depends on surface and atmospheric properties, which leads to strong directional variations associated with the angular geometry relative to the incoming sunlight and the observer. The effect of this anisotropy on global average measurements from wide-field-of-view radiometers has been the topic of some investigation in the past, and results from an earlier study suggest that this effect could potentially lead to substantial systematic biases in the context of the global mean reflected shortwave radiation. Here we simulate wide-field-of-view instruments on satellites in polar, sun-synchronous and precessing orbits, as well as constellations of these types of satellite orbits, with both Lambertian and anisotropic shortwave reflection. Reference top-of-atmosphere radiative fluxes are taken from the Clouds and the Earth's Radiant Energy System synoptic data product and combined with angular dependence models that were developed for the Earth Radiation Budget Experiment. We find that the polar regions appear brighter and the midlatitudes appear dimmer with anisotropic reflection, but both the estimated global annual mean and the estimated interannual trend only exhibit limited sensitivity to whether Lambertian or anisotropic reflection is assumed. With anisotropic reflection, the estimated global annual mean root-mean-square sampling error is at most 0.11 W m−2 provided that at least two complementary satellites are used, compared with at most 0.09 W m−2 in the case of Lambertian reflection. The magnitude of the difference in the estimated interannual trend is at most 0.07 W m−2 per decade, and typically only ∼ 0.01 W m−2 per decade. Analysis of the angular sampling of these satellites reveals that the anisotropic reflection requires sufficient sampling of viewing zenith angle and relative azimuth angle, in addition to the solar zenith angle. However, we conclude that it is possible to choose satellite orbits so that the sampling error is not substantially affected by reflected shortwave anisotropy.

<strong class="journal-contentHeaderColor">Abstract.</strong> We present a fully implicit finite element method for viscous free surface Stokes problems which are common in glaciology and geodynamics. Standard fully implicit iterative solvers diverge for such problems when using large time steps. We address this by incorporating a stabilization term that vanishes upon convergence. This approach enables the use of large time steps, thereby significantly improving the efficiency of simulations. Furthermore, we leverage the fully implicit solver to develop second-order time-stepping schemes that mitigate the high errors associated with first-order methods for large time steps. The methods are tested and evaluated on model domains relevant to geodynamics and glaciology.