The long-term net sink of carbon (C), nitrogen (N) and greenhouse gases (GHGs) in the northern permafrost region is projected to weaken or shift under climate change. But large uncertainties remain, even on present-day GHG budgets. We compare bottom-up (data-driven upscaling, process-based models) and top-down budgets (atmospheric inversion models) of the main GHGs (CO2, CH4, and N2O) and lateral fluxes of C and N across the region over 2000-2020. Bottom-up approaches estimate higher land to atmosphere fluxes for all GHGs compared to top-down atmospheric inversions. Both bottom-up and top-down approaches respectively show a net sink of CO2 in natural ecosystems (-31 (-667, 559) and -587 (-862, -312), respectively) but sources of CH4 (38 (23, 53) and 15 (11, 18) Tg CH4-C yr-1) and N2O (0.6 (0.03, 1.2) and 0.09 (-0.19, 0.37) Tg N2O-N yr-1) in natural ecosystems. Assuming equal weight to bottom-up and top-down budgets and including anthropogenic emissions, the combined GHG budget is a source of 147 (-492, 759) Tg CO2-Ceq yr-1 (GWP100). A net CO2 sink in boreal forests and wetlands is offset by CO2 emissions from inland waters and CH4 emissions from wetlands and inland waters, with a smaller additional warming from N2O emissions. Priorities for future research include representation of inland waters in process-based models and compilation of process-model ensembles for CH4 and N2O. Discrepancies between bottom-up and top-down methods call for analyses of how prior flux ensembles impact inversion budgets, more in-situ flux observations and improved resolution in upscaling.

The northern permafrost region has been projected to shift from a net sink to a net source of carbon under global warming. However, estimates of the contemporary net greenhouse gas (GHG) balance and budgets of the permafrost region remain highly uncertain. Here we construct the first comprehensive bottom-up budgets of CO2, CH4, and N2O across the terrestrial permafrost region using databases of more than 1000 in-situ flux measurements and a land cover-based ecosystem flux upscaling approach for the period 2000-2020. Estimates indicate that the permafrost region emitted a mean annual flux of 0.36 (-620, 652) Tg CO2-C y-1, 38 (21, 53) Tg CH4-C y-1, and 0.62 (0.03, 1.2) Tg N2O-N y-1 to the atmosphere throughout the period. While the region was a net source of CH4 and N2O, the CO2 budget was near neutral with large uncertainties. Terrestrial ecosystems remained a CO2 sink, but emissions from fire disturbances and inland waters largely offset the sink in vegetated ecosystems. Including lateral fluxes, the permafrost region was a net source of C and N, releasing 136 (-517, 821) Tg C y-1 and 3.2 (1.9, 4.8) Tg N y-1. Large uncertainty ranges in these estimates point to a need for further expansion of monitoring networks, continued data synthesis efforts, and better integration of field observations, remote sensing data, and ecosystem models to constrain the contemporary net GHG budgets of the permafrost region and track their future trajectory.

The Arctic is one of the regions in our planet with strongest warming observed and it is also almost certain to continue to change in the near future. The continuous change in key indicators of Arctic climate change (e.g. increase of temperature, intensification of the hydrological cycle, and shortening of the spring snow cover) will have marked consequences on ecosystem carbon (C) sink-source functioning. Such consequences are, however, broadly uncertain. Comprehensively integrated ecosystem models with long-term in-situ data are essential to understand the Arctic C cycle sensitivity to climate change and explore robust future scenarios. Our aim is to quantify the relative sensitivity of Greenland’s C balance to climate change based on regional variation in C and N cycling in a tundra gradient. The key roadblocks to this understanding have been limited time series of C fluxes, and limited regional data. Now with observations from multiple data streams measured by the Greenland Ecosystem Monitoring (GEM) program over the last two decades in conjunction with proven ecosystem and climate models we 1) analyse the underlying processes and links between present climate and terrestrial C and N cycling and 2) forecast the variation of plant phenology, productivity, and respiration forward in time. We use an established but novel C cycle model, the Soil-Plant-Atmosphere model, applied to two GEM wetlands relying on previous substantiated efforts on source-code model implementation, model calibration, and validation based on quality-controlled long-term data. Additionally, our modelling framework is now forced with future projections from the regional climate model HIRHAM5 specifically designed to characterize the Greenland domain (typically left behind in global modelling analyses) following the IPCC greenhouse gas emission scenarios. We ask the ecological question: How sensitive is the C balance expected to be under warmer and wetter conditions forecasted for the 21st century? Although still preliminary, we found strong evidence that the net C exchange will be significantly exposed to higher temperatures and intensified precipitation levels increasing 10-80% the C sink strength by the end of the century, but lengthening of the growing season and nutrient availability will also play a significant role.