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Variations in solar output (yellow) show only the slightest correlation with changes in global mean temperatures and El Niño cycles.

Mike Lockwood/University of Southampton

Not Much Warming Under the Sun

Don't blame the sun for recent global warming. A new analysis, based on historical data rather than computer simulations, shows that our star's role in climate change has been vastly overtaken by other factors, particularly the human-induced buildup of greenhouse gases.

We get our warmth from the sun, sure, but our climate results from a complex and precarious balance of additional factors, including ocean currents, winds, the amount of snow and ice cover, and even Earth's orbit and rotational wobble. It's well-known that our climate has been warming over the past century--a situation most researchers blame on human-induced buildup of carbon dioxide, methane, and other greenhouse gases. Studies have shown that the sun was a driving factor in climate change in preindustrial times, but some researchers have wondered whether changes in the sun's intensity are continuing to play a major role, possibly by hitting the planet with more heat than normal.

To help nail down the effect of solar radiation, geophysicist Mike Lockwood of the University of Southampton, U.K., examined data available since 1955 on the monthly average output of the sun, including sunspots, magnetic activity, and cosmic-ray variations. Then he compared those data, month by month, with average global temperature records, as well as El Niño- and La Niña-induced weather cycles and the atmospheric effects of major volcanic eruptions. The result, Lockwood and colleagues report in two papers published online this week in the Proceedings of the Royal Society A, is that for the past half-century, the sun has exerted only a small influence on climate--about 3% compared with the warming influence of greenhouse gases and natural climate cycles (see illustration).

Lockwood says a key advantage of his approach is that he relied on hard data rather than computer models. "One problem that crops up [in the climate discussion] is that scientists use complex models that nonspecialists don't understand and therefore don't trust," he explains.

Lockwood's research represents "a solid look at whether global temperature increases are being driven by changes in the brightness of the sun," says geophysicist Dáithí Stone of the University of Oxford in the U.K. The work suggests that "there is basically no way that this can be the case," he says.

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