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Although El Niño and La Niña are the largest single sources of global interannual climate variability, climate shifts on longer time scales than El Niño's 2 to 7 years are also drawing the attention of researchers. On multidecadal time scales of 40 to 80 years, a restless North Atlantic seems to be at work, alternately countering and enhancing humankind's alterations of climate. The evidence for this is turning up in such records as tree rings, ice cores, and corals.
Richard A. Kerr (Fri,) studied this question.