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

Solar geoengineering that manipulates the amount of sunlight Earth absorbs is increasingly discussed as an option to counter global warming. However, we demonstrate that solar geoengineering is not a fail-safe option to prevent global warming because it does not mitigate risks to the climate system that arise from direct effects of greenhouse gases on cloud cover. High-resolution simulations of stratocumulus clouds show that clouds thin as greenhouse gases build up, even when warming is modest. In a scenario of solar geoengineering that is sustained for more than a century, this can eventually lead to breakup of the clouds, triggering strong (5°C), and possibly difficult to reverse, global warming, despite the solar geoengineering.