It is widely known and agreed upon within the scientific community and our elected representatives that Mankind has to limit the increase of temperatures to two degrees Celsius (3.6°F) by 2100 compared to 1850 levels.
However, we have recently seen that the least developed nations and many island nations in the Pacific Ocean call for more drastic climate action in order to limit the rise of temperatures to 1.5°C (2.4°F).
Since temperatures already increased by 0.6°C since the 19th century we understand how much we need decisions and measures to be taken in Copenhagen
This limit was fixed by 1996 after the report of the IPCC published one year earlier. Since then events like the Katrina Hurricane in the United States in 2003 or the Cyclone Nargis in 2008 (which killed 140,000 people in Asia) have shown that climate change could have catastrophic consequences.
Regions of the world won’t be harmed the same with a 2°C increase of temperatures. What would bring certain doom to the arctic or snow in the Alps or the Himalayas won’t have catastrophic consequences for some other regions. For example, the WWF pointed as early as 2005 that 2°C could be too much for the Arctic.
Last but not least : in February an update of the Burning embers diagram of the IPCC in 2001 have shown that a lessened increase of temperatures could trigger more problems. Here is the diagram which puts in correlation risks and temperatures :
(cf. this article from Dot Earth for more)