Investigating the effects of a subtropical stratocumulus cloud breakup in warm climates using cloud-locking experiments

In the warm, equable climate of the Eocene, constraints on CO 2 levels and low-latitude temperatures have generally precluded climate models from recreating key features of the climate, especially the above-freezing winter temperatures in the continental interiors suggested by fossil evidence of frost-intolerant species at high latitudes. Several cloud feedbacks have been suggested as mechanisms for enhanced wintertime warming, including a breakup of subtropical stratocumulus cloud decks at high CO 2 concentrations. It has been suggested that this breakup could lead to 8 K of global average warming, but how this low-latitude cloud feedback translates to high-latitude warming is not obvious and warrants quantification with a global climate model. In this study, we use cloud-locking experiments in the Community Earth System Model, version 2 (CESM2), in which homogeneous subtropical stratocumulus clouds are prescribed or completely removed to investigate the maximum warming achieved by a breakup of subtropical stratocumulus clouds in both a preindustrial and a high CO 2 climate. We use the present-day continental configuration and vegetation to make these results applicable to a future warm climate. We find that in the most dramatic case, the stratocumulus breakup leads to a significant globally averaged warming of about 4.5 K and contributes to some reduction in below-freezing days in the continental interiors at high latitudes. The resulting warming is limited by stabilizing low-cloud feedbacks induced elsewhere following the breakup. We conclude that a stratocumulus breakup may have played a nonnegligible role in past warm climates, even if it cannot, on its own, explain the key features of the Eocene.