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Arctic permafrost is yet the largest non-seasonal component of Earth's cryosphere and has been proposed as a climate tipping element. Already today, permafrost thaw and ground ice loss have detrimental consequences for Arctic communities and are affecting the global climate via carbon-cycle–feedbacks. However, it is an open question whether climatic changes drive permafrost loss in a way that gives rise to a tipping point, crossing of which would imply abrupt acceleration of thaw and disproportional unfolding of its impacts.Here, we address this question by geospatial analyses and a comprehensive literature review of the mechanisms and feedbacks driving permafrost thaw across spatial scales. We find that neither observation-constrained nor model-based projections of permafrost loss provide evidence for the existence of a global-scale tipping point, and instead suggest a quasi-linear response to global warming. We identify a range of processes that drive rapid permafrost thaw and irreversible ground ice loss on a local scale, but these do not accumulate to a non-linear response beyond regional scales.We emphasize that it is precisely because of this overall linear response, that there is no „safe space“ for Arctic permafrost where its loss could be acceptable. Every additional amount of global warming will proportionally subject additional land areas underlain by permafrost to thaw, implying further local impacts and carbon emissions.

Within the framework of the project IMPOSE (Emit now, mitigate later? IMPlications of temperature OverShoots for the Earth system) six idealized emission-overshoot simulations have been performed with the Earth System Model NorESM2-LM2 and used as forcing for the 2nd generation dynamic global vegetation model LPJ-GUESS with its fire-model SIMFIRE-BLAZE to investigate the impact of different CO2 overshoots on global wildfire regimes.The simulations describe a set of scenarios with high, medium, and low accumulative CO2 emissions and each of which has a short (immediate) and a long (100 years) peak of accumulative CO2 emissions before declining towards a baseline simulation of 1500 PgC accumulatively emitted within the first 100 years.The results show that the height of the overshoot has an impact on global fire regimes while its duration does not seem to play a significant role 200 years after peak CO2. Overall, we can see that changes in vegetation composition following the temperature anomaly are the main driver for changes in global wildfire frequency. While in the low overshoot scenarios burnt area has almost converged towards the baseline simulation, the extremest scenarios show the lowest burnt area at the end of the simulation period, indicating that vegetation changes, especially in low latitudes, have been most significant and/or are still ongoing.

There is relevant research on temporal carbon accumulation changes, mostly in arctic permafrost peatlands in Norway, but little is known about the differences and comparisons with more oceanic and lower latitude peatlands in the region, where rainfall is one of the main climatic drivers. Climate projections in Norway for 2031-2060 and 2071-2100 show a rise in mean temperature and an increase of annual rainfall with more intense seasonal events in western, eastern, and northern parts. Under this rationale, this study hypothesizes that temporal variability of temperature and precipitation during the Holocene led to weaker and stronger evapotranspiration and moisture signals affecting local and regional vegetation in peatland ecosystems, water-table changes, and carbon accumulation capacity. This study will contribute to the generation of evidence of the roles and interactions of hydrology, temperature, vegetation, and land use changes on peatbogs carbon accumulation capacity during the Holocene. It will help to disentangle the responses of the carbon budget at different time scales. Methods involve the use of a set of proxies such as pollen, testate amoeba, LOI and bulk density to reconstruct the peat composition rate, organic matter, water table, and local (and regional) vegetation. A generation of an age-depth model and further multivariate analysis will allow to investigate the relationship between the proxies and carbon accumulation rate over the Holocene to further understand the temperature/precipitation correlation and the effects of a changing climate on the carbon budget.

With the current rate of climate change, exceeding the remaining global carbon budget for a warming target of 1.5 °C becomes increasingly likely. Hence, attention has been put towards carbon dioxide removal (CDR) strategies that, given the required socio-economic feasibility and sustainability, allow temporary emission overshoots. The balance between the potential viability of overshoot scenarios and the associated risks depends on their characteristics and how the global ecosystems respond during and after the overshoot. Here we investigate the global climate and regional ecosystem responses to emission-driven overshoot scenarios of different magnitude, duration and timing, imposing the potential for irreversible changes. Our analysis focuses on the behavior and reversibility of thermodynamic, hydrological, and biogeochemical processes and their impacts on a regional scale. Based on results from state-of-the-art Earth system models, physically driven mechanisms appear to be mostly reversible for moderate overshoot scenarios considering a response lag in the range of decades. However, feedbacks in high overshoot scenarios and biogeochemical processes show signs of irreversibility on a larger spatial scale – some of which have been brought into connection with the crossing of tipping points for certain elements of the Earth system. Our analysis informs about the reversibility of climate system processes, which allows refining thresholds for global climate change mitigation policies. Furthermore, the aspect of partial reversibility of temperature overshoot scenarios might not be the main concern, but rather the impacts and risks occurring during the periods of elevated temperatures during the overshoot.

