Releasing global forests from management: how much more carbon could be stored?

Global forests play a key role in the global carbon cycle and are a cornerstone in international policy-making to prevent global warming from exceeding 1.5°C and reach carbon neutrality. In line with recent climate science, the actions taken in the current decade are crucial for obtaining the goals laid out in international agreements. One forest-based strategy with high short-term climate benefits is the return of global forests to their carbon storage potential, by ceasing forest management, but the ecological boundaries of increasing biomass in existing forests remain poorly quantified. Recent studies preferentially focus on the mitigation potential of reforestation, without explicitly accounting for the carbon dynamics in existing forests, thus providing an incomplete evaluation of the possible expansion of the forest carbon stock. Here we integrate satellite remote sensing estimates of current forest biomass with a machine learning framework to show that existing global forests could increase their above-ground biomass by 44.1 PgC at most (an increase of 16% over current levels) if allowed to reach their natural equilibrium state. In total, the maximum carbon storage potential in this hypothetical scenario equates to just about 4 years of global anthropogenic CO2 emissions (at the 2019 rate). This maximum potential would require the complete stop of forest management and harvesting for decades. Therefore, without first strongly reducing CO2 emissions, this strategy holds low climate change mitigation potential. This urges to view storing additional carbon in existing forests as an effective strategy to offset carbon emission from sectors that will be hard to decarbonise, rather than as a tool to compensate all business-as-usual emissions.