In his Environmental Research Letters paper Hansen claims that the IPCC 2007 figures are low because the IPCC says it is unable to evaluate possible dynamical responses of the ice sheets and therefore its figures do not include any possible rapid dynamical changes in ice flow. (environmentalresearchweb.org)
This is the result of different cloud and convection formulations used in the models, as well as differing atmospheric and oceanic dynamical responses associated with changing temperature gradients. (giss.nasa.gov)
Dynamical Response of the Tropical Pacific Ocean to Solar Forcing During the Early Holocene, Thomas M. Marchitto et al., 12/2010, Science, Volume 330, Number 6009, pp. 1378 - 1381 (notrickszone.com)