Climate trail goes cold at sun? -- Warming concerns not linked to star

Union Tribune (2007-07-19) Brian Handwerk

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By Brian Handwerk NATIONAL GEOGRAPHIC NEWS

July 19, 2007

Cyclical changes in the sun's energy output are not responsible for Earth's recent warming, a new study asserts.

The findings put the blame for climate change squarely on human-created carbon dioxide and other greenhouse gases, reinforcing the beliefs of most climate scientists.

The sun's output waxes and wanes due to a variety of mechanisms. Its power rose during much of the 20th century, but it has declined. “Up until 1985, you could argue that the sun was (trending) in a direction that could have contributed to Earth's rising temperatures,” said study author A. Mike Lockwood of the University of Southampton in Britain.

Two decades ago, “it did a U-turn. If the sun had been warming the Earth, that should have come to an end, and we should have seen temperatures start to go the other way,” Lockwood said.

Yet temperatures have continued to climb since that date, making a strong solar role in warming appear unlikely.

“I think it's quite conclusive,” said Lockwood, a co-author of the report in the current issue of the U.K. journal Proceedings of the Royal Society.

For centuries scientists have pondered the link between the sun's intensity and Earth's climate. When global warming became an issue in recent years, the debate heated up.

Recently one hypothesis suggested that cosmic rays from the sun could be responsible for significant warming. The cosmic rays deliver particles to Earth's atmosphere, around which cooling clouds may form. During periods of high solar strength, the sun's magnetic field blocks some of these rays, which in turn could hamper cloud formation on Earth and cause the planet to warm.

But the new data seem to be at odds with that theory.

“In terms of the last 30 years, I would have to agree that there's nothing in these records that suggests solar variability could be giving rise to warming global temperatures,” said Carl Wunsch, a climate expert at the Massachusetts Institute of Technology who was not involved in the study.

“Nobody can show that there is no solar-Earth climate connection,” he added. “But having said that, if there is one and if these records are representative, whatever connection exists is weak.”

In the latest study, co-author Lockwood does report evidence for a solar role in shaping Earth's preindustrial climate: “I even believe that you can detect in the climate record a solar influence up until about 1940,” he said. “The trouble is that (in) about 1960, solar variability started to become dominated by fossil fuel burning and greenhouse gases.”

Gavin Schmidt, a climate scientist at NASA's Goddard Institute for Space Studies and an editor of the blog http://www.realclimate.org, agreed.

“Basically, there simply isn't a trend in solar activity that would explain the rapid warming that has occurred in recent decades,” said Schmidt, who was unaffiliated with the study.

That's not to say that there hasn't been a solar influence on climate change in the past, or that there isn't still uncertainty in possible mechanisms, he added.

“But for what we are concerned about now, the sun is not to blame. Think of the sun as a criminal suspect who has a long record, but a cast-iron alibi for the latest crime,” Schmidt said.

“And meanwhile, the fingerprints of CO2 are all over the murder weapon.”