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Lowering urban temperatures
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7 August 2026
As trees disappear from urban areas, lives may be lost with them.
In a new study, Northwestern University scientists tracked death rates and fluctuations in tree canopy cover across an 11-year period in Chicago.
They found that areas experiencing greater losses in tree canopy – land covered by trees’ leafy crowns – also experienced higher mortality rates, particularly in the city’s hottest neighbourhoods.
The findings suggest cities should prioritise protecting mature trees alongside planting new ones.
Given the myriad health benefits associated with trees, tree maintenance has become even more crucial as human-caused climate change drives more extreme weather and intensifies heat risks in cities.
Daniel Horton, the study’s lead author, is an associate professor of Earth, environmental and planetary sciences at Northwestern’s Weinberg College of Arts and Sciences, where he leads the Climate Change Research Group (CCRG). He also is a faculty affiliate of the Paula M. Trienens Institute for Sustainability and Energy and the Roberta Buffett Institute for Global Affairs.
‘Previous studies have mostly looked at how health outcomes are associated with static tree canopy coverage. These snapshot-style studies looked at mortality rates and the tree canopy at one moment in time. But cities and their tree canopies constantly change, so we looked at how the canopy and death rates change over the years. We found that as tree canopy decreases, mortality increases.’
DANIEL HORTON
Senior author of the study, Northwestern University
The study grew out of the Buffett Institute’s Defusing Disasters Working Group, an interdisciplinary initiative that unites climate scientists, physicians, public health experts, city leaders and community partners to explore how extreme weather affects vulnerable populations in Chicago.
Co-led by Horton, the group also develops practical strategies to help cities reduce health risks and strengthen climate resilience.
‘The Defusing Disasters Working Group set out to determine what makes Chicago residents vulnerable to extreme heat. We found that a number of different factors contribute to heat vulnerability, and trees are one part of the equation.’
DANIEL HORTON
Senior author of the study, Northwestern University
A walking tour of Chicago’s North Lawndale and Douglas Park areas inspired Harrison Garcia, the study’s lead author and a medical student at Northwestern University Feinberg School of Medicine, to consider canopy coverage.
‘I noticed how much more exhausting it felt to walk through areas with little tree cover compared to nearby blocks lined with mature trees’, said Garcia, who also is a member of the Defusing Disasters Working Group. ‘I remember thinking that if the heat was affecting me so noticeably during a short walking tour, it must be shaping people’s health over the long term.’
To capture canopy changes, the team analysed annual satellite measurements of tree canopy across every Chicago census tract from 2011 to 2021. Satellite data were then combined with more than 220,000 death records from the Illinois and Chicago Departments of Public Health.
To isolate the relationship between tree canopy and mortality, the researchers developed a statistical model that accounted for differences in summer temperatures, air pollution, income, population size, racial/ethnic residential segregation and other neighbourhood-level factors that can influence health.
This approach enabled the research team to distinguish the effects of changing tree canopy from differences that already existed among neighbourhoods.
The team found that a neighbourhood’s overall tree canopy within each year was not significantly associated with mortality. Instead, what mattered was the year-to-year change in tree canopy.
Increases in canopy were consistently associated with fewer deaths from cardiovascular disease, respiratory disease, several musculoskeletal conditions and mental health-related illnesses.
‘We used a technique called K-means clustering to group Chicago neighbourhoods based on three factors: tree canopy, temperature and mortality. That allowed us to identify patterns across Chicago and to pinpoint where protecting and expanding tree canopy is needed most.’
BENJAMIN BARRETT
Study co-author, Ph.D. candidate in health and biomedical informatics at Feinberg and member of the Defusing Disasters Working Group
The geographic disparities were striking. According to the model, neighbourhoods on the South and West sides tended to experience hotter summer temperatures, greater absolute canopy losses and higher mortality than other parts of the city.
Neighbourhoods along Lake Michigan generally experienced cooler temperatures and lower mortality, while areas near the Forest Preserves had the city’s largest tree canopy and experienced relatively lower mortality.
The study coauthors underscore an often-overlooked point: planting trees and increasing tree canopy are not the same.
While young saplings take years to develop broad, leafy crowns, mature trees already provide extensive shade and cooling.
Losing those trees — to disease, storms or development — can erase decades of canopy growth almost overnight.
‘Chicago must both maintain the canopy it has and expand the canopy where it’s needed most. That means protecting mature trees from unnecessary removal, providing ongoing care for existing trees, replacing removed trees and investing in their long-term survival.’
RAED MANSOUR
Study coauthor, member of the Defusing Disasters Working Group and of the Openlands Board of Directors
To illustrate the potential benefits, the research team modelled what might happen if Chicago increased tree canopy by about 0.03% each year, which is roughly equivalent to planting 14,000 trees annually. The model estimated this increase was associated with roughly 105 fewer deaths annually.
While trees can cool neighbourhoods by providing shade and reducing the effects of urban heat islands, they also have numerous other health benefits. Trees improve air quality, encourage outdoor activity and contribute to healthier neighbourhoods.
‘Trees provide dozens of benefits. They decrease stress, anxiety and blood pressure and increase social cohesion. They also reduce soil erosion and support biodiversity. Maintaining the urban canopy and planting trees where they are needed are both important. Many folks see trees primarily as aesthetics features or amenities, but they should be recognised as part of our public health infrastructure.’
RAED MANSOUR
Study coauthor, member of the Defusing Disasters Working Group and of the Openlands Board of Directors
The study, ‘Increasing urban tree canopy associated with reduced mortality: a longitudinal analysis of Chicago neighbourhoods’, was supported by the Buffett Institute for Global Affairs.
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