Stratocumulus clouds cover 20% of the low-latitude oceans and are especially prevalent in the subtropics. They cool the Earth by shading large portions of its surface from sunlight. However, as their dynamical scales are too small to be resolvable in global climate models, predictions of their response to greenhouse warming have remained uncertain. Here we report how stratocumulus decks respond to greenhouse warming in large-eddy simulations that explicitly resolve cloud dynamics in a representative subtropical region. In the simulations, stratocumulus decks become unstable and break up into scattered clouds when CO2 levels rise above 1,200 ppm. In addition to the warming from rising CO2 levels, this instability triggers a surface warming of about 8 K globally and 10 K in the subtropics. Once the stratocumulus decks have broken up, they only re-form once CO2 concentrations drop substantially below the level at which the instability first occurred. Climate transitions that arise from this instability may have contributed importantly to hothouse climates and abrupt climate changes in the geological past. Such transitions to a much warmer climate may also occur in the future if CO2 levels continue to rise.
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The source code for the simulations is available at climate-dynamics.org/software/#pycles.
The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files. The raw data in the figures are available from the corresponding author upon request.
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This research was supported by C. Trimble. The computations were performed on ETH Zurich’s Euler cluster and on Caltech’s High Performance Cluster, which is partially supported by a grant from the Gordon and Betty Moore Foundation. We thank M. Hell for assistance with the figures. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
The authors declare no competing interests.
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Time evolution of the stratocumulus instability. The shading shows the liquid water path in the subtropical LES domain in the 1300 ppm simulation between days 255 and 275, which is during the time of the stratocumulus breakup (Fig. 1). Blue for zero LWP, and the colorscale saturates white at a LWP of 300 μm. The time series at the bottom show the cloud fraction (blue) and SST (red) in the LES domain. The breakup of the stratocumulus clouds is more rapid than it would be in nature because of the unrealistically small thermal inertia of the underlying slab ocean.
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Schneider, T., Kaul, C.M. & Pressel, K.G. Possible climate transitions from breakup of stratocumulus decks under greenhouse warming. Nat. Geosci. 12, 163–167 (2019). https://doi.org/10.1038/s41561-019-0310-1
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