Global Climate More Sensitive Now Than Ever Before

ARLINGTON, Virginia, May 21, 2004 (ENS) - Earth's climate system is more sensitive to changes now than it was millions of years ago, new research published this week in the journal "Nature" has found. A previously unrecognized role for tropical and subtropical regions in controlling the sensitivity of the climate to change has been identified.

Ana Christina Ravelo, an ocean scientist at the University of California, Santa Cruz (UCSC), and her coauthors at UCSC and Boise State University, Idaho, focused on the Pliocene epoch, from about five million to 1.8 million years ago, when the climate was warmer, sea levels were higher, and polar ice sheets were smaller than they are today.

During the late Pliocene, the climate shifted to the much cooler regime of the Pleistocene, when periods of extensive glaciation swept the Northern Hemisphere.

Ana Christina Ravelo is an associate professor in the Ocean Sciences Department at the University of California, Santa Cruz (Photo courtesy UCSC)

The Pliocene is the most recent period in Earth's history with warmer temperatures than today and comparable concentrations of greenhouse gases, but the Pliocene was very different in terms of circulation patterns and sensitivity to climate change, Ravelo said.

Her findings contribute facts to the debate taking place within the scientific community researching paleoceanography over the mechanisms and rates of climate change from the warm Pliocene to the cool Pleistocene.

Traditional explanations for the transition have focused on single events - such as the uplifting of mountain ranges or separation of ocean basins - that may have altered global circulation patterns and tipped the climate system beyond some threshold, resulting in a new climate regime.

But Ravelo's findings indicate a gradual process with shifts in major components of the climate system occurring at different times in different regions.

"We found evidence of regional responses that can't be explained by a domino effect," Ravelo said. "The transition took about two million years, and there is no way one event could have led to that."

Amos Winter is program director in the National Science Foundation marine geology and geophysics program, which funded the research.

"Using deep-sea sediment cores to reconstruct climate over the last five million years, Ravelo and colleagues demonstrate that the transition can't be explained by a single event, as previously had been thought," explained Winter.

The ocean floor holds clues to the climate millions of years ago. (Photo courtesy NOAA)

When they compared climate trends at different latitudes, the researchers found that tropical conditions remained stable while a major shift took place at higher latitudes. The onset of significant glaciation in the Northern Hemisphere took place about 2.75 million years ago, accompanied by cooling in subtropical regions. Significant changes in the tropics were not seen until a million years later, when conditions in the tropics and subtropics switched to the patterns of ocean temperatures and atmospheric circulation that persist today.

With this transition to the modern mode of circulation in the tropics and subtropics, the global climate system seems to have become much more sensitive to small perturbations. On short timescales, for example, dramatic swings in climate known as El Niño and La Niña are triggered by periodic changes in the equatorial waters of the Pacific.

On longer timescales, the comings and goings of the glacial ice sheets over hundreds of thousands of years during the Pleistocene correlate with cyclical changes in solar heating of the planet related to cycles in Earth's orbit around the Sun.

Climatologists refer to such effects as "solar forcing." But during the Pliocene, the same cyclic changes in solar heating took place without corresponding swings in the global climate.

Ravelo in her lab at University of California Santa Cruz (Photo courtesy UCSC)

The ultimate cause of the transition from Pliocene to Pleistocene climate regimes is still unknown. A likely candidate, however, is a gradual decline in the concentration of greenhouse gases in the atmosphere, Ravelo said.

"The forcing must have been gradual, and different places went through this major transition in the climate at different times because of distinct regional responses to the global forcing.

"If we use that time period as an analogy for the future, we need to understand that we are looking at a climate system that is really quite different than today," she said. "And whatever happens in the future, if there are significant changes in the lower latitudes, that could have major effects on the global climate system."

Ravelo's coauthors include Dyke Andreason, formerly a graduate student at UCSC and now at Rutgers University; Mitchell Lyle and Annette Olivarez Lyle of Boise State University; and UCSC graduate student Michael Wara.