Janet Cole told herself she didn’t need to take any more pictures of trees at The Morton Arboretum. She had more than enough.
But an amur corktree standing alone with a mass of vibrant leaves called to Cole, 75, and she found herself once again grabbing photos.
“I just wanted to come over and see it with its beautiful, yellow colors,” Cole said. “It’s a gorgeous tree.”
The Downers Grove resident has volunteered at the arboretum for six years, since retiring from her job as a librarian. She does everything from digging out weeds to assisting with events like wine tastings.
Cole even rides her bike around and answers visitors’ questions. It often gets too cold for that in the winter, but this year, she said it might be warm enough to keep cycling.
The abnormal warmth is also delaying the changing of many tree leaves, and could bring health implications for plants across the Midwest and certain organisms living in the Great Lakes.
In the coming months, Chicago and much of the world are likely to continue to experience above-average temperatures due to El Niño, a naturally occurring weather phenomenon, according to the National Oceanic and Atmospheric Administration.
Illinois state climatologist Trent Ford said El Niños are not reliable predictors of precipitation in Illinois. They can say a lot about temperatures, though.
From December through February, he said the Great Lakes region faces a roughly 70% chance of above-average temperatures.
“This is about as close to a slam dunk prediction for winter as we can get, as far as temperature is concerned,” Ford said.
Temperature plays a crucial role in when and how trees change color, said Michael Jesiolowski, The Morton Arboretum’s plant clinic manager. Often leaves begin to change as temperatures drop below freezing, or 32 degrees, he said.
As of Thursday, October lows had remained above 45 degrees, but later in the week they began dropping into the low 40s and high 30s, according to the National Weather Service. The city and surrounding suburbs had hit 88 degrees Oct. 12, shattering the area’s record high for the date set in 2008.
On average, maples develop their brightest and most consistent color in mid-October, and oaks in late October in the Chicago region, according to the arboretum.
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Although foliage may turn yellow and red later in the season this year, the trade-off is longer-lasting colors, Jesiolowski said. This depends on a lot of factors, though, such as precipitation and how much sunlight plants get.
“One of the determinants of fall color is the cool mornings, cool evenings,” he said Tuesday. “And since we haven’t had those freezing mornings or really anything close to a freezing morning, we’re not seeing an abundance of fall color right now.”
A mixed bag
The El Niño-Southern Oscillation, a years-long cycle of warming and cooling in the Pacific Ocean, is arguably the most influential climate driver on Earth, according to NOAA. It’s made up of a warmer period known as El Niño, which is currently affecting weather globally, and a cooler period known as La Niña.
Amplified by human-caused climate change, many experts are forecasting an historic El Niño.
This winter will see the “strongest El Niño on record or very close to it,” said Zack Leasor, state climatologist at the University of Missouri. The last El Niño occurred in 2023.
While the effects of El Niño could be severe in other parts of the world, the winter forecast from the National Oceanic and Atmospheric Administration, released Thursday, predicts most of the U.S. will see either normal or above-average temperatures. Illinois is expected to have warmer than normal temperatures, and the region has an equal chance of getting more or less precipitation.
El Niño will be a mixed bag when it comes to agriculture. It could help address years of drought in some parts of the country but exacerbate adverse crop conditions such as pests and weeds.
“For the crops, there is a kind of trend that El Niño years have higher corn and soybean production, especially corn,” said David Zaya, a botanist at the Illinois Natural History Survey. “And that has to do with a longer growing season and easier planting.”
Cold springs can make it harder for farmers to plant in fields that aren’t fully thawed, he said, which is much less of a problem during El Niño years.
Leasor also said that this year’s weather could be a game changer for some farmers. So much heat is over the Pacific Ocean during an El Niño that it changes global weather patterns. In the U.S., this means more rain in the South and parts of the Midwest.
“A wetter winter, a wetter spring is really just what we need,” Leasor said. “And, like many other parts of the Midwest, we’ve had five consecutive growing seasons in Missouri with substantial drought.”
