Uneven future greening across the northern high latitudes: Regional responses to rising CO2

With polar amplification warming the northern high latitudes at an unprecedented rate, understanding the future dynamics of vegetation and the associated carbon-nitrogen cycle is increasingly critical. This study uses the dynamic vegetation model LPJ-GUESS 4.1 to simulate vegetation changes for a future climate scenario, generated by the EC-Earth3.3.1 Earth System model, with the forcing of a 560 ppm CO 2 level. Using climate output from an earth system model without coupled dynamic vegetation, to run a higher resolution dynamic vegetation standalone model, allows for a more in depth exploration of vegetation changes. Plus, with this approach, the drivers of high latitude vegetation changes are isolated, but there is still a complete understanding of the climate system and the feedback mechanisms that contributed to it. Our simulations reveal an uneven greening response. The already vegetated Southern Scandinavia and western Russia undergo a shift in species composition as boreal species decline and temperate species expand. This is accompanied by a shift to a carbon sink, despite higher litterfall, root turnover and soil respiration rates, suggesting productivity increases are outpacing decomposition. The previously barren or marginal landscapes of Siberia and interior Alaska/Western Canada, undergo significant vegetation expansion, transitioning towards more stable, forested systems with enhanced carbon uptake. Yet, in the previously sparsely vegetated northern Scandinavia, under elevated CO 2 temperate species quickly establish, bypassing the expected boreal progression due to surpassed climate thresholds. Here, despite rising productivity, there is a shift to a carbon source. The deeply frozen soils in central Siberia resist colonisation, underscoring the role of continuous permafrost in buffering ecological change. Together, these results highlight that CO 2 induced greening does not always equate to enhanced carbon sequestration. The interplay of warming, nutrient constraints, permafrost dynamics and disturbance regimes creates divergent ecosystem trajectories across the northern high latitudes. These findings illustrate a strong need for regional differentiation in climate projections and carbon budget assessments, as the Arctic’s role as a carbon sink may be more heterogeneous and vulnerable than previously assumed. • A CO 2 level of 560 ppm drives uneven greening across the northern high latitudes. • Greening does not always equate to more carbon storage. • Permafrost, nutrients and disturbance drive ecosystem paths. • Siberia and interior Alaska green rapidly, forming stable forests with high C uptake. • S. Scandinavia and W. Russia shift to temperate species, losing soil C despite growth. • Central Siberian permafrost resists change, limiting vegetation expansion.