Carbon cycle feedbacks from tropical forest restoration in an Earth System Model

To achieve the Paris Agreement commitment to 'pursue efforts' to limit global surface air temperatures to 1.5℃ above pre-industrial levels, aggressive mitigation strategies and negative emission technologies need to be implemented to reduce greenhouse gas emission to net zero over the next few decades. As well as fossil fuel emissions the land surface plays a major role: deforestation and forest degradation are the second largest source of CO₂ emissions, while forest restoration is a viable, if limited, negative emissions technology. Here we use a fully-coupled Earth System Model (HadGEM2-ES) with interactive CO₂ emissions to quantify the maximum potential for temperature and CO₂ benefits from global tropical reforestation under the aggressive mitigation scenario RCP 2.6 by 2100. This allows us to see how stopping tropical deforestation and allowing forests to recover impacts the global carbon cycle. We find a small CO₂ ( 12 ppm) benefit relative to the standard RCP 2.6 control simulation by 2100, but this relatively modest change does not translate to an detectable reduction in global temperature. One feature of the results is a 20% decline in the atmosphere-ocean carbon flux in response to lower atmospheric CO₂ following forest restoration. This completely offsets the CO₂ uptake from forest restoration after a response time of 25 years. This response time is supported by analysis of the historical forest regrowth that occured after the European arrival in the Americas, which serves as a potential historical analogue for the impact of future large-scale forest restoration on the carbon cycle. We find that a stop of tropical deforestation and restoration of 18 million km² removes 40 Pg C, but feedback processes mean that atmospheric carbon only reduces by 25 Pg C, a value equivalent to less than three years of fossil fuel emissions at 2017 rates. We conclude that forest restoration is vital for staying within a 1.5℃ target, but when all feedback processes are considered it can only be a modest contribution to negative emissions.