Midwesterners sweltering in hot, muggy weather this summer are hearing that their agricultural mainstay—corn—is at least partly responsible for their discomfort. This is because the corn plants transpire, drawing moisture into the air, which adds to the humidity. One can think of transpiration as the crop “sweating”—the plant’s means of staying cool. And high summer, when corn plants are tasseling, or setting silk, corresponds with peak sweat.
When hot, moisture-laden air hovers over an area, it’s referred to as a “heat dome.” Added humidity makes it feel even hotter. Moisture transpired by plants can raise the dew point—the temperature threshold of saturation, below which condensation will form. Is this inevitable when growing corn, or just the result of how we grow it?
One factor is plant density, says agricultural meteorologist Eugene Takle, director of the Climate Science Initiative at Iowa State University. “When I grew up in southwest Minnesota, there were 12,000 to 14,000 plants per acre,” he recalls. “We’ve doubled or tripled what we had 50 years ago.” The composition of plants matters, too. “We’ve changed the landscape from tall-grass prairie to bi-culture, mostly corn,” he says. “Corn stays green into August and part of September. It’s photosynthesizing—as an annual plant it does so intensely and all at once.” A plant that’s photosynthesizing will draw down atmospheric carbon—a plus climate-wise—yet will also be emitting moisture. Come mid-July heat, those tightly spaced plants can mean steamy conditions.
Corn-growing practices geared to large yields also tend to generate heat, says David Yarrow, a soil and biochar consultant based in Kansas City. “Years of clean tillage and heavy doses of nitrogen fertilizers will deplete the soil of carbon, and carbon is a principle factor to increase soil macro-structure, porosity and water holding,” he says, noting that bare, mineralized soils will absorb heat and evaporate more moisture.
Jerry L. Hatfield, laboratory director and supervisory plant physiologist at USDA-ARS, says extreme temperatures, particularly at night, mean the corn itself is taking a hit. This is due to higher plant respiration, whereby the plant releases CO2. “You can think of plant growth as an equation: photosynthesis minus respiration,” he says, adding that since respiration is driven by temperature rather than sunlight, it can carry on through the night. “With higher humidity the plant can’t cool itself as much. Say it’s 95 degrees outside, and the plant is at 90 degrees. The respiration rates are going to be extremely high. This means very little energy leftover for growth” and, therefore, lower yields.
Christopher J. Anderson, assistant director of Iowa State’s Climate Science Program, emphasizes that corn sweat is but one small factor in weather. And that, surprisingly, it is likely lowering temperatures, albeit slightly. For transpiration, which converts heat energy into latent heat suspended in water vapor, is a cooling mechanism. On hot summer days, he says, corn could be putting back into the air a quarter inch or more of water.
Still, as Eugene Takle says, “Water vapor is the atmosphere’s largest greenhouse gas. When you put a lot of moisture in the air, you’re trapping heat. Even if more is allocated to transpiration than sensible heating, what sensible heating you have is trapped.”
Plus the all-important humidity factor. “Corn can change the dew point from 75 to 80 degrees,” says Christopher Anderson, noting that this could bolster the “apparent temperature”—what it feels like—from 100 to 105. As the Corn Belt looks forward to a string of hot days, he says, “It’s going to be a sauna.”
Judith D. Schwartz is an environmental journalist based in Vermont (though she did spend a year in Iowa). Her new book, Water In Plain Sight: Hope for a Thirsty World, will be out from St. Martin’s Press on July 26.