We have to be able to quantify the benefits, risks, and costs of geoengineering, and compare them to the benefits, risks, and costs of our other options, so that we can make an informed decision. SRM might be needed in a planetary emergency – say, if continued warming rapidly accelerates ice-sheet melting and sea-level rise, or if it accelerates methane and CO2 emissions from thawing tundra, which would then accelerate the warming itself.