Over at RealClimate they are talking about geoengineering: that’s the intentional manipulation of the global climatic system with the intent to counteract the effects of greenhouse gasses. Generally, it consists of efforts to either reflect more solar energy back into space or enhance the activity of biological carbon sinks. It has been mentioned here before.
The fundamental problem with all geoengineering schemes (from sulfite injections to plankton tubes to giant mirrors) is that they risk creating unexpected and negative side-effects. That said, it does seem intelligent to investigate them as a last resort. Nobody knows at what point critical physical and biological systems might tip into a cycle of self-reinforcing warming. Plausible examples include permafrost melting in the Arctic, releasing methane that heats the atmosphere still more, or the large-scale burning of tropical rainforests, both producing emissions and reducing the capacity of carbon sinks. If physical or biological systems became net emitters of greenhouse gasses, cutting human emissions to zero would not be sufficient to stop warming; it would simply continue until the planet reached a new equilibrium.
Given linear projections of climate change damages, we would probably be wisest to heed the Stern Review and spend adequately on mitigation. Given the danger of strong positive feedbacks, it makes sense to develop some fallback options for use in desperate times. It seems to me that various forms of geoengineering should be among them. Let us hope they never need to be used.