Amid one of the warmest springs on record in Chicago, and renewed worries about our warming planet, how is it that late summer days across the Midwest are cooling?
The answer may be in the towering, tightly packed rows of corn that blanket Illinois at harvest and the ripple effects from industrialized farming that scientists are only beginning to understand.
In the last 80 years, those lazy, late summer days in July and August have been getting cooler and wetter throughout much of the Midwest. In Chicago, temperatures reached 90 degrees or higher 344 days during the 1930s, but only 172 days in 2000-09.
In place of those dry, 90-degree scorchers are the kind of lingering 80-degree days with higher humidity that don't cool down much at night. Climate scientists say the cause is rising dew-point levels — the measure of water vapor in the atmosphere.
These high dew-point levels are important, said David Changnon, a climate scientist at Northern Illinois University who helped pioneer this research, because even though the temperature is lower, the heat index is higher. And that's bad news for many city dwellers, since those conditions contributed to the deadly heat waves that hit Chicago in 1995 and 1999.
Already, these cooler but muggy late-summer days are likely to be producing more powerful thunderstorms and periods of heavier rain that bear watching, Changnon said.
"While we're seeing fewer really hot days, we've created dew points in Chicago and around the Midwest that are unheard of," Changnon said. "And it begs the question, 'How the heck can we do that?'"
Changnon's theory, backed by more than a decade of research, is that more densely planted corn and soybean fields scattered across the Midwest are changing the regional climate by releasing more water vapor into the atmosphere. The more water vapor that reaches the atmosphere, the higher the dew point, and the fewer extremely hot summer days.
In other words, while some still question whether people are to blame for changing weather patterns around the globe, farmers around the Midwest are already altering the region's climate in significant ways, Changnon said.
"It's a different type of human-induced climate change that has certainly played a role in the changes to Illinois' weather," said Jim Angel, a climatologist at the Illinois State Water Survey in Champaign. "It's kind of an interesting way to look at all this."
Interesting, but also crucially important, Changnon said, as climate scientists ponder two intriguing questions related to this research: Have Midwest farmers accidentally created a barrier to soften the most severe effects of global warming? And if so, can it be repeated elsewhere?
Changnon said scientists don't yet know the answers to these questions, but said they will clearly form the groundwork for the next phase of research.
"We've literally seen with our climate models a cool hole in the middle of the country," Changnon said. "Perhaps what we're learning is that this intense agricultural process is helping mask the type of climate change we'd expect to see from a warming globe.
"That's purely speculation at this point, but that's where the research has to go."
Thanks to improved technology, changes in farming practices, more effective herbicides and genetic improvements to corn stock, farmers are able to grow corn in increasingly tighter spaces, said Emerson Nafziger, a professor of crop sciences at the University of Illinois at Urbana-Champaign.
Those advancements have narrowed the width between rows of corn from an average of 40 inches in the 1950s to about 30 inches today, enabling farmers to maximize their land and grow corn in numbers not thought possible decades ago, Nafziger said. Some counties now average more than 200 bushels of corn per acre today, more than twice what the most productive counties were growing in the 1970s.
For scientists, these record yields create a vexing environmental problem. Corn and soybean roots pull moisture out of the ground, then release it as water vapor into the air. This process, called transpiration, intensifies as crops become more dense, meaning today's corn and soybeans are using greater amounts of water for irrigation, Nafziger said.
By sending more water vapor into the atmosphere, the plants are actually improving their own growing conditions, researchers said. But the unintended consequences are the cooler and wetter days that could have far-reaching implications for the region, Changnon said.
Changnon is interested in the big picture. If denser corn and soybean crops are, indeed, responsible for Chicago's cooler summers, could denser sugar cane rows stave off some of the effects of global warming in South America? Could wheat and barley affect the climate in Russia? Lotus seeds in China?
"These are the types of questions we're now asking," Changnon said. "Who knows if we'll ever be able to find the answers, but they're important things to think about as we see our world changing."