Understanding the Impact of El Niño on the Midwest and Natural Environment

Janet Cole visited The Morton Arboretum, telling herself she had no need for more tree pictures. Yet, the sight of an amur corktree with vibrant leaves changed her mind. “I wanted to see its beautiful yellow colors,” said Cole, 75, a Downers Grove resident and volunteer at the arboretum. Since retiring as a librarian, Cole has volunteered for six years, helping with tasks from weeding to assisting with events. Despite the cold winters, she doesn’t ride her bike often, though this year may be different.

Unusually warm temperatures affect the timing of leaf color changes, with potential health impacts for Midwest plants and Great Lakes organisms. The National Oceanic and Atmospheric Administration (NOAA) indicates that above-average temperatures worldwide, including Chicago, are due to El Niño. According to Illinois state climatologist Trent Ford, El Niño isn’t a reliable precipitation predictor in Illinois but does indicate temperature trends. There is a 70% chance of above-average temperatures from December to February in the Great Lakes region, Ford noted.

Temperature affects tree color change, explained Michael Jesiolowski, Morton Arboretum’s plant clinic manager. Leaves usually change as temperatures fall below 32 degrees. As of mid-October, Chicago lows stayed above 45 degrees before dipping into the low 40s and high 30s, per the National Weather Service. The area reached 88 degrees on October 12, the highest record for that date since 2008. Typically, maple trees display their brightest colors in mid-October and oaks in late October in Chicago, according to the arboretum. However, leaves were still starting to change color at the arboretum by mid-October 2026.

Delays in leaf color change might result in longer-lasting colors, as Jesiolowski stated, depending on factors like precipitation and sunlight. Cooler mornings and evenings typically influence fall colors. Without freezing mornings, abundant fall colors aren’t evident yet.

The El Niño-Southern Oscillation (ENSO), a Pacific Ocean warming and cooling cycle, is a major global climate driver, NOAA says. ENSO includes the warm El Niño and cool La Niña phases. Human-caused climate change may intensify this El Niño, predicted to be historic by experts. NOAA forecasts mild or above-average temperatures for most of the U.S. this winter, with Illinois expecting warmer temperatures and potential changes in precipitation.

El Niño affects U.S. agriculture, offering benefits and challenges. It could help alleviate drought conditions in some regions but increase crop issues like pests and weeds. David Zaya, a botanist at the Illinois Natural History Survey, noted that El Niño years may enhance corn and soybean production due to a longer growing season. Cold springs pose planting challenges, which less often occur in El Niño years. According to state climatologist Zack Leasor from the University of Missouri, increased Pacific Ocean heat alters global weather patterns, bringing more rain to the South and Midwest. Missouri has faced five consecutive drought seasons, highlighting the need for wetter periods, Leasor added.

However, warm temperatures may impact crops like cherries, apples, and peaches in Southern Illinois, posing frost risk. Illinois’ major crops, corn and soybeans, might face higher pest survival rates into spring, as explained by Kelly Estes, an entomologist at the University of Illinois. Weeds, including marestail, resistant to some herbicides, can appear earlier with warmer winters. Zaya stressed concern over increased herbicide use nearby natural areas, potentially damaging them. In his study, 99% of surveyed natural areas in Illinois showed herbicide damage.

Warmer winters could also hasten the spread of invasive plants like Japanese chaff flower in Southern Illinois. Cold temperatures typically slow their spread.

Aquatic environments are similarly affected by warmer temperatures. Harmful algae, depleting oxygen or clogging fish gills, are prevalent in the Great Lakes. Studies found high toxin levels in algal blooms in the Illinois River in 2021. Heat accelerates algae growth, likened to compound interest by Casey Godwin, a research professor at the University of Michigan.

El Niño patterns also influence lake whitefish, whose eggs need ice for protection. Godwin explained ice buffers eggs from waves and wind.

While other regions might face flooding or droughts, the Midwest is expected to experience milder effects per climatologist Ford. Gardeners should monitor plant moisture as warmer conditions could stress them. Jesiolowski advises watering plants deeply weekly and using mulch or leaf litter to retain moisture, fostering root depth for better drought resilience.

As autumn progresses at the Morton Arboretum, Cole enjoys the gradual transition and finds it a welcome distraction. “It’s inspiring,” she remarked, appreciating fall’s beauty to escape worldly concerns.