The IPCC’s fourth climate assessment in 2007 estimated that the Earth would warm 3 degrees Celsius by the end of the century — this estimate was in a range of warming from 2 to 4.5 degrees Celsius. But as the 2000s wore on and little warming occurred, climate scientists began to lower their climate sensitivity estimates from 3 to 2 degrees Celsius in a century and only 1.5 degrees of warming in the next 70 years. But this revelation was only hinted at in the IPCC’s 2013 climate assessment. Instead of lowering their central climate sensitivity measure down from 3 degrees Celsius, the IPCC simply did not give a central estimate and just reduced its lower-bound warming estimate from 2 to 1.5 degrees Celsius. Now the IPCC’s warming range for the next hundred years is 1.5 to 4.5 degrees Celsius.