For some time, global warming enthusiasts have been at a loss to explain why actual measurements of temperatures around the world have not confirmed the warming trend confidently predicted by climate models.
The problem, explains New York Times reporter William Stevens, could lie with the North Atlantic Oscillation (NAO), which “may be in the process of switching to a new state, one with far-reaching implications for the study of global climate.”
First discovered by scientists in the 1930s, the NAO is essentially the difference in pressure between the Azores (Bermuda) High and the Icelandic Low. When both features are strong, the NAO is positive (the low is deeper and the high pressure system stronger) and the westerly winds across the Atlantic Basin strengthen. As University of Virginia climatologist Patrick Michaels explains it, the faster westerly winds minimize polar outbreaks over Europe, creating above-normal temperatures in winter. A negative NAO is associated with weaker westerly winds and lower European temperatures.
After noting that the NAO has been positive for a while, Stevens recently informed the Times’ readers that, last year, the NAO “made its sharpest reversal on record, cooling off Eurasia and casting Northern Europe into two harsh winters in a row.” “If the overall rise in global temperatures continues in the face of continental cooling,” he continues, “it would offer strong support to the theory that human activity–particularly the emission of heat-trapping gases in the atmosphere–is disrupting the global climate system. . . . Average global temperatures remained at a near-record level despite the cooler continents.”
Stevens’ implication is clear, notes Michaels. If warming occurs in the face of a negative NAO, global warming is a reality. Even the lack of warming may be proof of warming when the all-powerful NAO is negative.
The key to this issue, according to Michaels, is the strength of the relationship between the NAO and temperatures. Upon closer inspection, the relationship is not as strong as Stevens suggests. The NAO is subject to natural circulation shifts that regularly occur across the globe, the origins of which remain a mystery to climatologists.
Taking these shifts into consideration, Michaels correlated the winter (December through February) NAO index since 1885 with winter surface temperature departures over consecutively smaller regions: the entire globe, the Northern Hemisphere, the eastern half of the Northern Hemisphere (roughly Eurasia), and Europe. The global correlation turned about to be so small that it was not statistically significant. Correlations were also weak for the Northern Hemisphere and Eurasia. However, the correlation is strong and significant over Europe, where strong westerly winds produce above-average European temperatures.
Michaels’ findings were borne out in another study published last year by James Hurrell. Hurrell’s work related wintertime temperatures over the non-tropical portion of the Northern Hemisphere with the NAO. Hurrell found that one-third of the variation in winter temperatures over the past 60 winters is attributable to the NAO. This means that two-thirds of the variation is due to other causes.
PF: For further information, see Patrick Michaels, “NAO(W) What? Will Lack of Cooling Become Evidence for Global Warming?” World Climate Report, April 28, 1997. The four-page article is available through PolicyFax; call 847/202-4888 and request document #2329706.