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.