Christina Ravelo, an associate professor of ocean sciences at the University of California, Santa Cruz, 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 than today, sea levels were higher, and polar ice sheets were smaller. 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. Our current climate happens to be a relatively warm period within this generally cool climate regime.
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 climatic 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.
The researchers analyzed sediment cores from the ocean floor for evidence of climatic conditions during the Pliocene. The 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 still 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. For example, on short timescales, we see the dramatic swings in climate known as El Niño and La Niña, 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 corrrelate 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" of the climate. 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," she said.
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.
"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. Their research was funded by the National Science Foundation.