NIU researcher gets heated publicity

by Tom Parisi

You might have seen NIU climatologist David Changnon recently on TV, in the newspaper or in a cornfield. That's because the latest research study by Changnon and two of his students has been attracting some major media attention. Their study finds that the northern Illinois region is more apt now than in decades past to experience heat waves accompanied by extreme and dangerous spikes in humidity. And Changnon said corn crops, which are particularly efficient at transpiring water vapor into the lower atmosphere, could be at the root of the problem.

NIU Professor David Changnon discusses his research with WGN Meteorologist Tom Skilling, who recently visited campus to film a segment on the findings.

"Our research findings at NIU suggest that a new, more dangerous breed of heat wave has become established in northeastern Illinois," said Changnon, an NIU professor of meteorology.

"Heat waves today are different than they were a half-century ago because they are more frequently accompanied by extreme spikes in humidity," Changnon said. "I strongly suspect that changes in agricultural methods — particularly in the area of corn production — are playing a major role in this by adding more water vapor to the lower atmosphere of the Upper Midwest." WGN-TV's Tom Skilling broadcast a featured segment Aug. 13 on the research of Changnon and graduate students Jesse Sparks and Jason Starke. The Chicago Sun-Times followed the next day with a front-page headline, "Could corn be to blame for humidity?" Local newspapers, the national public radio station in Chicago (WBEZ) and Farm Week magazine also are picking up on the story.

Jesse Sparks (left) and Jason Starke, both graduate students in meteorology at NIU, monitor air temperatures and humidity at a weather station in a DeKalb cornfield.

All plants transpire, that is, release water vapor into the atmosphere through their leaves. Corn is unique in that it belongs to a family of plants that transpire both day and night.

Changnon points out that average corn yields in Illinois have increased from about 50 bushels per acre in 1950 to more than 130 bushels per acre in 2000. Planting densities climbed dramatically as well in the 1970s, when farmers started planting crop rows closer together.

"Overall, the amount of water transpired to the atmosphere in our region is greater today than it was a half century ago just based on changes in agricultural practices and corn production," Changnon said. "More plants and greater yields per acre imply an increased need for and use of water by corn.

"I'm not knocking the agricultural industry — corn and other crops are absolutely vital to our region, our nation and the world," Changnon added. "These higher dew points represent a product of a complex agricultural process. We don't want to go backward in terms of production techniques, but we need to investigate and deal with the impacts related to what appears to be a significant factor in the regional climate of the Midwest."

Changnon began to suspect a link between agricultural production and humidity after he and his student researchers studied historical trends in northeast Illinois dew-point values. The results of that study are published in the August edition of the Journal of Applied Meteorology.

The dew point is the temperature at which condensation begins, and dew-point measurements provide an indication of how much water vapor is in the air.

Initially, the study was undertaken in response to concerns of NIU's physical plant supervisor, who saw a relationship between dew points and the efficiency of the campus cooling system. But while extreme hot and humid weather can take its toll on air-conditioning systems and increase electrical demand, it also can be deadly.

The northern Illinois heat waves of 1995 and 1999 claimed hundreds of lives. Previous studies have noted that both high air temperatures and high dew points characterized those hot spells. "The higher the dew-point value, the more difficult it is for the body to cool itself through evaporation from the skin," Changnon said.

Changnon and his students analyzed hourly dew-point readings recorded from 1959 to 2000 at Chicago's O'Hare Airport and the Greater Rockford Airport.

Analysis of the 10 most extreme heat waves in the region showed that the number of high dew-point hours was much greater after 1980. The top four high dew-point frequency years were 1983, 1987, 1995 and 1999. Additionally, the researchers discovered that three different dew-point indices at both airports showed general increases over time.

The Gulf of Mexico plays a large role in humidity levels in northeast Illinois. Normally during the summer, tropical air masses originating in the Gulf of Mexico move into and remain in the Midwest for days or weeks at a time. Because that source of water vapor hasn't changed over time, Changnon ruled out the Gulf as being the source for the growing frequency in high humidity levels. He noted that during the 1995 and 1999 heat waves, the dew-point levels were greater in the Upper Midwest than in those areas between the Midwest and the Gulf Coast.

Changnon also ruled out urban heat island effect as a primary cause of humidity spikes because similar trends were identified at both the suburban O'Hare weather station and the rural Rockford site. For transpiration to occur at levels that cause such high dew points, crops such as corn and soybeans must have access to sufficient soil moisture. Over the past 100 years, precipitation in the Midwest has increased, Changnon said.

He said the link between adequate soil moisture, increased transpiration and a greater number of high dew points was evident in the 1995 and 1999 heat events, which were preceded by average to above average precipitation across northern Illinois.

"In contrast, the heat wave of 1988 was accompanied by a drought," Changnon said. "Corn yields dropped by nearly 50 percent. And although it was an extremely hot summer, with Chicago experiencing temperatures of 90 degrees or greater on more than 40 days, very few high dew points occurred."