While the reality of human-induced global warming is beyond doubt, a question of intense interest is whether the recent acceleration in warming and the midcentury cooling are due to internal variability or changes in natural and anthropogenic forcing (including volcanic and solar forcing). By internal variability we mean variability that occurs in the absence of natural or anthropogenic forcing; that is, variability that occurs solely due to the internal dynamics of the coupled atmosphere-ocean- biosphere-cryosphere system. The purpose of this paper is to ...