Defending climate targets under threat of forest carbon impermanence

Carbon dioxide removal (CDR) can support mitigation efforts and help to limit the footprint of the hardest-to-abate sectors. Forests are one of the most cost-effective solutions to provide this CDR service at scale. Therefore, reforestation has become a major pillar supporting climate targets in scenarios and action plans such as the Nationally Determined Contributions. In addition, forests provide an unassisted aid to climate mitigation, removing a quarter of annual emissions as part of the terrestrial carbon cycle. As a result, today’s mitigation pathways have become a bet on the perpetual growth and permanence of the forest’s carbon storage. However, recent studies are raising doubt about the impeccable future productivity of forests we came to depend on. Forest resilience, especially in biomass hotspots like the Amazon, is in decline. An unexpected carbon stock loss becomes more likely as almost a quarter of primary forests reach critical resilience thresholds. Further, forest disturbances by fire, windfall, and pests become more widespread under changing climatic conditions. Moreover, nutrient limitation might regionally negate positive feedbacks we had hoped for, like CO2 fertilization and prolonged growing seasons. We use the integrated assessment model REMIND-MAgPIE to explore 1.5°C and 2°C mitigation scenarios assuming a range of forest disturbance levels and response timings. Here we show that forest disturbances call for more stringent mitigation targets in all sectors to maintain climate goals. Postponing action instead of preparation risks spiraling costs. Reacting only five years after the disturbance is introduced to the scenario doubles the GDP cost of mitigation action under the same disturbance level. In addition, twice the carbon price is required to reach the same climate goal in 2050. We conclude that even disturbed forests can provide carbon removal services. However, the promise of forest CDR may not be misused to delay decarbonization. Over-relying on forest CDR heightens the risk of unplanned future emissions and leaves us with few options to cope with it.