This is a slightly edited version of a comment Richard Betts left on the discussion forum. I thought it was quite challenging to much of what we hear about climate change in the mainstream media and therefore worthy of posting here as a header post. (Richard, for anyone visiting for the first time, is head of climate change impacts at the Met Office).
Most climate scientists* do not subscribe to the 2 degrees "Dangerous Climate Change" meme (I know I don't). "Dangerous" is a value judgement, and the relationship between any particular level of global mean temperature rise and impacts on society are fraught with uncertainties, including the nature of regional climate responses and the vulnerability/resilience of society. The most solid evidence for something with serious global implications that might happen at 2 degrees is the possible passing of a key threshold for the Greenland ice sheet, but even then that's the lower limit and also would probably take centuries to take full effect. Other impacts like drought and crop failures are massively uncertain, and while severe negative impacts may occur in some regions, positive impacts may occur in others. While the major negative impacts can't be ruled out, their certainty is wildly over-stated.
While really bad things may happen at 2 degrees, they may very well not happen either - especially in the short term (there may be a committment to longer-term consequences such as ongoing sea level rise that future generations have to deal with, but imminent catastrophe affecting the current generation is far less certain than people make out. We just don't know.
The thing that worries me about the talking-up of doom at 2 degrees is that this could lead to some very bad and expensive decisions in terms of adaptation. It probably is correct that we have about 5 years to achieve a peak and decline of global emissions that give a reasonable probability of staying below 2 degrees, but what happens in 10 years' time when emissions are still rising and we are probably on course for 2 degrees? If the doom scenario is right then it would make sense to prepare to adapt to the massive impacts expected within a few decades, and hence we'd have to start spending billions on new flood defences, water infrastructure and storm shelters, and it would probably also make sense for conservationists to give up on areas of biodiversity that are apparently "committed to extinction" - however all these things do not make sense if the probability of the major impacts is actually quite small.
So while I do agree that climate change is a serious issue and it makes sense to try to avoid committing the planet to long-term changes, creating a sense of urgency by over-stating imminent catastrophe at 2 degrees could paint us into a corner when 2 degrees does become inevitable.
*I prefer to distinguish between "climate scientists" (who are mainly atmospheric physicists) and "climate change scientists" who seem to be just about anyone in science or social science that has decided to see what climate change means for their own particular field of expertise. While many of these folks do have a good grasp of climate science (atmospheric physics) and the uncertainties in attribution of past events and future projections, many sadly do not. "Climate change science" is unfortunately a rather disconnected set of disciplines with some not understanding the others - see the inconsistencies between WG1 and WG2 in IPCC AR4 for example. We are working hard to overcome these barriers but there is a long way to go.