A volcanic source of variation
The Atlantic Multidecadal Oscillation (AMO), a 50- to 70-year near-period variation in climate centered in the North Atlantic region, has long been viewed as an internal oscillation of the climate system. Mann et al. now show that this variation is forced externally by episodes of explosive volcanism of high amplitude. They used a set of climate models to assess the causes of AMO, finding that volcanoes are the most important influence and that there is no evidence to show that it was generated internally during of the last millennium.
Science, this issue p. 1014
Previous research argues for a multidecadal internal oscillation (40 to 60 years) distinct from climate noise. Rather, recent studies have argued that this so-called Atlantic multidecadal oscillation is a manifestation of time-varying competing effects of anthropogenic greenhouse gases and sulphated aerosols. This conclusion is supported by the absence of robust multidecadal climatic oscillations in the control simulations of current generation models. Paleoclimate data, however, demonstrate multidecadal oscillatory behavior during the pre-industrial era. By comparing the control …
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