Increased multidecadal variability of the North Atlantic Oscillation since 1781

The North Atlantic Oscillation is a meridional oscillation of atmospheric mass measured between Iceland and the Acores(1,2), which drives winter climate variability in eastern North America and Europe. A prolonged period of the positive phase during the 1990s led to the suggestion that anthropogenic warming was affecting the behaviour of the North Atlantic Oscillation(3,4). However, instrumental records(1,5) are too short to compare observations during periods of extended warm and cold hemispheric temperatures, and existing palaeoclimate reconstructions(6,7) primarily capture terrestrial variability. Here we present a record of Sr/Ca, a sea surface temperature proxy, from a Bermuda coral from 1781 to 1999. We use this monthly resolved record to reconstruct past variability of the North Atlantic Oscillation at multiple frequencies. Our record shows enhanced multidecadal scale variability during the late twentieth century compared with the end of the Little Ice Age (1800-1850). We suggest that variability within the North Atlantic Oscillation is linked to the mean temperature of the Northern Hemisphere, which must be considered in any long-term predictions.