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The current generation of Earth system models exhibits large inter-model differences in the simulated climate of the Arctic and subarctic zone, with differences in model structure and parametrizations being one of the main sources of uncertainty. One particularly challenging aspect in modelling is the representation of terrestrial processes in permafrost-affected regions, which are often governed by spatial heterogeneity far below the resolution of the models' land surface components. Here, we use the Max Planck Institute (MPI) Earth System Model to investigate how different plausible assumptions for the representation of permafrost hydrology modulate land–atmosphere interactions and how the resulting feedbacks affect not only the regional and global climate, but also our ability to predict whether the high latitudes will become wetter or drier in a warmer future. Focusing on two idealized setups that induce comparatively “wet” or “dry” conditions in regions that are presently affected by permafrost, we find that the parameter settings determine the direction of the 21st-century trend in the simulated soil water content and result in substantial differences in the land–atmosphere exchange of energy and moisture. The latter leads to differences in the simulated cloud cover during spring and summer and thus in the planetary energy uptake. The respective effects are so pronounced that uncertainties in the representation of the Arctic hydrological cycle can help to explain a large fraction of the inter-model spread in regional surface temperatures and precipitation. Furthermore, they affect a range of components of the Earth system as far to the south as the tropics. With both setups being similarly plausible, our findings highlight the need for more observational constraints on the permafrost hydrology to reduce the inter-model spread in Arctic climate projections.

Under increased warming from ongoing anthropogenic climate change, the land acts as an energy sink for the climate system, interacting with the atmosphere at a wide range of time scales. Based on CMIP multi-model comparisons, the latest estimates of the global energy budget quantify the land contribution to be 2% in the last six decades, whereas other studies based on borehole temperature profiles scale it up to 5%. This discrepancy is suspected to stem from state-of-the-art CMIP land surface models using a shallow zero flux bottom boundary condition placement (BBCP) that severely constrains land energy storage by halting ground heat flux penetration at the BBCP depth and biasing subsurface thermal structure. A 2000-year-long (past2k) forced simulation using a version of the Max Planck Institute (MPI) Earth System Model (ESM) with a deep BBCP (1417 m) was performed to assess the behavior of subsurface temperature and energy storage at long-term scales. Results show that land energy uptake is 4 times higher in a coupled MPI-ESM simulation with a deep version of the land component compared to standard shallow (~10m) simulations. These estimates are well above those provided by CMIP6 models and are much closer to observations, underlining the importance of BBCP-depth in correctly representing the role of the land component in the global energy budget. The results of the analysis of the past2k simulation also allow for deriving reliable estimates of land energy uptake from other observational and reanalysis products as well as providing corrected estimates for the shallow LSM CMIP6 historical and scenario simulations. Land energy uptake estimates rendered from this new approach are much closer to previous BTP-based estimates and agree with the value derived from MPI-ESM deep simulation.

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.

Under current climate-change conditions, the energy imbalance at the top of the atmosphere results in an uptake of energy by the Earth system. Previous efforts have identified the magnitude and proportions of this energy excess and how it is distributed among the different components of the climate system. However, the bulk of the Earth System Models (ESMs) participating in CMIP5/6 deliver Earth energy inventory estimates that differ substantially from recent observations. Particularly for the land component, there is a significant underestimation of simulated continental energy uptake, which was hypothesized to be caused by too shallow land surface model components in current-generation ESMs. Support for the latter was given by previous modeling estimates based on analytical heat conduction models and standalone land surface model simulations. Here we use a suite of current-generation fully-coupled CMIP6 ESMs and a version of the MPI-ESM that includes a deep land model component, accommodating the required space for increased terrestrial energy storage. The simulations show that a sufficiently deep land model leads to more realistic subsurface energy storage - correlating with model depth rather than climate sensitivity, and an adjusted estimate of energy uptake ratios among the Earth system components compared to observational estimates. However, the impact of changes in the land energy budget from the perspective of the entire Earth system appears to have only a marginal influence due to its relatively small fraction of the Earth energy inventory.

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.

