Evidence from North Atlantic deep sea cores reveals that abrupt shifts punctuated what is conventionally thought to have been a relatively stable Holocene climate. During each of these episodes, cool, ice-bearing waters from north of Iceland were advected as far south as the latitude of Britain. At about the same times, the atmospheric circulation above Greenland changed abruptly. Pacings of the Holocene events and of abrupt climate shifts during the last glaciation are statistically the same; together, they make up a series of climate shifts with a cyclicity close to 1470 ± 500 years. The Holocene events, therefore, appear to be the most recent manifestation of a pervasive millennial-scale climate cycle operating independently of the glacial-interglacial climate state. Amplification of the cycle during the last glaciation may have been linked to the North Atlantic's thermohaline circulation.

The climate of the Atlantic sector exhibits considerable variability on a wide range of time scales. A substantial portion is associated with the North Atlantic Oscillation (NAO), a hemispheric meridional oscillation in atmospheric mass with centers of action near Iceland and over the subtropical Atlantic. NAO-related impacts on winter climate extend from Florida to Greenland and from northwestern Africa over Europe far into northern Asia. Over the last 3 decades, the phase of the NAO has been shifting from mostly negative to mostly positive index values. Much remains to be learned about the mechanisms that produce such low frequency changes in the North Atlantic climate, but it seems increasingly likely that human activities are playing a significant role. W hen the North Atlantic Oscillation (NAO) is in its positive phase, low-pressure anomalies over the Icelandic region and throughout the Arctic combine with high-pressure anomalies across the subtropical Atlantic to produce stronger-thanaverage westerlies across the midlatitudes. During a positive NAO, conditions are colder and drier than average over the northwestern Atlantic and Mediterranean regions, whereas conditions are warmer and wetter than average in northern Europe, the eastern United States, and parts of Scandinavia (Fig. 1 top). Walker and Bliss (1) were among the first to recognize and study this pattern of climate anomalies, which is most pronounced during boreal winter (December through March).A remarkable feature of the NAO is its trend toward a more positive phase over the past 30 years, with a magnitude that seems to be unprecedented in the observational record (2). Some of the most pronounced anomalies have occurred since the winter of 1989, when record positive values of the NAO index have been documented (Fig. 1 Lower). Moreover, the trend in the NAO accounts for a myriad of remarkable changes in the climate over the middle and high latitudes of the Northern Hemisphere, as well as in marine and terrestrial ecosystems. Among these changes are:Milder winters in Europe downstream and across Asia, juxtaposed against more severe winters over eastern Canada and the northwest Atlantic (2); Pronounced regional changes in precipitation patterns, resulting in the advance of some northern European glaciers and the retreat of Alpine glaciers (3, 4); Changes in sea-ice cover in both the Labrador and Greenland Seas as well as over the Arctic (5); Pronounced decreases in mean sea level pressure (SLP) over the Arctic and changes in the physical properties of Arctic sea water (6, 7); Changes in the intensity of convection in the Labrador and the Greenland-Iceland Seas, which in turn influence the strength and character of the Atlantic meridional overturning circulation (8);Stratospheric cooling over the polar cap and total column ozone losses poleward of 40°N (9); Changes in the production of zooplankton and the distribution of fish (e.g., ref. 10), and changes in the length of the growing season over Europe (11).All of these changes seem to be strongly r...

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