That's far from the worst flaw in his calculation, since his two biggest blunders are the neglect of the radiative cooling due to sulfate aerosols (known to be a critical factor in the period in question) and his neglect of the many links in the chain of physical effects needed to translate a top of atmosphere radiative imbalance to a change in net surface energy flux imbalance. (realclimate.org)
Nazarenko, L., and S. Menon, 2005: Varying trends in surface energy fluxes and associated climate between 1960 and 2002 based on transient climate simulations. (pubs.giss.nasa.gov)
The release of vapor into the air alters the surface energy fluxes and leads to potential cloud formation. (sciencedaily.com)