The U.S. Drought Monitor on Thursday identified portions of Missouri as facing exceptional drought, its highest designation. Southern Illinois reached moderate drought, and northern Illinois did not face drought conditions.
Ford, the Illinois climatologist, said elevated temperatures in southern Illinois are especially concerning because of the potential effect on crops that continuously produce yields without having to be replanted, such as cherries, apples and peaches.
These crops normally stay dormant throughout the winter but could wake up early during a warm stretch. If temperatures then drop, the plants risk freeze damage.
For Illinois’ staple crops: corn and soybeans, more pests may survive into the spring, putting the plants at increased risk, said Kelly Estes, an entomologist at the University of Illinois.
There are also weeds. Some, especially marestail (also known as horseweed), can survive mild winters and appear earlier in the spring when it gets warmer. Marestail commonly spreads in corn and soybean fields, where it is resistant to some weedkillers and can crowd out crops, according to the University of Illinois Extension.
“That doesn’t mean it will hurt crop production, necessarily, because there are effective treatments for them, but what it might mean is that more effort needs to go into controlling those very early season weeds,” Zaya said.
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He said he worries about the impact that increased herbicide use could have on natural areas near farms. If warmer temperatures mean more weeds, in turn leading to more herbicides, then these chemicals could drift over to nearby natural areas, already a significant problem in Illinois, Zaya said.
Of the many natural areas in Illinois that Zaya has visited, about 99% showed visible signs of herbicide damage, according to a study he co-authored.
Mild winters also mean some invasive plants could spread at faster rates, Zaya said. One example he gave was the Japanese chaff flower, an invasive species in southern Illinois that crowds out native plants and spreads aggressively. Cold winters slow the spread.
Algae and fish
Warm temperatures this winter could also affect organisms living in the water.
The spread of harmful algae, which eat oxygen in the water or clog fish gills, is a growing problem in every Great Lake, according to NOAA.
Algal blooms sampled in 2021, showing as neon green splotches in the Illinois River, revealed levels of a potent toxin more than 30 times above the advised state recreational standard.
Heat makes harmful plankton divide faster, said Casey Godwin, an associate research professor at the University of Michigan. El Niño could be a significant contributor to the growth of harmful algal blooms, according to a 2022 study published in Nature Communications Earth and Environment.
Godwin compared the effect of warm water temperatures on algae over weeks to the way money can compound exponentially.
“Think of an interest rate. A slow interest rate, even a modest interest rate, over a longer period of time leads to a lot of growth of money, the same way plankton grow,” Godwin said.
El Niño patterns also affect lake whitefish, which Godwin says rely on ice to protect their fragile eggs. Ice acts as a buffer, limiting the presence of wave and wind energy that can destroy the eggs.
“The success of their eggs, which are deposited close to shore, is actually enhanced by having ice cover,” he said.
Mild impacts locally
While other parts of the world will have to contend with flooding and droughts, Chicago and the Midwest will likely only experience mild impacts, Ford said.
But homeowners with gardens and trees should be alert as warmer temperatures mean many plants could struggle to retain moisture.
Jesiolowski said people should make sure to water deeply at least once a week, as opposed to watering lightly every day. This causes plant roots to grow further down, he said, which allows them to better seek out water during droughts.
People should also apply mulch and tree leaves to retain moisture in gardens, he said. Following a gardening practice known as “Leave the Leaves,” many nationwide are opting not to remove leaves from their yards for conservation reasons.
“In terms of the mulch and the leaf litter, having those in your garden, having those over plants is, basically, it forms a carpet that, the water is not being evaporated to the air so quickly,” Jesiolowski said.
At the Morton Arboretum, leaves continue to build toward peak viewing as low temperatures are now dropping into the 40s. Cole is content to wait.
“It’s inspiring,” she said. “It’s a distraction from events in the world.”