Limiting global warming to 1.5°C by the end of the century currently seems to be an ambitious target which will potentially be accompanied by a period of temperature overshoot. Achieving this climate goal might require massive carbon dioxide removal on large scales. Regardless of the feasibility of such removals, their effects on biogeochemical cycles and climate are not well understood. Changes in atmospheric CO2 concentration ([CO2]) and climate alter the CO2 exchange between the atmosphere and the underlying carbon reservoirs of land and ocean. Carbon-concentration and carbon-climate feedback metrics are useful tools for quantifying such changes in the carbon uptake by land and ocean, currently acting as a sink of carbon. We investigate the changes in carbon feedbacks under overshoot scenarios that could influence mitigation pathways to achieve the temperature goal. An ensemble of Coupled Model Intercomparison Project 6 (CMIP6) Earth system models that conducted an idealized ramp-up and ramp-down experiment (1pctCO2, with increasing and later decreasing [CO2] at a rate of 1% per year) has been used and compared against a scenario simulation involving negative emissions (SSP5-3.4-OS). The analyses are based on results from biogeochemically coupled (where land and ocean respond to rising CO2 levels but the climate is kept constant) and fully coupled simulations. For the positive emission phases, the model-mean global average carbon-climate feedback looks roughly similar between the SSP5-3.4-OS and the 1pctCO2 simulations, with a gradual monotonic decreasing behavior in absolute values which translates to a reduction in land and ocean uptakes. The carbon-concentration feedback in SSP5-3.4-OS is larger than in the 1pctCO2 simulations over the ocean. Both the ocean and land simulate an increase in carbon uptake during the ramp-up, while during the ramp-down, their uptakes show a hysteresis behavior. This feature is more prominent in the idealistic 1pctCO2 experiment with a higher [CO2] growth rate and without land use change effects than in the more realistic SSP5-3.4-OS scenario. Also, the time evolution of the global annual carbon-concentration and carbon-climate feedbacks seem to be very similar over natural land areas. In addition, changes in carbon fluxes are compared over the high latitude permafrost and non-permafrost regions in the Northern Hemisphere. Over land, the carbon-concentration feedback metric is decomposed into different terms to investigate the contributions from changes in live vegetation carbon pools and soil carbon pools. This indicates that the feedback is dominated by the residence time of carbon in vegetation and soil. Furthermore, building on previous studies, feedback metrics are also calculated using an alternative approach of instantaneous flux-based feedback metrics to further compare differences between models. The difference between the two approaches can be seen more obviously in the geographical distribution of the two feedbacks, especially for the negative emission phases of the 1pctCO2 experiment.

Abrupt permafrost thawing is expected to release large amounts of greenhouse gasses to the atmosphere, creating a positive feedback to climate warming. There is, however, still large uncertainty in the timing, duration, magnitude, and mechanisms controlling this process, which hampers accurate quantification of permafrost carbon climate feedback cycles. The current understanding supports that abrupt permafrost thaw will lead to surface inundation and create anaerobic landscapes, which dominantly produce methane during the decomposition process. Over time, natural succession and vegetation growth may decrease methane release and increase net carbon uptake. We investigated how rapid permafrost thawing and subsequent natural succession over time affect CO2, CH4, and N2O release at a field site in northern Norway (69ᵒN), where recent abrupt degradation of permafrost created thaw ponds in palsa peat plateau-mire ecosystems. The site exhibits a natural gradient of permafrost thaw, which also corresponds to a strong hydrological gradient (i.e. dry peat plateau underlain by intact permafrost, seasonally inundated thaw slumps, thaw ponds, and natural succession ponds covered by sphagnum and sedges). Since 2017, we used a range of manual and automated techniques to measure changes in vegetation, soil and water microclimate, biogeochemistry, and soil CO2, CH4, and N2O concentrations and fluxes across the permafrost thaw gradient. In the three-year observations, we show that abrupt permafrost thaw and land surface subsidence – both intermediate slumping and pond formation – increase net annual carbon loss. Permafrost thaw accelerated CO2 release greatly in thaw slumps (177.5 gCO2 m-2) compared to intact permafrost peat plateau (59.0 gCO2 m-2). During the growing season, peat plateau was a small sink of atmospheric CH4 (-2.5 gCH4 m-2), whereas permafrost thaw slumping and pond formation increased CH4 release dramatically (ranging from 9.7 to 36.1 gCH4 m-2). Furthermore, CH4 release continues to increase even in natural succession pond likely due to aerenchyma transport of CH4 from deeper soil. The overall N2O release was negligeable except in the bare soil peat plateau. The net radiative forcing of ecosystem carbon balance will depend on the carbon uptake from the natural succession of vegetation, but we show that greenhouse gas emissions continue to increase beyond abrupt permafrost thaw event towards natural succession.

