Possible climate transitions from breakup of stratocumulus decks under greenhouse warming
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 CO_2 levels rise above 1,200 ppm. In addition to the warming from rising CO_2 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 CO_2 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 CO_2 levels continue to rise.
© The Author(s), under exclusive licence to Springer Nature Limited 2019. Received 07 September 2018; Accepted 16 January 2019; Published 25 February 2019. 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. Data availability: 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. Code availability: The source code for the simulations is available at climate-dynamics.org/software/#pycles.
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