While temperature records dating back thousands of years already exist for certain regions of the United States, like the East Coast and the Northwest, no such record exists for the North American Great Plains. But now, a Baylor University researcher along with a team of scientists has developed a new method to measure temperature fluctuations in the Great Plains, creating a temperature record for that area of the country dating back 12,000 years. It is the first such record for the Great Plains, a region in the central part of North America stretching from southern Canada to north Texas.
The research results were published by the Geological Society of America in the February issue of GEOLOGY, one of the premier scholarly journals. Dr. Lee Nordt, a geology professor at Baylor, is the lead author of the paper with contributions by Joseph von Fischer, a special assistant professor of biology at Colorado State University, and Larry Tieszen, a researcher with the U.S Geological Survey.
Temperature records are usually created by assessing fossil pollens, which are better preserved in areas with a wetter climate and more swamps. But in the Great Plains, pollen preservation is virtually nonexistent, so scientists had to come up with another way to develop a temperature scale.
Nordt developed the first comprehensive temperature record for the Great Plains by assessing the behavior of the stable isotopic composition of buried soils. Using this methodology, Nordt examined the relationship between the fluctuations in the abundance of warm-season grasses and the mean July temperature from 61 modern native prairies. He then applied a mathematical equation of what he found in the modern record to the buried soil record from the same region. The result was a reliable temperature curve for the past 12,000 years.
"Grasslands make up a significant portion of the world's ecosystems, but we never had a way to create a reliable temperature scale," Nordt said. "This new method will provide the base. We can now go to many other grassland regions, apply the new method and create a temperature curve."
In the Great Plains, Nordt found the annual mean temperature fluctuated about 10 degrees Fahrenheit over the last 12,000 years. Between 12,000 and 7,000 years ago, the annual mean temperature was significantly cooler than what it is today, Nordt said, even though the earth was coming out of an ice age and the sun's intensity on earth was steadily increasing. From 7,000 to 5,000 years ago, temperatures finally warmed to the point that it was about 2 degrees hotter than it is today. It has since gradually declined to present day temperatures.
"The results really surprised us, especially between 12,000 and 7,000 years ago," Nordt said. "Earth temperatures should have been getting warmer during that time, but they weren't. We concluded it was caused by negative feedback from the melting glaciers. The ocean water temperature was colder because the glaciers were melting. That, in turn, caused temperatures to drop."
Nordt said the sun's intensity on earth is the main reason why temperatures generally warmed over the last 12,000 years, but that intensity is now decreasing. Yet temperatures are not decreasing.
"Is this caused by global warming or are temperatures just lagging behind? We don't know," Nordt said. "At some point in the next few thousand years, I would expect temperatures to fall, but for now we have to be sensitive to both Earth's natural cycles and global warming produced by anthropogenic emissions of carbon dioxide."
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