Maintaining atmospheric CO2 levels at 450 ppm risks a 20 percent chance of global temperatures rising nearly 4 degrees Celsius. However, the relationships between temperature and carbon sinks such as the world’s oceans and forests are hard to predict:
For instance, melting Arctic sea ice heated up the ocean in the far north 
much faster because ice reflects a large portion of the sun’s radiation, while open oceans absorb the sun’s heat. In addition, a rise of several degrees Celsius could also mean the Amazon rainforest drying out, turning it into a huge source of carbon dioxide emissions rather than a vast sink for the gas as it is now.
The Stern Review on the Economics of Climate Change, published in Oct 2006, concludes that climate change will result in the loss of 5% global GDP every year, possibly rising to 20% GDP if a broader range of impacts is considered. Combined with the findings that a “mitigation strategy” for GHG is estimated to cost 1% of global GDP, there should be no delay in a worldwide positive action.