By Brian Handwerk for National Geographic News
July 12, 2007

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

Instead the findings put the blame for climate change squarely on human-created carbon dioxide (CO2) 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 over the past two decades.

"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 Earth's temperatures have continued to climb since that date -- making a strong solar role in warming appear unlikely. (What is global warming?)

"I think it's quite conclusive," said Lockwood, who co-authored the report appearing in the current issue of the U.K. journal Proceedings of the Royal Society A.

Ancient Climate Change

For centuries scientists have pondered an intuitive link between the sun's intensity and Earth's climate. When global warming became an issue in recent years, the debate heated up. (Related: "Mars Melt Hints at Solar, Not Human, Cause for Warming, Scientist Says" [February 28, 2007].)

Recently one hypothesis suggested that cosmic rays from the sun could be responsible for significant warming. The star's 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. (Explore a virtual solar system.)

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

"In terms of the last 30 years I'd 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 in Cambridge, 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 past 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, climate scientist at NASA's Goddard Institute for Space Studies and an editor of the blog 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 isn't to say that there isn't 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 too 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."