Scientists have found evidence that tropical Atlantic Ocean
temperatures may have once reached 107°F (42°C)—about 25°F (14°C)
higher than ocean temperatures today and warmer than a hot tub. The
surprisingly high ocean temperatures, the warmest estimates to date for
any place on Earth, occurred millions of year ago when carbon dioxide
levels in Earth’s atmosphere were also high, but researchers say they
may be an indication that greenhouse gases could heat the oceans in the
future much more than currently anticipated. The study suggests
that climate models underestimate future warming.
“These temperatures are off the charts from what we’ve seen before,”
said Karen Bice, a paleoclimatologist at Woods Hole Oceanographic
Institution (WHOI). Bice reported the findings Feb. 17 at the annual
meeting of the American Association for the Advancement of Science
(AAAS) in St. Louis and is also lead author of a study to be published
in an upcoming issue of the journal Paleoceanography, published by the American Geophysical Union.
Bice and a multi-institutional team of scientists studied three long
columns of sediment cored from the seafloor in 2003 off Suriname,
on the northeast coast of South America, by the drillship JOIDES Resolution, operated by the international Ocean Drilling Program.
The sediments contained an unusually rich and well-preserved
accumulation of both carbon-rich organic matter and the fossilized
shells of microscopic marine organisms that had settled and piled up on
the seafloor over tens of millions of years. The deeper down in the
core the scientists analyzed, the further back in time they went.
The team analyzed the shells’ isotopic and trace element chemistry,
which changes along with temperature changes in the surface waters
where they lived. They determined that ocean temperatures in the region
ranged between 91° and 107°F (33° and 42°C) between 84 million and 100
million years ago in an era when dinosaurs roamed the Earth.
Temperatures range between 75° and 82°F (24° and 28°C) in the same
region now. The approximate uncertainty in the paleotemperature
estimates is +/-2°C.
Using organic matter from the sediments, the group also estimated
atmospheric carbon dioxide concentrations during the same time span.
They were 1,300 to 2,300 parts per million (ppm), compared with 380 ppm
today.
The findings, if confirmed, create a dilemma for scientists seeking to
forecast how Earth’s climate and environment will change in response to
the rising amounts of heat-trapping carbon dioxide in the atmosphere,
caused by deforestation and the burning of oil, coal, and other fossil
fuels. When 1,300 to 2,300 ppm of carbon dioxide is factored into
current computer models that simulate global climate, it does not
produce such high ocean temperatures.
“The climate models underestimate temperatures and the amount
of warming that would accompany an increase in CO2 of more than 1,000
ppm above today’s level.” Bice said..
If the scientists’ interpretations of past ocean temperatures and
carbon dioxide levels prove accurate, actual future warming from
elevated atmospheric carbon dioxide concentrations may be much greater
than predicted by the models, the scientists reported.
“One of the most important impacts this evidence suggests is the change
to the Earth’ hydrologic cycle,” Bice said. “Higher tropical temperatures will
increase the intensity of hurricanes and winter storms. In addition,
precipitation patterns will change, moving even more rain that now
falls on the central U.S. – an area known as the breadbasket of the
U.S. for its food production – to higher latitudes where the quality of
the soil may not be as conducive to agriculture”
“Policymakers use these models to predict likely climate change with increasing CO2 levels,
and if the models are not right, society is not well informed or well
served.”
Alternatively, the models used to predict future climate may be missing
a critical factor that amplifies heating, Bice said. During past warm
periods, oceans and wetlands may have released much more methane gas to
the atmosphere. Methane traps heat 10 times more effectively than
carbon dioxide.
However, extraordinarily high concentrations of methane in the model
still fail to produce the tropical Atlantic and Arctic Ocean
temperatures inferred for 91 million years ago. This supports the idea
that the model’s response to increased greenhouse gas concentrations
underestimates the actual climate system’s response.
The research team included Bice and Kristina Dahl of WHOI, Philip A.
Meyers of the University of Michigan, Daniel Birgel and Kai-Uwe
Hinrichs of the University of Bremen, and Richard D. Norris of Scripps
Institution of Oceanography. Bice’s work was supported by private
funding from the Woods Hole Oceanographic Institution through the Ocean
and Climate Change Institute and The Andrew W. Mellon Foundation
Endowed Fund for Innovative Research. Funding for this research was
also provided by the Joint Oceanographic Institutions U.S. Science
Support Program and Deutsche Forschungsgemeinschaft through the
DFG-Research Center Ocean Margins.
