Here is a brief excerpt:
The phrase "tipping point" is heard a lot more from scientists. This is where a small amount of warming sets off unstoppable changes, for example the melting of the ice caps. Once the temperature rises a certain amount then all the ice caps will melt. The tipping point in many scientists' view is the 2˚C [about 4˚F] rise that the EU has adopted as the maximum limit that mankind can risk. Beyond that, as unwelcome changes in the earth's reaction to extra warmth continue, it is theoretically possible to trigger runaway climate change, making the earth's atmosphere so different that most of life would be threatened.