Imbalances in climate outcomes in net-zero pathways with fossil fuel CO2 emissions and reforestation-based CO2 removals

Reforestation is considered an important nature-based climate solution to help achieve net-zero CO2 emissions. However, strategies using reforestation-based CO2 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 pathway, and net-zero pathways where additional fossil fuel CO2 emissions relative to the reference pathway are balanced by reforestation-based CO2 removals (“Reforestation Net-zero pathways”). Results show that model simulations of Reforestation Net-zero pathways yield a higher atmospheric CO2 and warmer climate outcome than the reference simulation. The higher atmospheric CO2 results from carbon cycle feedbacks. The additional global warming from higher atmospheric CO2 is further amplified by biogeophysical effects of reforestation. These findings highlight the need for improved methods to account for the carbon cycle and climate effects of reforestation. Higher atmospheric CO2 and warmer climate are found in net-zero pathways that balance fossil fuel emissions with reforestation-based removals compared to an idealized mitigation pathway, as revealed by scenario simulations with an Earth system model.