June 27, 2005
New findings show
persistent El Niño-like conditions during past global
By Tim Stephens
During the most recent period in Earth's past with a climate
warmer than today, the tropical Pacific was in a stable state
of El Niño-like conditions, according to a new study
by UCSC researchers.
Whether this represents a likely scenario for the future, given
the current rise in global temperatures, is uncertain. Nevertheless,
the study has important implications for scientists trying to
understand the global climate system and how it might respond
to global warming, the researchers said.
El Niño is a temporary disruption of normal circulation
patterns in the ocean and atmosphere in the tropical Pacific,
with important consequences for weather around the globe. Because
the system always returns to the normal circulation patterns
after an El Niño event, many scientists have considered
these patterns to be the only state in which the system can
remain stable over long periods of time. The new study, published
this week in the journal Science, shows that there is
another stable state for the ocean-atmosphere system that is
dramatically different from today's.
"The stable configuration that we've gotten used to is
not the only stable configuration for the tropical Pacific,
and this suggests that the Earth's system for global heat transport
functioned in a fundamentally different way the last time the
climate was warmer than it is today," said the study's
lead author, Michael Wara.
Wara, who earned a Ph.D. in ocean sciences at UCSC, is now
studying law and environmental policy at Stanford University.
His coauthors are Christina Ravelo, associate professor of ocean
sciences at UCSC, and Margaret Delaney, professor of ocean sciences
The researchers based their findings on an analysis of hundreds
of samples from sediment cores drilled from the ocean floor
on opposite sides of the tropical Pacific Ocean. The sediment
cores were obtained by the international Ocean Drilling Program
from a site near Indonesia in the western Pacific and another
site near the Galapagos Islands in the eastern Pacific.
The sediments contain the microscopic shells of tiny sea creatures
called foraminifera that lived in the surface waters of the
ocean. The chemistry of these shells--in particular, the ratio
of magnesium to calcium--is highly sensitive to the temperature
of the water in which they formed. By analyzing the composition
of the shells, the researchers were able to reconstruct a detailed
record of sea-surface temperatures in the tropical Pacific during
the Pliocene epoch, which lasted from about 5 million years
ago to about 1.7 million years ago.
Currently, the normal sea-surface temperatures in the tropical
Pacific show a strong gradient from cool temperatures in the
eastern Pacific off South America, where upwelling of cold deep
water occurs, to much warmer temperatures in the west, where
the trade winds pile up warm surface waters. During an El Niño,
the trade winds slacken and warm water spreads eastward across
the tropical Pacific, drastically weakening the temperature
gradient. The UCSC researchers found that sea-surface temperatures
during the Pliocene were much like those seen during an El Niño
"It looks like a permanent El Niño," Ravelo
said. "We know El Niños have far-reaching global
climate effects today, so that gives us an idea of what the
global climate system may have been like during the Pliocene."
The UCSC group's findings contradict a study published earlier
this year in Science, which used the same methods but
found cooler rather than warmer temperatures in the eastern
Pacific. Ravelo said the difference is probably due to the much
smaller number of samples analyzed in the earlier study. The
UCSC group obtained more than 400 data points for the same time
period covered by six data points in the earlier paper.
"Maybe they were unlucky and got a couple of samples that
don't represent that time period well," Ravelo said.
Previous research by Ravelo and others has shown that conditions
outside the tropics during the Pliocene were also consistent
with a permanent El Niño-like state. The global consequences
of El Niño events include dramatic changes in rainfall
patterns, causing serious flooding in some areas while other
regions experience droughts. Shifts in ocean temperatures also
spread beyond the tropics, affecting fisheries along the California
coast, for example.
According to Ravelo, however, the El Niño-like conditions
of the Pliocene should not be regarded as a direct analogy for
the future effects of global warming. Rather, the Pliocene climate
should serve as a target for global climate models to test their
ability to reproduce the full range of possible climate states.
Climate experts use computer-driven climate models to help them
understand how the climate system works and how it is likely
to respond to changes such as the increasing concentration of
greenhouse gases in the atmosphere.
"The current climate models are very good at reproducing
stable conditions in the tropics like we have today, but they
should also be able to reproduce this very different tropical
climate state that was stable in the past. If they can't, we
know there is something missing," Ravelo said.
The new study reinforces the notion that the coupled systems
of oceanic and atmospheric circulation that drive the global
climate are capable of dramatic shifts from one stable state
"Many aspects of the climate system that appear stable
within a certain range of temperatures can shift dramatically
when a particular threshold is passed," Wara said. "We
can't say where that threshold is, but it is a concern as we
continue this ongoing global experiment of adding greenhouse
gases to the atmosphere."
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