Future glacier retreat and forest expansion in the Swiss Alps provide limited benefits for carbon sinks

Glacier retreat as a consequence of climate change creates new ice-free terrain and soil development that prompt plant colonization and ecological succession. These processes impact terrestrial ecosystems with profound ecological and societal consequences. However, quantification of how the carbon cycle evolves in deglaciated areas in response to these processes remains limited. We examined the impacts of forest expansion and soil development on the carbon cycle under climate change in a deglaciated area in the Swiss Alps. Using the mechanistic ecohydrological T&C model, we computed the changes in vegetation, soil, and carbon from 1981 to 2099 under climate change, revealing complex carbon cycle responses in deglaciating ecosystems. Vegetation growth, soil organic matter, and plant nutrient uptake are projected to increase by mid-century and then stabilize, indicating that plant growth is relatively limited by nutrient availability. The amount of carbon stored in plant biomass will increase toward the end of the century at a faster rate than that of carbon stored in soil and litter. The carbon cycle is projected to continue its current accelerating trend characterized by enhanced vegetation photosynthesis, increased plant and soil respiration, and higher net ecosystem production (NEP) by mid-century. Alpine ecosystems have already been serving as carbon sinks and have the potential to increase their carbon sink capacity, but at varying rates depending on how climate will evolve: NEP will stabilize around 24 gC m^−2 y^−1 in RCP4.5 or might elevate to 55 gC m^−2 y^−1 by end-century in RCP8.5. Even under the most extreme scenario, this increase in stored carbon in the proglacial areas of the Swiss Alps is still a drop in the ocean, as it represents only 0.9% of overall Swiss carbon emissions, thus highlighting the need for additional carbon management and mitigation efforts.