Solar geoengineering may not prevent strong warming from direct effects of CO2 on stratocumulus cloud cover

Discussions of countering global warming with solar geoengineering assume that warming owing to rising greenhouse gas concentrations can be compensated by artificially reducing the amount of sunlight Earth absorbs. However, here we show that solar geoengineering may not be fail-safe to prevent global warming because CO\textsubscript{2} has direct effects on cloud cover: it reduces cloud cover by modulating the longwave radiative cooling within the atmosphere, which is not mitigated by solar geoengineering. We use idealized high-resolution simulations of clouds to show that under a sustained solar geoengineering scenario, the direct CO\textsubscript{2} effects gradually thin subtropical stratocumulus clouds and may eventually lead to their loss at concentrations above 1700~ppm. Because stratocumulus clouds cover large swaths of subtropical oceans and cool Earth by reflecting incident sunlight, their loss would trigger strong (about 5~K) global warming. Thus, the results highlight that, at least in this extreme and idealized scenario, solar geoengineering may be incapable of countering greenhouse-gas driven global warming.