Cooling, warming of oceans may be linked to sun cycles

WASHINGTON -- The brightening and dimming of the sun may account for a 1,500-year cycle of cooling and warming on parts of the Earth, a study of ice in the North Atlantic suggests.

Researchers found that a very slight difference in the amount of solar energy reaching the Earth can have a powerful chilling effect on the climate: ice builds up in lands bordering the North Atlantic, the average temperature drops in Europe and North America.

"Whether the whole Earth is affected, we don't know for sure yet, but it is certainly implied," said Gerard C. Bond, a researcher at the Lamont-Doherty Earth Observatory of Columbia University in Palisades, N.Y.

"The effect does extend from the high northern latitudes down, maybe even to the tropics," said Bond, first author of a study that appeared Friday in the journal Science.

The cycle of sunlight intensity roughly follows a 1,500-year pattern, based on analysis of the past 12,000 years. But the difference from the top of the cycle to the bottom is very small, with less than a 0.1 percent difference in energy levels, he said.

Bond and his colleagues believe this is enough to trigger severe climate changes, such as the Little Ice Age, a 490-year period starting in 1400 that dramatically chilled Europe and the North Atlantic.

The study is an effort to determine if small changes in sunlight over centuries can cause the Earth's climate to warm or cool. Other experts working on the same problem said Bond and his team have made a strong case.

"It shows that the connection is real," Jeffrey Park of Yale University said in Science. To David Thomson of Queen's University in Ontario, Canada, it seems like "a fairly convincing case."