Just how much warming should we expect from rising levels of atmospheric greenhouse gases (GHGs)? The answer largely hinges on how much extra warming might be generated by the initial warming—that is, how strong (and in what direction) are the feedbacks from water vapor and clouds.
By most estimates (including climate model outcomes), these feedbacks are positive and result in about a doubling of the warming that would result from greenhouse gas increases alone. By others, however, the total feedbacks are negative,…