May 24, 2004
New findings on climate portray a gradual shift
to modern mode with increased sensitivity to perturbations
By Tim Stephens
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.
Christina Ravelo's climate study focused on the Pliocene epoch.
While the laughably rapid climate change portrayed in the soon-to-be-released
movie The Day After Tomorrow remains safely in the realm of Hollywood
fiction, the new findings do suggest a previously unrecognized role
for tropical and subtropical regions in controlling the sensitivity
of the climate to change.
Christina Ravelo, an associate professor of ocean sciences at 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 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
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,
"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.
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