Cropping patterns and irrigation could be cooling hot summers, as well as increasing midsummer rainfall.
Despite global warming advocates' figures that temperatures have been on the rise during the past century, average temperatures in the Midwest have declined during the past 50 years, says David Changnon, a climatologist at Northern Illinois University. During the same time period, more rain fell in July and August. He credits at least part of the change to cropping patterns and increased irrigation.
"Average July and August temperatures statewide in Iowa and Illinois exhibit 1⁄2° cooling from the 1930 to 1969 time period to the 1970 to 2009 period. Rainfall is up 1⁄3" to 1⁄2" in those months, as well,” Changnon says.
"Irrigation is enhancing the regional hydrologic cycle. What we've done is add more water to the surface, then the plants transpire that back into the atmosphere. It requires energy to do that, which reduces temperatures at that location,” he explains.
"In irrigation we put water into the plant when it needs it for growth. Then, the vast majority is pumped right back out by the plant.”
In addition, cropping patterns may have created a regional microclimate conducive to cooler temperatures.
"In Illinois, Iowa, Nebraska and Indiana, if you go back to 1950 in a typical county and look at how agricultural land was parceled, the vast majority was what I call general farmland. A lot more acreage then was used for pasture and livestock. Now, most of the animals are either gone or produced in a very different manner. In DeKalb County, Ill., for example, about 97% of the land is now in corn and soybeans,” Changnon says.
"Those two crops generally use the greatest amount of water from early July to Labor Day. We've seen increases in high dew-point temperatures and regional precipitation in those months. There may be some relationship between this incredible agricultural process and feedback into the atmosphere. We've tended to create a situation where there are cooler maximum temperatures,” he says.
Genetic impact. Higher plant population per acre can also add to the effect by creating a more dense canopy.
"Illinois, Iowa, Nebraska and Indiana have always been ag states. But you have to look at what's changed (at the genetics, the ability of the plant to produce more per acre, at higher plant populations, at the places we plant now where we didn't plant anything 40 years ago) and you see that this is a lot different from agriculture in 1950,” Changnon says.
This scenario can also make for more extreme weather events, he says.
- March 2010