Simulated responses and feedbacks of permafrost carbon under future emissions pathways and idealized solar geoengineering scenarios

The carbon-rich northern high-latitude permafrost is a potential climate tipping point. Once triggered, it is thawing and release of carbon dioxide and methane might unleash irreversible changes in the Earth's climate system. We investigate the response of permafrost under three Shared Socioeconomic Pathways (SSPs) with no mitigation (SSP5-8.5), moderate mitigation (SSP2-4.5) and delayed mitigation (SSP5-3.4-OS), and three solar geoengineering scenarios applied to each experiment to prevent global warming from exceeding 2 degrees C above pre-industrial. The long-term negative emissions in SSP5-3.4-OS preserves much more frozen soil than SSP5-8.5, but shows nearly as much permafrost carbon loss this century as SSP2-4.5 due to its mid-century temperature overshoot. Solar geoengineering to meet the 2 degrees C target above pre-industrial effectively suppresses permafrost thawing and reduces subsequent carbon release from the soil. However, the carbon emission from permafrost still continues after the temperature is stabilized, due to the decomposition of thawed permafrost carbon. More solar insolation reduction is required to compensate the positive permafrost carbon feedback, which exerts greater impacts on the efficiency of solar geoengineering under a scenario with strong climate policy and lower carbon emissions.