http://www.nature.com/nature/journal/v505/n7481/full/nature12829.html 4+-1 C, more precisely. That's 6.4+-1.6 Fahrenheit. Previous estimates had been 1.5 to 5C. The reduction in the error bar is because the errors among the previous estimates are about half explained by a mechanism involving low versus high clouds, and water-vs-altitude mixing. So upon fitting that to observations this error mechanism can be reduced. And note, this error reduction now unfortunately for humanity takes us to the high end of the previous prediction range.