Can Polar Stratospheric Clouds Explain Arctic Amplification?

Climate models underestimate the magnitude of Arctic warming in past warm climates, like the early Creta-ceous and Paleogene periods, implying that certain physical processes might be missing or poorly represented. Previous studies suggest that a large increase in wintertime Arctic polar stratospheric clouds (PSCs) might have promoted Arctic amplification through additional greenhouse warming. High methane concentrations in warm climates might have in-creased stratospheric water vapor providing favorable conditions for PSCs. However, methane concentrations in past warm climates are extremely uncertain. Here, we revisit the PSC hypothesis by exploring PSC changes under very high methane levels, 43 preindustrial carbon dioxide, and strong polar-amplified surface warming, using a whole-atmosphere model with fully interactive chemistry. We find that with polar-amplified warming there is a large increase in Arctic outgo-ing longwave radiation (OLR) that reduces as the methane concentration is increased. PSCs increase monotonically with methane concentration. A large radiative cooling and an increase in water vapor in the stratosphere increases Arctic PSCs, which follow a power law with respect to relative humidity. Using a two-way partial radiative perturbation technique, we show that the OLR reduction due to PSCs is similar to the direct radiative forcing of methane for high methane levels. Thus, we find that PSCs could play an important role in Arctic warming in a warmer-than-present-day climate, but only if methane levels were higher than suggested by previous modeling studies for past warm climates.