Summary/Abstract
The Atlantic Multidecadal Oscillation (AMO), a 50- to 70-year quasiperiodic variation of climate centered in the North Atlantic region, was long thought to be an internal oscillation of the climate system. Mann et al. now show that this variation is forced externally by episodes of high-amplitude explosive volcanism. They used an ensemble of climate models to evaluate the causes of the AMO, finding that volcanos are the most important influence, and that there is no evidence to show that it has been internally generated during the last millennium.