4 Degrees is the New 2 Degrees
Conservative estimates by scientists suggested a change in the global temperature of more than 2 degrees Celsius would lead to catastrophic effects (today we are 0.8 degrees). This was the target that world leaders agreed to in Copenhagen in 2009.
In September 2013, a report by the Intergovernmental Panel on Climate Change found that the international community is failing to meet the 2 degree Copenhagen commitment.
To make matters worse, acclaimed climate scientist James Hansen warned in a paper published in December 2013 that even a 2 degree change would lead to “disastrous consequences.” As per Grist's analysis of the report, the consequences ranged from "rampant sea-level rise to widespread extinction."
The news doesn't get better. Now a new report is out that says runaway global warming is likely to lead to a 4 degree change by 2100. In short, if greenhouse emissions are left unchecked the world is definitely in trouble.
Interesting quote around climate change modeling from Professor Steven Sherwood, who led this work:
Climate skeptics like to criticize climate models for getting things wrong, and we are the first to admit they are not perfect. But what we are finding is that the mistakes are being made by the models which predict less warming, not those that predict more.