  The ultimate climate emergency is a "runaway greenhouse": a hot and water
vapour rich atmosphere limits the emission of thermal radiation to space,
causing runaway warming. Warming ceases only once the surface reaches ~1400K
and emits radiation in the near-infrared, where water is not a good greenhouse
gas. This would evaporate the entire ocean and exterminate all planetary life.
Venus experienced a runaway greenhouse in the past, and we expect that Earth
will in around 2 billion years as solar luminosity increases. But could we
bring on such a catastrophe prematurely, by our current climate-altering
activities? Here we review what is known about the runaway greenhouse to answer
this question, describing the various limits on outgoing radiation and how
climate will evolve between these. 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.
