Projected impact of combined high-end atmospheric carbon dioxide levels and tree restoration on albedo, forest emissions and carbon uptake

Abstract Tree restoration is seen as a nature-based solution to climate change, because trees remove carbon from the atmosphere. However, tree cover can influence surface temperatures in other ways, for example by changing albedo and enhancing evapotranspiration. These impacts may, in turn, be affected by increasing atmospheric carbon dioxide concentrations. Here, we present simulations with a coupled atmosphere-land-slab-ocean model to investigate how doubled atmospheric carbon dioxide levels affect warming in a high-end scenario where afforestation covers a land area 35% larger than the USA. Changes in albedo, changes in evapotranspiration, and forest biogenic volatile organic compound emissions combined result in reduced warming by 0.06+/-0.04 K in the afforestation scenario with doubled carbon dioxide scenario, compared with afforestation in a baseline scenario with present-day carbon dioxide levels. This reduced warming is largely due to less snow in the Northern Hemisphere and thus less surface darkening. Similarly, tree carbon sequestration enhances cooling by 0.20 K in the afforestation/doubled carbon dioxide scenario, compared to an afforestation/present-day carbon dioxide scenario . We conclude that in a world with doubled atmospheric carbon dioxide concentrations, the climate mitigation potential of tree restoration is only minimally affected.