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Analyzing tree-ring data from 5,000-year-old living bristlecone pines and even older dead ones, Eddy reported in 1976 that their carbon-14 content seemed to vary in rhythm with sunspot numbers. When sunspots were rare, as they were during the Maunder minimum, the amount of carbon 14 in the tree rings increased markedly; when they were numerous, the amount decreased. The explanation: during the sun's more active periods, its magnetic field, which ordinarily deflects some cosmic rays away from the earth, expands and becomes an even greater barrier to the rays. As a result, less carbon 14 is created in the atmosphere and less finds its way into trees.
Eddy's tree-ring data revealed other 50-to-100-year intervals in the past when carbon-14 production was high and the sun apparently quiescent. But did this mean that all of these periods were times of extreme cold? Many scientists doubted it, suggesting that the correlation between the Maunder minimum and the little ice age might be nothing more than sheer coincidence. Changes in solar cyclic activity, the doubters argued, were not necessarily accompanied by variations in the sun's output of heat and light and probably did not affect terrestrial weather and climate.
Solar Max has undermined those arguments. A sensitive radiometer aboard the satellite has confirmed that between 1980 and 1986 average solar output declined one-tenth of 1%, then leveled off, and now has begun to climb. The finding strongly suggests that solar radiation varies with the sunspot cycle and that the solar constant is not that constant after all.
But is so small a cyclic change able to have a noticeable effect on weather? Two scientists suspect it may. Karin Labitzke of Berlin's Free University and NCAR's Van Loon have discovered a relationship between the solar cycle and the stratospheric winds over the tropics. During a 28-month period, these winds reverse direction, blowing half the time from the east, the other half from the west, a phenomenon meteorologists call the QBO, or quasi-biennial oscillation. Depending on the direction of the QBO flow, Labitzke and Van Loon found, solar maximums and minimums seem linked to changes in air pressure, temperatures, the number of storms and perhaps even the size of the notorious hole in the Antarctic ozone layer.
"It's pure statistics," Van Loon concedes. "We have no physical explanation for what we've found." That explanation may be hard to come by. Experts have calculated that the tiny change in the solar constant detected by Solar Max can supply less than a millionth of the energy needed to produce the observed changes in weather. "If there really is an effect," says Van Loon, "there must be an enhancing mechanism, and we don't have the foggiest idea of what that enhancing mechanism might be." Yet the statistical evidence is so compelling that many scientists are taking it seriously. The QBO data have persuaded meteorologist Anthony Barnston, of the National Climate Analysis Center, to incorporate the solar cycle into the computer algorithms for his monthly and 90-day seasonal forecasts.