Sir John Beddington's contribution to a paper on climate change for the Chartered Institute of Insurance is worth a read. Here he is on tipping points:
There is wide acceptance in the scientific community that there are also likely to be ‘tipping points’ in the climate system which, if crossed, could result in long-term or irreversible changes to our climate. The warmer our climate gets, the greater the likelihood of passing a ‘tipping point’; be it accelerated (and/or irreversible) melting of the Greenland or West Antarctic Ice Sheets, which could significantly increase sea levels, or changes to large-scale atmospheric and oceanic circulatory systems, such as the Gulf Stream, which could potentially fundamentally alter regional climates.
I'm aware of Tim Lenton's paper (with co-authors such as Schellnhuber and Rahmstorf) on tipping points - the result of a confab at the British Embassy in Berlin. But what other support is there for the idea of tipping points in the climate? Do these emerge unbidden from climate models? I certainly seem to remember someone reporting that changes to the Gulf Stream were now being downplayed - see this from Wikipedia for example:
Modelling suggests that increase of fresh water flows large enough to shut down the thermohaline circulation would be an order of magnitude greater than currently estimated to be occurring, and such increases are unlikely to become critical within the next hundred years; this is hard to reconcile with the Bryden measurements.