The Atlantic meridional overturning circulation (AMOC) transports large amount of heat northwards, where this heat contributes to warm the atmosphere. It is well established that an AMOC collapse, or strong reduction would substantially cool the northern high latitudes. New Earth system model (ESM) simulations indicate the possibility that such cooling, due to the large inertia of the climate system, could also happen when CO2 emissions are phased out after AMOC has been weakened by preceding climate change. Our simulations show a temporally and spatially strongly variable zero emission commitment (ZEC), which is first negative (up to -0.4K) and then turns positive (up to 0.5K) as AMOC recovers. Regionally, the strongest cooling-warming pattern is found in the northern high latitudes north of 40°N, where the amplitude of this cycle can exceed several degrees. The mechanism of this cooling can be understood by the fact that the radiative forcing of CO2 is reduced when emissions become zero while the Earth system is in a state of reduced northward heat transport. We further show that this northern high-latitude cooling could be amplified under deliberate CO2 removal and result in a temporary undershoot of a targeted temperature level. Such “stronger than intended” cooling north of 40°N under net negative emissions is a robust feature of Earth system models that show a strong AMOC decline in response to warming, both in idealized simulations but also in an overshoot scenario created for CMIP6 by an integrated assessment model. The future fate of ocean circulation is highly uncertain in ESM projections, and the amplified northern high latitude cooling under zero or net negative emissions is a feature of ESMs with a high sensitivity of AMOC to warming. Nevertheless, or results underscore (i) the importance of understanding the physical climate system’s response to phasing out emissions to avoid surprises, (ii) the importance of coordinated experimentation with ESMs such as CMIP6 ZECMIP and CDRMIP, and finally (iii) the importance of assessing zero and net negative emissions beyond simple metrics like global mean surface temperature.

Permafrost in the north Polar Regions stores more than 1,500 Pg of organic carbon, which is nearly twice as much as the atmospheric carbon pool. As the Arctic region is experiencing unprecedented warming, accelerated decomposition in permafrost is potentially switching it to a hotspot of carbon emissions. In addition to the widely studies carbon dioxide and methane, permafrost may also be a source of biogenic volatile organic compounds (BVOCs), a reactive group of trace gases which have so far received much less attention. BVOCs can prolong the lifetime of methane through the depletion of hydroxyl radicals, contribute to ozone formation, and lead to the formation of secondary organic aerosol, and thus exert significant impact on climate forcing, especially in unpolluted Arctic region.Here, we conducted in situ measurements of soil BVOC emissions on an actively degrading permafrost peatland during a growing season. We compared emissions along a gradient of landscape units from soil palsa and vegetated palsa to thaw slump, thaw pond and vegetated thaw pond. BVOC samples were collected onto absorbent cartridges using dynamic enclosure chamber method, and then analyzed with a gas chromatograph coupled with a mass spectrometer (GC/MS), based upon which the emission rates were calculated.Results suggested that all landscapes units across the peatland showed net emissions of BVOCs during the summertime. Major BVOC groups included monoterpenes, sesquiterpenes, isoprene, hydrocarbons, methanol, acetone, other oxygenated VOCs and other compounds, and these groups were present in all landscape units. All VOC groups also exhibited seasonal and spatial variations across the different sampling months and landscape units. For example, the actively degrading thaw slump showed higher monoterpene emissions that other landscape units, while sesquiterpene emissions were highest from the vegetated thaw ponds. Principal component analysis further revealed temporal and spatial patterns in the relative compositions of the BVOC profiles. Our results show that soil BVOC emissions change in response to active permafrost thaw.

