tured not only the warming of the last century, but also much of the preceding Little Ice Age (Jones and Bradley, 1992). Long cool periods, such as the summers throughout the nineteenth century, were punctuated by extreme events. The cold phases were known to have had important effects on human activity, as in the abrupt European cooling from the unusually warm 1730s to the cold 1740s. Famine occurred across western Europe, especially in Ireland and France, where farmers who depended on wheat and potatoes were slow to adapt. In Ireland, this is known as the “forgotten famine”; as many people died as in the famed “potato famine” of the 1840s.

In much of the northern United States, 1816 was the “year without a summer” (Stommel and Stommel, 1983); it was linked to the atmospheric veil from the volcanic eruption of Tambora in Indonesia. Snow fell in July in northern New England, farming was disrupted, and crops failed. The event might have spurred the westward migration of farmers from the thin soils of the hill farms of New Hampshire. Without suggesting a causal link, La Niña occurred in late 1815; by late 1816, a strong El Niño developed. There was severe drought in Brazil in 1816-1817. Single volcanic eruptions often have a widespread cooling effect (e.g., Grootes and Stuiver, 1997; Zielinski et al., 1997). Such effects ordinarily would be considered “noise” in a climate state; however the compounding of such noise with longer climate trends, such as the Little Ice Age, can increase the impacts, as observed. Volcanic forcing also represents an experiment in which the response of global climate modes can be observed in great detail, helping understand how modes might respond to other forcings.

AO/NAO Trends and Variability

The winter atmospheric circulation in the Northern Hemisphere has undergone some remarkable changes during the last several decades. Sea-level pressures over the Arctic have fallen by about 6 hPa (Walsh et al., 1996), and the subpolar westerly winds have strengthened, particularly over the Atlantic sector. Related circulation changes have favored mild winters over most of Russia, China and Japan, and drought over southern Europe and parts of the Middle East (Hurrell, 1995; Thompson et al., 2000). Warming during the period has been concentrated in central and northern Asia and northwestern North America. Retreating glaciers, warming permafrost, and decreasing sea-ice cover have been observed in Alaska, where temperatures increased abruptly in the late 1970s, distinct from the post-1980 acceleration of globally averaged temperature rise. The patterns in these

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