Feedbacks Set the Size of the Response

Carbon dioxide acting alone would warm Earth by roughly one degree for each doubling of its concentration. The projected range of two to four and a half degrees comes almost entirely from feedbacks: warmer air holds more water vapour, itself a powerful greenhouse gas, and melting ice replaces a surface reflecting about 80% of sunlight with ocean absorbing over 90%. The uncertainty in a climate projection is therefore mostly uncertainty about feedbacks, not about the carbon dioxide itself.