2 Degree Disaster

A paper by James Hansen et al concludes that 2 degrees Celsius of warming would spur an eventual warming of 3 to 4 degrees Celsius with "disastrous consequences".

Rapid emissions reduction is required to restore Earth’s energy balance and avoid ocean heat uptake that would practically guarantee irreversible effects.

Update (December 17):  NASA reports that the mean global temperature for November was the highest ever recorded for that month since records began in 1880.  The November mean was 0.78 degrees Celsius above the base mean.  It was the 345th consecutive month with a mean temperature above the long-term average.

Update (December 31):  A study published in Nature suggests that cloud formation would decrease with higher temperatures which in turn would enhance the warming.  A minimum warming of 4 degrees Celsius is expected by 2100.

Update (April 4, 2015):  Petra Tschakert with the Earth and Environmental Systems Institute at Pennsylvania State University argues that the 2 degrees Celsius limit on warming is inadequate.

A low temperature target is the best bet to prevent severe, pervasive and potentially irreversible impacts, while allowing ecosystems to adapt naturally, ensuring food production and security, and enabling economic development to proceed in a sustainable manner.