Unique Weather A Factor In Record 2004 Midwest Crop Yields

"Never before have corn, soybeans, sorghum, and alfalfa hay all achieved record yields in the same year," said Stanley A. Changnon, chief emeritus of the Illinois State Water Survey (ISWS) and an adjunct professor of geography at the University of Illinois at Urbana-Champaign.

In Illinois, the average corn yield in 2004 was 180 bushels per acre -- 16 bushels an acre higher than the record set in 2003. Soybean yields was 50.5 bushels per acre, beating a record set in 1994 by five bushels per acre. Record high corn yields also were reported in Iowa, Indiana, Missouri, Nebraska and Ohio. Nationally, the corn yield was 160 bushels per acre -- 18 bushels an acre above the 2003 record.

"Planting during the 2004 growing season was early," Changnon said. "Summer temperatures were below normal with no hot days. Rainfall was adequate. Crop-yield predictions issued during the growing season and up through August 2004 did not anticipate the high magnitude of the corn and soybean yields that actually occurred."

Sophisticated crop-weather models relying on daily temperature and rainfall values of 2004 also did not calculate yields as high as the actual yields. Predictions and model-generated yields were 7 percent to 15 percent lower than final corn yields for the 11 Corn Belt states, and 15 percent to 33 percent lower than final soybean yields of the Midwest.

Those outcomes, Changnon said, help to reveal that weather conditions critical to generating extremely high yields of all Midwest crops were not detected. He and his son David Changnon, a geography professor at Northern Illinois University in DeKalb, decided to take a closer look at the weather conditions during 2004.

"A climatological evaluation revealed that summer 2004 conditions were unlike any experienced during the past 117 years," Stanley Changnon said.

They found that an unusually high number of sunny days had occurred, aiding photosynthesis. The frequency of summer days with clear skies was a critical, beneficial factor for all the crops grown in the Midwest.

"When a large number of clear days occurred in most previous summers, conditions were hot and dry with much above average temperatures and below average rainfall. Temperatures in 18 of the 33 summers between 1888 and 2003 with frequent clear skies averaged between 1.2 degrees Farenheit and 4.5 degrees Farenheit above the long-term average," Stanley Changnon said.

Summers with frequent clear skies, well below average temperatures, and above average rainfall occurred in just two years in the past 117 years: 1927 and 2004. Skies were clear on many more days in 2004 than in 1927, and June and August rainfall in both years had different magnitudes. Thus, the 2004 weather conditions were anomalous.

Summers with below average temperatures in all three months (June, July and August), as in 2004, occurred in 18 previous summers between 1888 and 2003. Sky conditions during those cool summers were mostly cloudy, quite different than in 2004.

Sunny, cool conditions in 2004 were a result of 20 cold Canadian fronts that crossed the Midwest, followed by strong high-pressure systems for several days. Each such intrusion dropped temperatures 5-15 degrees, followed by several clear days. High-pressure centers dominated the atmospheric circulation and kept warm, stationary fronts with their attendant penetrations of warm, moist air masses away from the Midwest.

"The atmospheric circulation pattern during summer 2004 was unusual, but these conditions and their crop impacts are not considered indicative of those expected with a change in climate due to global warming," Stanley Changnon said.

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The report detailing the study is available from the ISWS Web site (http://www.sws.uiuc.edu). The research was conducted as part of the weather-climate impacts program of the Midwestern Regional Climate Center (http://sisyphus.sws.uiuc.edu/) and the ISWS climate program. The climate center is funded by the National Oceanic and Atmospheric Administration and the state of Illinois.