Impact of Smoky Skies Is Hazy

Canada Fire Smoke May Affect Pollinating Corn

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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Thick smoke and haze that made U.S. air quality across the central Midwest the worst on Earth in late June may affect pollinating corn. However, the extent of the impact is uncertain. This map shows the extent of smoke coverage in the U.S. at about 3 p.m. CDT on Tuesday, June 27. (College of DuPage graphic).

It's deja vu all over again when it comes to wildfire smoke filling the skies over the central U.S. in summer. For the second time in three seasons, haze from Canadian wildfires has blanketed Midwest skies. And that again raises the question of what impact -- if any -- smoky skies could have on developing crops.

In 2021, the fire source was in the Western U.S., along with western and south-central Canada. This year, the main fire source is eastern Canada, mainly the forests of Quebec. Circulation around stationary high pressure in central Canada has carried that smoke into the central and eastern U.S. and turned Midwest skies into the dirtiest on Earth. Air quality bulletins have been numerous during the month of June across the Midwest and Northeast because of the smoky haze. A comment in the July 3, 2021, issue of the New Yorker magazine by Dhruv Khullar noted that "... the number of Americans who experience at least one day of 'extreme smoke' a year has increased twenty-seven-fold since 2006."

The threat to human health from the thick haze is widely publicized, especially regarding the health impact on children. But what about crop health? Anything that might interfere with corn pollination is important.

However, crop impact appears to be mixed, depending on the thickness of the hazy sky screen and how long it lasts. And there is even the potential for some benefit from the haze. Writing in the Journal of Geophysical Research in January 2020, environmental scientists Kyle Hemes of Stanford University, Joseph Verfaillie of the University of California-Berkeley and Dennis Baldocchi, also of UC-Berkeley, noted that haze can benefit plant leaves by scattering the incoming sunlight. The published article reads in part:

"While we know that wildfire smoke has negative health impacts for humans, there is evidence that it could increase plant productivity. This occurs due to the way that smoke scatters incoming sunlight, allowing the sun's energy to reach further into dense plant canopies. ... We find that smoky conditions increased the efficiency by which these plant canopies photosynthesized, leading to productivity increases, depending on trade-offs with total light and other pollutants."

An article with more details on the study that concluded crop benefit is available at this link:….

One feature of the potential for crop damage from smoke appears to be the ozone content in the smoke. The Minnesota Extension Service notes this aspect in an online article titled "Managing Wildfire Smoke: Impacts to Crops and Workers" published in August 2021. Specifically, the article notes: "Plant stomata are pores on the leaf surface where gas exchange occurs, and while we've probably all learned that plants intake CO2 and exhale oxygen, other gases also enter plants through their stomata. When high concentrations of ozone are present in the atmosphere, it enters plants through their stomata, and can interfere with photosynthesis. These impacts can occur hundreds of miles from the area that's actively burning (Yue & Unger, 2018)."

Midwest smoke is forecast to thin out during the latter part of this week. Uncertainty over whether this will be the end of smoky skies for the summer, and how much crop health was affected by what has gone on, will stay around through the end of the season -- truly a hazy outlook.

Bryce Anderson can be reached at

Bryce Anderson

Bryce Anderson
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