Microtopography can be a key driver of heterogeneity in the ground thermal and hydrological regime of permafrost landscapes. In turn, this heterogeneity can influence plant communities, methane fluxes and the initiation of abrupt thaw processes. Here we have implemented a two-tile representation of microtopography in JULES (the Joint UK Land Environment Simulator), where tiles are representative of repeating patterns of elevation difference. We evaluate the model against available spatially resolved observations at four sites, gauge the importance of explicitly representing microtopography for modelling methane emissions and quantify the relative importance of model processes and the model’s sensitivity its parameters. Tiles are coupled by lateral flows of water, heat and redistribution of snow. A surface water store is added to represent ponding. The model is parametrised using characteristic dimensions of landscape features at sites. Simulations are performed of two Siberian polygon sites, Samoylov and Kytalyk, and two Scandinavian palsa sites, Stordalen and Iškoras. The model represents the observed differences between greater snow depth in hollows vs raised areas well. The model also improves soil moisture for hollows vs the non-tiled configuration (‘standard JULES’) though the raised tile remains drier than observed. For the two palsa sites, it is found that drainage needs to be impeded from the lower tile, representing the non-permafrost mire, to achieve the observed soil saturation. This demonstrates the need for the landscape-scale drainage to be correctly modelled. Causes of moisture heterogeneity between tiles are decreased runoff from the low tile, differences in snowmelt, and high to low-tile water flow. Unsaturated flows between tiles are negligible, suggesting the adequacy of simpler water-table based models of lateral flow in wetland environments. The modelled differences in snow depths and soil moistures between tiles result in the lower tile soil temperatures being warmer for palsa sites. When comparing the soil temperatures for July at 20 cm depth, the difference in temperature between tiles, or ‘temperature splitting’, is smaller than observed (3.2 vs 5.5 °C). The mean temperature of the two tiles remains approximately unchanged (+0.4 °C) vs standard JULES, and lower than observations. Polygons display small (0.2 °C) to zero temperature splitting, in agreement with observations. Consequently, methane fluxes are near identical (+0 to 9 %) to those for standard JULES for polygons, though can be greater than standard JULES for palsa sites (+10 to 49 %). Through a sensitivity analysis we identify the parameters resulting in the greatest uncertainty in modelled temperature. We find that at the sites tested, varying the parameters can result in the modelled July temperature splitting being at most 0.9 or 3 °C larger than observed for palsa or polygon sites respectively. Varying the palsa elevation between 0.5 and 3 m has little effect on modelled soil temperatures, showing that having only two tiles can still be a valid representation of sites with a large variability of palsa elevations. Lateral conductive fluxes, while small, reduce the temperature splitting by ~1 °C, and correspond to the order of observed lateral degradation rates in peat plateau regions, indicating possible application in an area-based thaw model.

Global scale warming has led to permafrost thaw, which may release large amounts of carbon to the atmosphere as CO2 and CH4, potentially accelerating global warming (i.e. a positive feedback). However, uncertainty in the mechanisms controlling carbon mineralization is compounded by concurrent changes in soil hydrology associated with permafrost thaw. Thawing permafrost can lead to surface water accumulation in some areas and seasonal or permanent soil drying in areas where permafrost thaw opens up new channels of water to penetrate into the groundwater system. The complexity of the hydrologic response to permafrost thaw increases the challenge in generating reliable estimates of the permafrost carbon climate feedback. Furthermore, limited observational data exist to i) quantify the effects of permafrost thaw on net tundra carbon budgets, particularly on an annual basis, and ii) as well as constrain the underlying processes governing carbon release under aerobic and anaerobic conditions.Here, we investigated how changes in local hydrology affects CO2 and CH4 release from permafrost soils by establishing a field gradient study in northern Norway (69ᵒ N), where recent abrupt degradation of permafrost created thaw ponds in palsa-mire ecosystems. The site exhibits a natural gradient of permafrost thaw, which also corresponds to a strong hydrological gradient (i.e. dry palsas with intact permafrost, seasonally inundated thaw slumps, and thaw ponds). Since 2017, we have used a range of manual and automated techniques to measure changes in vegetation, soil and water microclimate, biogeochemistry, and soil CO2 and CH4 concentrations and efflux across the permafrost thaw gradient.Our preliminary results show that abrupt permafrost thaw and landscape subsidence – both intermediate slumping and thaw pond formation – increase net annual carbon loss from this type of subarctic wetland. Permafrost thaw approximately doubles CO2 emissions from thaw slumps compared to vegetated or soil palsas. Furthermore, CH4 release greatly increased across the permafrost thaw gradient. While vegetated palsas were small sinks of atmospheric CH4 during the growing season, permafrost thaw slumping and pond formation led to a dramatic increase in CH4 efflux compared to bare palsas. In contrast, bare soil palsas on were the most important source of N2O. Soil profile CO2 and CH4 concentrations in thawed permafrost plots were overall highly enriched relative to palsa profiles, reflecting soil conditions with inundated pore space and low oxygen availability along the permafrost thaw gradient. We therefore conclude that abrupt thaw will increase annual carbon loss in subarctic palsa wetlands.

