Temperature record for Midwest shows impact of global warming

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Temperature record for Midwest shows impact of global warming
mongabay.com
January 29, 2007

Researchers have developed a new method to create a temperature record for the Great Plains region of the United States. The model, based on analysis of ancient soils, could help predict the impact of global warming on American agricultural production.

The researchers, lead by Dr. Lee Nordt, a geology professor at Baylor University, "developed the first comprehensive temperature record for the Great Plains by assessing the behavior of the stable isotopic composition of buried soils," according to a news release from Baylor. "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."

NASA illustration showing average temperatures from 2001-2005 compared to a base period of temperatures from 1951-1980.

The work, published in the February issue of GEOLOGY, found that the annual mean temperature varied by about 10 degrees Fahrenheit over the last 12,000 years in the Great Plains, a region that extends from southern Canada to north Texas. The Baylor release elaborates:

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 that warming temperatures over the past 12,000 years are mostly the result of increasing intensity of the sun but that current declining intensity should be producing lower temperatures. He suspects that anthropogenic emissions of greenhouse gases might be stalling the expected fall in Midwest temperatures.

"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."

This article is based on a news release written by Matt Pene from Baylor University

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