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Getting Warmer? Don't Blame It on the Sun, Experts Say

A study of 1,000 years of variations in solar activity finds little or no effect on climate change.

September 16, 2006|Robert Lee Hotz | Times Staff Writer

Seeking another cause of global warming, some climate experts long suspected that the sun itself could be at fault.

Changes in the sun's luminosity might be more to blame for the world's rising temperatures than industrial greenhouse gases such as carbon dioxide and methane, they speculated.

Not so, scientists reported Wednesday in the journal Nature.

By evaluating patterns of solar activity during the past thousand years, a team of experts concluded that variations in the sun's energy output played little or no role in global warming.

"We have answered a classic question that goes back several hundred years: Does the changing brightness and darkness of the sun drive climate?" said lead researcher Peter Foukal, a solar physicist at Heliophysics Inc. in Nahant, Mass. "The answer is that it probably doesn't."

Foukal and colleagues from the Max Planck Institute for Astrophysics in Garching, Germany, and the University Corporation for Atmospheric Research in Boulder, Colo., assessed long-term variations in the total amount of energy emitted by the sun, which normally rises and falls slightly in step with an 11-year cycle of sunspot activity, back to the year 1000.

To reconstruct the relationship between the sun's energy output and climate, they used computer models, temperature changes as recorded in natural sources such as tree rings, and measurements of beryllium-10, an isotope produced naturally in the atmosphere as a consequence of solar activity.

They found that any long-term solar variations were simply too small to explain the way that temperatures and climate had changed "on centennial, millennial and even million-year timescales," the researchers reported. They did not rule out the possibility that some unknown behavior of the sun could be involved in climate change.

"The influence of the sun is utterly negligible," said Tom Wigley, a climate expert at the University Corporation for Atmospheric Research. "Compared with the human influence on climate, it is a very minor effect."

lee.hotz@latimes.com

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