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.
NSF is an independent federal agency that supports fundamental
research and education across all fields of science and
engineering, with an annual budget of nearly $5.58 billion. NSF
funds reach all 50 states through grants to nearly 2,000
universities and institutions. Each year, NSF receives about
40,000 competitive requests for funding, and makes about 11,000
new funding awards. NSF also awards over $200 million in
professional and service contracts yearly.
Receive official NSF news electronically through the e-mail
delivery system, NSFnews. To subscribe, send an e-mail message
to join-nsfnews@lists.nsf.gov. In the body of the message, type
“subscribe nsfnews” and then type your name. (Ex.: “subscribe
nsfnews John Smith”)
Useful NSF Web Sites:
NSF Home Page: http://www.nsf.gov
News Highlights: http://www.nsf.gov/home/news.html
Newsroom: http://www.nsf.gov/od/lpa/news/media/start.htm
Science Statistics: http://www.nsf.gov/sbe/srs/stats.htm
Awards Searches: http://www.fastlane.nsf.gov/a6/A6Start.htm
Media Contact:
NSF, Cheryl Dybas, cdybas@nsf.gov, 703-292-7734.
UCSC, Tim Stephens, stephens@ucsc.edu, 831-459-2495.
National Science Foundation
4201 Wilson Blvd.
Arlington, VA 22230
“Where discoveries begin”
