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 7 the subtropics. They cool Earth by shading large portions of its surface from sunlight. But 8 because their dynamical scales are too small to be resolvable in global climate models, pre9 dictions of their response to greenhouse warming have remained uncertain. Here we report 10 how stratocumulus decks respond to greenhouse warming in large-eddy simulations that ex11 plicitly resolve cloud dynamics in a representative subtropical region. In the simulations, 12 stratocumulus decks become unstable and break up into scattered clouds when CO2 levels 13 rise above 1200 ppm. In addition to the warming from rising CO2 levels, the instability trig14 gers a surface warming of about 8 K globally and 10 K in the subtropics. Once stratocumulus 15 decks have broken up, they only re-form once CO2 levels drop substantially below the level at 16 which the instability first occurred. Climate transitions arising fromthis instabilitymay have 17 importantly contributed to hothouse climates and abrupt climate changes in the geological 18 past. Such transitions to a much warmer climate may also occur in the future if CO2 levels 19 continue to rise.