Abstract/Summary

The “double CO2” experiment has become a standard experiment in climate science, and a convenient way of comparing the sensitivity of different climate models. Double CO2 was first used by Arrhenius in the 19th century and in the classic paper by Manabe and Wetherald, published 50 years ago, which marked the start of the modern era of climate modeling. Doubling CO2 now has an iconic role in climate research. The equilibrium climate sensitivity (ECS) is defined as the global-mean surface temperature change resulting from a doubling of CO2, which is a headline result in Intergovernmental Panel on Climate Change (IPCC) assessments. In its most recent assessment IPCC concluded that the ECS “is likely in the range 1.5 to 4.5oC”. We show that we are now halfway to doubling of CO2 since pre-industrial times in terms of radiative forcing, but not in concentration.