The good news is that almost all lines of evidence lead us to believe that is unlikely to be possible, even in principle, to trigger full a runaway greenhouse by addition of non-condensible greenhouse gases such as carbon dioxide to the atmosphere. However, our understanding of the dynamics, thermodynamics, radiative transfer and cloud physics of hot and steamy atmospheres is weak. We cannot therefore completely rule out the possibility that human actions might cause a transition, if not to full runaway, then at least to a much warmer climate state than the present one. High climate sensitivity might provide a warning. If we, or more likely our remote descendants, are threatened with a runaway greenhouse then geoengineering to reflect sunlight might be life’s only hope. …[2 sentences cut to meet arXiv char limit]… The runaway greenhouse also remains relevant in planetary sciences and astrobiology: as extrasolar planets smaller and nearer to their stars are detected, some will be in a runaway greenhouse state.

Goldblatt, Colin; Watson, Andrew J. (8 January 2012). “The Runaway Greenhouse: implications for future climate change, geoengineering and planetary atmospheres”. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 370 (1974): 4197–4216

We have a huge geoengineering greenhouse experiment running on earth as we speak with unclear final outcome. But at least the science of climate will become clearer during this experiment that’s for sure.