New findings on climate show gradual shift to modern but increased sensitivity to perturbations

Arlington, Va. — Earth’s climate system is more sensitive to

perturbations now than it was in the distant past, according to a

study published this week in the journal Nature. The findings

suggest a previously unrecognized role for tropical and

subtropical regions in controlling the sensitivity of the climate

to change.

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 5 million to 1.8 million years ago, when the climate

was significantly 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, characterized by episodes of extensive glaciation in

the Northern Hemisphere. Today’s climate is a relatively warm

period within this generally cool climate regime.

The findings have implications for understanding modern climate

change. The Pliocene is the most recent period in Earth’s history

with warmer temperatures than today and comparable concentrations

of greenhouse gases, so it offers a tempting analogy for future

climate change. But the Pliocene was a very different time in

terms of circulation patterns and sensitivity to climate change,

Ravelo said.

Traditional explanations for the transition from the warm

Pliocene to the cool Pleistocene 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. Ravelo’s findings, however, point toward a

gradual process in which shifts in major components of the

climate system occurred at different times in different regions.

“We found evidence of regional responses that can’t be explained

by a domino effect. The transition took about 2 million years,

and there is no way one event could have led to that,” Ravelo

said.

Added Amos Winter, program director in the National Science

Foundation (NSF)’s marine geology and geophysics program, which

funded the research, “There is a big debate regarding the

mechanisms and rates of climate change from the warm Pliocene to

the cool Pleistocene. Using deep-sea sediment cores to

reconstruct climate over the last 5 million years, Ravelo and

colleagues demonstrate that the transition can’t be explained by

a single event, as previously had been thought.”

The researchers analyzed sediment cores from the ocean floor for

evidence of climate conditions during the Pliocene. Fossils of

microscopic plankton preserved in the sediments hold records of

ocean temperatures and seasonal variability. Even the extent of

glaciation on land can be determined from oxygen isotope ratios

in the calcite shells of marine plankton.

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

NiÒo and La NiÒa 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.

“Small changes in the solar budget gave large climate responses

during the Pleistocene, which we now think is related to

conditions in tropical regions that create strong feedbacks

between the ocean and the atmosphere,” Ravelo said. “During the

Pliocene, the system didn’t respond very strongly to small

perturbations, because there weren’t these feedback mechanisms

embedded in the atmospheric and oceanic circulation patterns.”

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.

NSF Program Contact: Amos Winter, awinter@nsf.gov,703-292-8580.

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