An assessment of the soil and bedrock thermal structure of the Sierra de Guadarrama, in Central Spain, is provided using subsurface and ground surface temperature data coming from four deep (20 m) monitoring profiles belonging to the Guadarrama Monitoring Network (GuMNet), and two shallow (1 m) from the Spanish Meteorology Service (AEMET), covering the time span of 2015–2021 and 1989–2018, respectively. An evaluation of air and ground surface temperature coupling shows soil insulation due to snow cover is the main source of seasonal decoupling, being especially relevant in winter at high altitude sites. Temperature propagation in the subsurface is characterized by assuming a heat conductive regime, by considering apparent thermal diffusivity values derived from the amplitude attenuation and phase shift of the annual cycle with depth. For the deep profiles, the apparent thermal diffusivity ranges from 1 to 1.3 10−6 m2s−1, consistent with values for gneiss and granite, the major bedrock components in the Sierra de Guadarrama. However, thermal diffusivity is lower and more heterogeneous in the soil layers close to the surface (0.4–0.8 10−6 m2s−1). An increase of diffusivity with depth is observed, being generally larger in the soil-bedrock transition, at 4–8 m depth. A new method based on the spectral attenuation of temperature harmonics allows for analyzing thermal diffusivity from high-frequency changes in the soil near the surface at short timescales. The results are relevant for the understanding of soil thermodynamics in relation to other soil properties and suggest that changes in heat diffusivity are related to soil moisture content changes, which makes this method a potential tool in soil drought and water resource availability reconstruction from soil temperature data.

Reversibility is, next to socio-economic feasibility and sustainability, key for assessing if carbon dioxide removal (CDR) could be considered to return the Earth system to a less dangerous state after a period of temperature overshoot above a level that is considered safe. Here, we use a state-of-the-art Earth system model that includes a representation of permafrost carbon to investigate the reversibility of the Earth system after overshoots of different duration and magnitude in idealized simulations. We find that atmospheric CO2 concentrations are slightly lower after an overshoot, compared to a reference simulation without overshoot, due to a near-perfect compensation of carbon losses from land by increased ocean carbon uptake during the overshoot periods. Many aspects of the Earth system including global average surface temperature, marine and terrestrial productivity, strength of the Atlantic meridional overturning circulation, surface ocean pH, surface O2 concentration, and permafrost extent are reversible on a centennial time scale except in the most extreme overshoot scenario considered in this study. Consistent with previous studies, we find irreversibility for permafrost carbon and deep ocean properties like sea water temperature, pH, and O2 concentrations. We do not find any indication of tipping points or self-reinforcing feedbacks that would put the Earth system on a significantly different trajectory after an overshoot. Hence, irreversibility might not be the main issue of CDR but rather the impacts and risks that would occur during the period of elevated temperatures during the overshoot.

The representation of the terrestrial thermal and hydrological states in current-generation climate models is crucial to have a realistic simulation of the subsurface physical processes and land-atmosphere coupling. This is particularly important for high-latitude permafrost regions since these areas are prone to the release of substantial amounts of carbon from degrading permafrost under climate-change conditions. Many current-generation climate models still have deficiencies in the representation of terrestrial structure and physical mechanisms, such as too shallow land depth or insufficient hydro-thermodynamic coupling. We therefore introduce a deeper bottom boundary into the JSBACH land surface model. The associated changes in the simulated terrestrial thermal state influence the near-surface hydroclimate when sufficient coupling between the thermodynamic and hydrological regimes is present. Hence, we also assess the influence of introducing various physical modifications for the representation of soil hydro-thermodynamic processes in climate projections of the 21st century. The results show significant impacts on terrestrial energy uptake, as well as changes in global near-surface ground temperatures when introducing the physical modifications. The resulting simulation of high-latitude permafrost extent is subject to large variations depending on the model configuration, reflecting the uncertainty of carbon release from permafrost degradation. We further use the modified model to assess the sensitivity of simulated high-latitude climate dynamics to different hydrological configurations in the coupled MPI-ESM. The differences in soil hydrological representation in permafrost regions could explain a large part of CMIP6 inter-model spread in simulated Arctic climate, with remote effects on subarctic dynamical systems.