An analysis of sediments from the subtropical Atlantic Ocean deposited during Earth's last glacial period indicate sudden temperature fluctuations were as large as those seen in the warming at the end of the last ice age, raising concerns about future climate change.
Scott Lehman, a research associate at CU-Boulder's Institute of Arctic and Alpine Research, said the study indicated the temperature of the Sargasso Sea between the West Indies and the Azores fluctuated repeatedly by up to 9 degrees Fahrenheit from 60,000 to 30,000 years ago.
"What is new here is clear evidence that the warm Atlantic, like the polar Atlantic, was undergoing very large and very rapid temperature changes during the last glacial period."
Instrumental climate records and models indicate changes in warm ocean temperatures are likely to produce widespread, global climate impacts, he said. The impacts are due in part to the vast surface area of Earth's warm oceans and the fact that warm oceans create much more water vapor, increasing atmospheric heat trapping.
"The temperature of the warm ocean realm regulates the water vapor content of the atmosphere and its greenhouse capacity," he said. Past temperature records and climate models suggest ocean circulation changes, like those in the last glacial period, can be triggered by human activity, showing that "the impact of possible future circulation changes may be more dramatic and widespread than suspected."
A paper on the subject by Lehman and Julian Sachs, a former CU-Boulder researcher at INSTAAR now at Columbia University's Barnard College, will appear in the Oct. 22 issue of Science, the nation's premier weekly science magazine.
Lehman and Sachs reached their conclusions after studying 50 meters of sediment cores hauled up from several miles deep in the Sargasso Sea near Bermuda by French scientists as part of an international project. The CU researchers analyzed the saturation state of organic molecules from planktonic algae over the past 100,000 years, providing sea-surface temperatures during that period.
"The warming at the end of the last ice age about 10,000 years ago was supported by the disappearance of enormous ice sheets, a one-third increase in atmospheric carbon dioxide levels and changes in the seasonal distribution of the sun's energy," Lehman said. "But the abrupt changes we documented during the last ice age seem to be almost entirely ocean driven."
Freshening of Earth's oceans is believed to have the ability to trigger abrupt and long-lasting cooling events, including ice ages, by interfering with the conveyor belt of water carrying heat from the tropics to temperate regions. "Numerical modeling studies show that similar changes can be triggered by warming associated with human emissions as well," said Lehman.
"Trapping more heat in the atmosphere has the potential to kill major parts of ocean circulation, with the effects reverberating throughout the world," he said.
A 1999 study by INSTAAR's Don Barber and colleagues showed the collapse of two gigantic glacial lakes near Hudson Bay about 8,000 years ago poured enough fresh water into the Northern Atlantic to shut down the ocean circulation for several centuries, cooling Europe and Greenland by some 6 degrees F.
The last 8,000 years have been remarkably stable in terms of climate, considering the large temperature fluctuations, said Lehman. "By altering the environment through greenhouse gas emissions, we will likely find out how fragile the stability of Earth's climate really is. We may well find out we are dealing with a hair trigger."
The next step is to determine if similar changes occurred in the much larger Pacific Ocean, said Lehman. "If so, any human-induced changes to the ocean's plumbing are likely to affect everyone on Earth, not just Greenlanders and Northern Europeans."
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