October 27, 2003
New evidence of global warming in Earth's past
supports current models of how climate responds to greenhouse gases
By Tim Stephens
Scientists have filled in a key piece of the global climate picture
for a period 55 million years ago that is considered one of the most
abrupt and extreme episodes of global warming in Earth's history.
|Researchers determined sea surface
temperatures 55 million years ago by analyzing the chemical composition
of the shells of microscopic plankton. This scanning electron microscope
image shows a shell of the plankton species Acaranina soldadoensis.
Photo: Amanda Brill, University of North
The new results from an analysis of sediment cores from the ocean
floor are consistent with theoretical predictions of how Earth's climate
would respond to rising concentrations of greenhouse gases in the atmosphere.
The new study, led by James Zachos, professor of Earth sciences at
UCSC, was published online by Science Express last week, and
will appear in a later print edition of Science magazine.
The researchers analyzed sediments deposited on the seafloor during
a period known as the Paleocene-Eocene Thermal Maximum, when a massive
release of heat-trapping greenhouse gases is thought to have triggered
a runaway process of global warming.
Climate theory predicts that the increase in greenhouse gases would
have caused temperatures to rise all over the planet, with greater increases
in sea surface temperatures at high latitudes than at low latitudes.
Zachos and a team of researchers at UCSC and several other institutions
have now obtained the first reliable estimates of the change in tropical
sea surface temperatures during this period. When combined with existing
records of sea surface temperatures at high latitudes, the findings
fit well with the predictions of computer simulations based on current
The study provides important backing for the climate models that scientists
are using to predict the effects of the current rise in atmospheric
carbon dioxide due to industrial emissions, Zachos said.
"The predictions from the models seem to be consistent with the
geologic record, so I'd say greenhouse climate theory is alive and well,"
he said. "People have raised questions about how accurate these
models are in terms of handling heat transport in response to rising
greenhouse gases, but this study indicates that the climate people have
got it right or close to right."
The Paleocene-Eocene Thermal Maximum, starting about 55 million years
ago and lasting about 150,000 years, is marked by dramatic changes in
the fossil record of life in the ocean and on land. Average global temperatures
increased by about 5 degrees Celsius (9 degrees Fahrenheit). The increase
in sea surface temperatures at high latitudes was 8 to 10 degrees Celsius,
and the new study shows a 4- to 5-degree Celsius increase in tropical
sea surface temperatures.
"This event is the best example of greenhouse warming in the geologic
record, and for the first time we have been able to document the climate
response on a relatively broad planetary scale, from the tropics to
polar latitudes," Zachos said.
The temperature estimates were derived from chemical analyses of the
shells of microscopic plankton preserved in the seafloor sediments.
The chemical composition of the plankton's calcite shells reflects the
temperature of the water in which they were formed. A key measurement
examined in this study was the ratio of magnesium to calcium, which
increases exponentially with the temperature at which the shells formed.
"The ratio of magnesium to calcium in seawater is relatively constant
over the timescale of this event, so the ratio in the shells is really
only sensitive to one variable, the calcification temperature,"
UCSC graduate students Michael Wara and Steven Bohaty performed most
of the chemical analyses. The researchers analyzed sediment cores recovered
from a site called Shatsky Rise in the tropical Pacific during an expedition
of the ship JOIDES Resolution in 2001 (Leg 198 of the Ocean Drilling
Program). The cores provided a complete sequence of deposits representing
the boundary between the Paleocene and Eocene epochs.
"There aren't many places in the Pacific where you can recover
sediments of this age in which the fossils are not so recrystallized
that they've lost their original geochemical signatures," Zachos
ODP Leg 198 and a complementary drilling expedition in the Atlantic
earlier this year (ODP Leg 208) were designed to test the leading explanation
for the Paleocene-Eocene Thermal Maximum, which attributes it to a massive
release of methane. Methane, a potent greenhouse gas, accumulates in
frozen deposits known as clathrates found in the deep ocean near continental
margins and also in the Arctic tundra. For reasons that remain unclear,
the clathrates suddenly began to decompose, releasing an estimated 2,000
gigatons (2 trillion tons) of methane.
Once released, the methane would have reacted with dissolved oxygen
in the ocean to produce carbon dioxide, another greenhouse gas. Large
amounts of both carbon dioxide and methane would have entered the atmosphere,
raising temperatures worldwide.
In addition to Zachos, Wara, and Bohaty, the coauthors on the Science
paper are Margaret Delaney, professor of ocean sciences at UCSC, Maria
Rose Petrizzo and Isabella Premoli-Silva of the University of Milan,
Amanda Brill of the University of North Carolina, and Timothy Bralower
of Pennsylvania State University. Bralower and Premoli-Silva were co-chief
scientists on ODP Leg 198.
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