Stratospheric Aerosol Injection to Stabilize Northern Hemisphere Terrestrial Permafrost Under the ARISE‐SAI‐1.5 Scenario

Permafrost, or ground that is continuously frozen for at least 2 years, contains vast stores of organic soil carbon. Stratospheric aerosol injection (SAI) may prevent tipping points that lead to widespread permafrost thaw and carbon release by cooling surface and soil temperatures, but it is unclear if or when permafrost could stabilize after SAI deployment. Here we use output from the ARISE‐SAI‐1.5 simulations to assess how permafrost may respond to a specific SAI strategy that maintains global mean surface temperature to 1.5°C above pre‐industrial levels. Permafrost responses under SAI are compared to responses under the control SSP2‐4.5 emissions scenario. We show that the rate of boreal permafrost thaw slows under SAI but does not fully stop, likely due to deep permafrost thaw processes that are resistant to surface temperatures changes. In both the ARISE‐SAI‐1.5 and SSP2‐4.5 simulations, permafrost completely thaws and disappears along the southern edge of the permafrost area by 2069, indicating that some permafrost loss may be inevitable even if SAI successfully stabilizes global mean surface temperatures. SAI does prevent a potential local tipping point (talik formation) in roughly 1 million km2 of permafrost. Most of the talik prevention occurs in permafrost peatlands. Thus, a more aggressive SAI strategy than that of ARISE‐SAI‐1.5 is likely required to prevent all future projected permafrost thaw. Plain Language Summary Permafrost is ground that stays frozen for at least 2 years. Permafrost thaw is a global concern because of how much soil carbon it stores, which can be released into the atmosphere as carbon dioxide and methane if permafrost thaws. One proposed method that could prevent permafrost thaw is to cool the Earth by injecting reflective particles into the stratosphere, which would block a small percentage of incoming sunlight and stabilize Earth's temperature to 1.5°C above pre‐industrial temperatures. We use climate projections from an Earth system model to assess how permafrost might respond to a proposed scenario for stabilizing Earth's temperature. We find that the total amount of permafrost still decreases even in a scenario where the Earth's temperature has stabilized, but pockets of unfrozen soil within and above permafrost are less likely to form, which reduces the likelihood of rapid and widespread permafrost thaw. A climate intervention strategy that starts sooner or aims to cool the Earth more may be required in order to fully