Enhanced warming in net-zero scenarios that balance fossil fuel CO2 emissions with CO2 removals through reforestation

Reforestation is increasingly considered an important nature-based climate solution for achieving net-zero CO ₂ emissions as it has the potential to sequester and store substantial quantities of atmospheric CO ₂ and generate co-benefits for ecosystems and ecosystem services. However, strategies using reforestation-based CO ₂ removal to offset fossil fuel emissions may not lead to the same climate outcome as avoiding the fossil fuel emissions. Here, we use an Earth System model of intermediate complexity to compare the climate outcome of different pathways: a reference net-zero pathway, and net-zero pathways where additional fossil fuel CO ₂ emissions relative to the reference pathway are balanced by reforestation-based CO ₂ removals (“Net-zero pathways”). Results show that model simulations of Net-zero pathways yield a higher atmospheric CO ₂ and warmer climate outcome than the reference simulation. The higher atmospheric CO ₂ results from carbon cycle feedbacks, which induce an imperfect compensation between the net land CO ₂ flux in reforestation areas that is accounted for as a removal, and the actual change in total land carbon storage. The additional global warming from higher atmospheric CO ₂ is further amplified by biogeophysical effects of reforestation. Our research highlights the need for improved methods to measure and track the carbon cycle and climate effects of reforestation, particularly when used to balance or offset fossil fuel emissions in net-zero pathways.