Plant phenology plays a fundamental role in land-atmosphere interactions, and its variability and variations are an indicator of climate and environmental changes. For this reason, current land surface models include phenology parameterizations and related biophysical and biogeochemical processes. In this work, the climatology of beginning and end of the growing season, simulated by seven state-of-the-art European land surface models, is evaluated globally against satellite observations. The assessment is performed using the vegetation metric leaf area index and a recently-developed approach, named four growing season types. On average, the land surface models show a 0.6-month delay in the growing season start, while they are about 0.5 months earlier in the growing season end. Difference with observation tends to be higher in the Southern Hemisphere compared to the Northern Hemisphere. High agreement between land surface models and observations is exhibited in areas dominated by broad-leaf deciduous trees, while high variability is noted in regions dominated by broad-leaf deciduous shrubs. Generally, the timing of the growing season end is accurately simulated in about 25 % of global land grid points versus 16 % in the timing of growing season start. The refinement of phenology parameterization can lead to better representation of vegetation-related energy, water, and carbon cycles in land surface models, but plant phenology is also affected by plant physiology and soil hydrology processes. Consequently, phenology representation and, in general, vegetation modelling is a complex task, which still needs further improvement, evaluation, and multi-model comparison.

A new generation of land surface models (LSMs) have been developed in the framework of the EU-funded CRESCENDO project aiming to improve understanding of the Earth system as part of the community CMIP6 effort. These new LSMs explicitly represent key processes in the carbon and nitrogen cycles, enabling more realistic vegetation-climate interactions to be simulated. For instance, vegetation phenology, the seasonality of vegetation, is explicitly represented in all these new LSMs. Intra- and inter-annual variations in vegetation phenology can substantially influence land-atmosphere exchanges of energy, moisture and carbon. Changes in phenological events also provide clear indicators of climate impacts on ecosystems. Results are presented on the evaluation of phenological variability from offline runs of this new generation of LSMs. In particular, the timing of growing season onset and offset at global scale, and the Leaf Area Index (LAI) peak timing are investigated using monthly mean outputs. Three satellite-derived LAI datasets are used as benchmark observations for this evaluation.In general, LSMs exhibit high skill in reproducing the observed phenology cycle in the North hemisphere mid- and high-latitudes, while lower skill is obtained in the South hemisphere. All LSMs simulate an offset in the timing of the active vegetative season characterized by later onset and LAI peak. Offset timings are slightly better captured by the LSMs. For these reasons, further development of the representation of phenology is required in LSMs, especially in the South hemisphere, where more complex vegetation and reduced in-situ observations are available.

Biological nitrogen fixation (BNF) is a key contributor to sustaining the terrestrial carbon cycle, providing nitrogen input that plants require. This is particularly salient for projections of carbon uptake under increased atmospheric carbon dioxide concentrations, which may allow for so-called ‘carbon dioxide fertilisation’ if other plant requirements, such as nitrogen, do not prevent increases in productivity. The amount, processes, and global distribution of BNF is highly disputed and consequently land surface models represent it in varying ways. Looking at the latest generation of CMIP6 earth system models with terrestrial nitrogen cycles, we consider their performance with regard to BNF. We assess models against a new comprehensive meta-analysis of BNF field measurements that gives a global range and site-specific values. We find that compared to the wide range of upscaled observations, the models still have a larger range, with under and overestimates.