The findings -- which may pin climate changes in the northern hemisphere to El Nino and La Nina-like conditions in the tropics -- contrast sharply with current theories that the climate oscillations were caused by events in the north Atlantic.

"Any studies of these abrupt climate changes have to take into account what's happening in the tropics. That's been a major void," said Lowell Stott, a USC associate professor of earth sciences and the paper's lead author.

The scientific context for Stott's research dates back more than a decade, when scientists extracted long "cores" from the ice sheet in Summit, Greenland, that gave them 70,000 years' worth of detailed climate information.

What they discovered caught the attention of the scientific community worldwide: Every 1,500 years or so Greenland's climate had undergone temperature changes of up to 59 degrees Fahrenheit.

"They're enormous changes," Stott said. "People have now found that these same climatic events were recorded throughout the northern hemisphere. That means that the global climate itself was changing on these time scales."

But what caused these changes, marked by heat spikes within a decade and cooling over a few thousand years?

Previous hypotheses focused on the role of the oceans, particularly the ocean circulation that originates in the north Atlantic.

Stott chose to look in the western Pacific for the answer and, with funding from the National Science Foundation and various European agencies, traveled there with an international team in 1998.

Like the researchers in Greenland, Stott and his team collected long cores that gave them roughly 70,000 years of climate history. But where researchers up north took cores of ice, Stott and his crew collected marine sediment that blankets the ocean floor.

The key lay in deciphering scientific clues trapped in the shells of microscopic organisms that lived a very brief life and then sank deep into the drink thousands of years ago.

The microorganisms, during a life span of days to weeks, essentially recorded information about the surface of the ocean in their calcium carbonate-rich shells.

By measuring the ratio of heavy to light oxygen isotopes -- atoms with different numbers of neutrons in their nuclei -- and magnesium to calcium in the shells, the scientists were able to piece together a picture of ocean temperature and salinity through the ages.

What they found was surprising.

The abrupt climate changes recorded in Greenland were matched by those in the tropics and seemed to correlate directly with El Nino or La Nina conditions.

When Greenland was experiencing warm conditions, for example, La Nina conditions were recorded in the tropics. Conversely, cold conditions in Greenland were matched with El Nino conditions in the tropics, Stott said.

"The implication is that changes in the tropical climate may have been a leading cause of the abrupt climate changes throughout the northern hemisphere," Stott said.

To understand this hypothesis, one must understand the role the tropics play in driving the weather around the globe.

The Earth receives much of the sun's energy along its equator and stores most of it in the western tropical Pacific around Indonesia. The warm water absorbs the solar radiation and emits the energy into the atmosphere in the form of steam vapor, which rises, cools and sinks again, causing large circulation systems.

"The tropics are where the climate system gets its energy," Stott said. "It's the engine for the weather and climate we experience on this planet."

So disruptions in these circulation systems -- known as El Nino and La Nina -- have a global effect.

The term El Nino, or "the little boy" in Spanish, was coined by Peruvian fishermen who noticed that a warm current appeared in the tropics every year around Christmas.

During an El Nino, the trade winds weaken over the tropical Pacific. Under normal conditions, they force warm water west, creating the atmospheric convection that drives the weather patterns.

But during an El Nino season, the weakened trade winds allow the pool of warm water in the western Pacific to slosh back toward the central and eastern Pacific, disrupting the atmospheric circulation system. The cold phase of El Nino is called La Nina.

Christopher Poulsen, a USC assistant professor of earth sciences who contributed to the paper, said the cause of the change between the El Nino- and La Nina-like conditions during cold and warm stages of the Pleistocene epoch is unknown.

"In the Science paper, the El Nino-like conditions correspond to the cold period. This is actually rather surprising," Poulsen said. "Most climate scientists would have guessed that the cold periods would correspond to La Nina-like conditions.

So, it is clear that we don't completely understand the physics that control the tropical climate variability between warm and cold periods."

Stott said studying these ancient patterns of warming and cooling may shed some light on the causes of modern-day global warming.

His current research indicates that the Pacific Ocean has steadily been warming since about 1700, long before any anthropogenic influences. That may mean that humans are not completely to blame for global warming, but may be compounding a natural cycle of warming.

"A substantial part of this late-20th-century warming appears to be associated with this natural climate variability," Stott said. "The important take-home message is that, for better or for worse, one of these abrupt warming events appears to coincide with the period of anthropogenic influence."

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