RealClimate has a great post up on climate "tipping points," a notion that has been used and abused with great frequency lately by laymen and journalists — including yours truly. It goes into detail picking apart positive feedbacks, tipping points, and points of no return.
The most valuable bit for me was clarifying what James Hansen has in mind when he says that we have ten years to fundamentally change course:
The ’10 year’ horizon is the point by which serious efforts will need to have started to move the trajectory of concentrations away from business-as-usual towards the alternative scenario if the ultimate warming is to stay below ‘dangerous levels’. Is it realistic timescale? That is very difficult to judge. Wrapped up in the ’10 year’ horizon are considerations of continued emission growth, climate sensitivity, assumptions about future volcanic eruptions and solar activity etc. … While the ’10 years’ shouldn’t be read as an exact timetable, it is surely in the right ballpark. 30 more years of business-as-usual will make it impossible to keep temperatures from rising beyond Eemian levels (see here for some discussion of stabilisation scenarios), and decisions (on infrastructure, power stations, R&D, etc.) that are being made now will determine the emissions for decades to come.
As the post goes on to explain, there isn’t so much a single tipping point as a variety of tipping points at various levels, some more dangerous than others, some more reversible than others.
Of course, what matters most at this point are not the purely physical tipping points but the ones that involve us. When will our concern pass the threshold of serious action? Nobody can predict that for us.