Abstract. The high-latitude regions of the Northern Hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance -those of the permanently frozen soils (permafrost), and of the great expanse of boreal forestare vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO 2 fluxes, in addition to a recently developed fire module. Outputs from ORCHIDEE-MICT, when forced by two climate input datasets, are extensively evaluated against (i) temperature gradients between the atmosphere and deep soils, (ii) the hydrological components comprising the water balance of the largest highlatitude basins, and (iii) CO 2 flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress andPublished by Copernicus Publications on behalf of the European Geosciences Union. 122M. Guimberteau et al.: ORCHIDEE-MICT, a LSM for the high latitudes a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcingdependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment.

Abstract. The high-latitude regions of the northern hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance – those of the permanently frozen soils (permafrost), and of the great expanse of boreal forest – are vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO2 fluxes, in addition to a recently-developed fire module. Outputs from ORCHIDEE-MICT, when forced by two climate input data sets, are extensively evaluated against: (i) temperature gradients between the atmosphere and deep soils; (ii) the hydrological components comprising the water balance of the largest high-latitude basins, and (iii) CO2 flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress and a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcing-dependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment.

Wildfires generally result in biospheric recovery approximating the pre-disturbance state. However legacy carbon(C) gains and losses that have until now been overlooked in global-scale theory and modelling indicate that post-fire C gains through pyrogenic carbon (PyC) production, and losses via fire regime shifts, post-fire mortality, topsoil loss and inland water export, may be central to whether 20 th century fires have imposed a net terrestrial C source or sink. Here, we integrate PyC production and soil accumulation into a global terrestrial model (ORCHIDEE-MICT) and estimate wildfire C-gains and losses over 1901-2010, quantifying the fire-C balance at global, regional and vegetation scales. Excluding the effect of PyC mineralisation, fires provide a land storage of +177 TgC yr -1 (63% PyC production), dominated by grasslands. The global balance is nuanced, with forest fires resulting in strong terrestrial net C loss:gain ratios (>-2:1) that are greatest in tropical regions (>-3:1). Frequent tropical grassland fires are responsible for the bulk of the land PyC sink and its environmental persistence, whose theoretical minimum mean residence time we quantify at 2760yrs. We highlight the dependency of the global fire-C balance on vegetation coverage and the potential role of preserving grasslands, particularly those in the tropics, in that regard.Wildfires are a key driver of disturbance-recovery cycles in many regions of the world. While fires emit large quantities of C to the atmosphere (~2 PgC yr -1 ) 1 , subsequent vegetation recovery re-captures the emitted C on decadal timescales and results in an uncertain but likely small net impact on atmospheric C in the long run. Natural shifts in fire regimes and vegetation occur infrequently and are largely driven by climatic and human perturbations, such that biomes tend towards quasi-steady state outside of these. It is thus assumed that on decadal to centennial time-, and biome to global spatial-scales:(1) Where 𝐸 ()* " is fire CO2 emissions due to vegetation combustion, and 𝑈 +)* " is uptake of atmospheric CO2 by post-fire vegetation recovery, poles referring to flux direction with respect to C stocks in the terrestrial biosphere.However, recent research on both sides of the flux complicates this perspective. A range of long-term 'legacy' C fluxes traced back to source fire events lead to either C accumulation or loss by land ecosystems, however their balance is yet