It's high tide for ice age climate change.

Earth's climate veers between warm and cool roughly every 1,500 years. Many scientists have thought that sunspots choreograph these fluctuations, but new research puts the spotlight on the moon.

A few years ago geochemist Charles D. Keeling and geophysicist Timothy P. Whorf, both of the Scripps Institution of Oceanography in La Jolla, Calif., suggested that Earth's climate doesn't swing with sunspot rhythms but instead syncopates to the beat of the tides.

When tides are higher than usual, the scientists say, they bring colder water to the surface from deep in the ocean and lead to an overall Earth cooling. In a report in the April 11 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, the researchers propose that the tides may sometimes be strong enough to tug Earth into an ice age.

Tidal cycles have been known for centuries, says Keeling, but most scientists assumed that such transient events couldn't influence climate. Scientists did not consider whether strong tides could cause the deep cooling that grips the North Atlantic about every 1,500 years. After all, says Keeling, a cycle of that period "can't depend on something that only happens for a few hours."

However, Keeling and Whorf say that clusters of tides can make a big difference in Earth's warming and cooling pattern.

Tides are unusually high every 411 days, when the moon comes closest to Earth. An eclipse also makes for strong tides, as the sun, moon, and Earth come into alignment, says Keeling. Tides swell as Earth moves closest to the sun, too, he adds.

When those three events take place at the same time--something that happens once every approximately 1,800 years--powerful tides could churn enough cold seawater to plunge the world into an ice age, the researchers say.

The scientists correlated the pattern of tides with seafloor sediments that record major climate changes. They found that many cooling events, including the Little Ice Age of about 500 years ago, happened near the peak of the tidal cycle.

The correlation isn't perfect, acknowledges Keeling. He points out that the sun, moon, and Earth change their alignments slightly from cycle to cycle. In d tail, there's nothing ever quite the same from one cycle to the next," he says. Other climate-changing conditions could also override the tides, he says.

The researchers had to discard four "exceptional events" in the sediment record to see the correlation with the tidal cycle. Their hypothesis "is really going to fly or fall on whether you can really throw those out," say Gerard Bond, a paleoclimatologist at the Lamont-Doherty Earth Observatory in Palisades, N.Y. Discarding some of the data may be valid because those records may come from climate-shifting events other than tides, Bond says.

"I think it's a very interesting idea," says climatologist Randall S. Cerveny of Arizona State University in Tempe, but it's a notion "that's going to bear some further analysis."