A hotter, more dangerous world
As an emergency physician in his native Australia, Dr. Ghosh treated patients burned in the 2009 Black Saturday bushfires, which killed 173 people in the state of Victoria. Later, at New York City’s Bellevue Hospital and in Puerto Rico, he worked through the aftermath of Hurricanes Sandy and Maria — events research suggests were intensified by a warming climate.
In addition to witnessing the acute human cost of these disasters, from severe burns to disruptions in critical psychiatric care, he has, through his research, chronicled associations that emerged years later. These include upticks in heart disease and death among dementia patients who lived through Hurricane Sandy flooding.
After these experiences, Dr. Ghosh set out to prevent this kind of suffering. His research on heat waves led him to trees.
Walk down a city street at noon in August, and you can’t help but appreciate the leafy canopies blocking the sun’s rays. But that’s not all. The water released by tree leaves evaporates, lowering the temperature in the surrounding air — similar to how sweat cools the body, says Dr. Dan Katz, assistant professor in Cornell’s School of Integrative Plant Science.
Trees’ benefits are well established. “We have hundreds of studies showing that areas with more trees are cooler,” he says. “But the problem is, we don’t have a good sense of what specific actions to take to best cool cities with them.”
Cool Trees got its start in 2022 when Dr. Ghosh reached out to Dr. Katz. Their team aims to create tools to guide city and community groups as they prepare to plant. They plan to create software capable of evaluating how planting scenarios — maybe adding a honey locust to a corner in Long Island City, Queens, or an oak mid-block in the Morris Park section of the Bronx — may alter residents’ risk of heat-related illness or even death.
Such a fine-grained level of detail matters, according to Cool Trees adviser Emily Nobel Maxwell, an urban forest expert who has advocated for making tree cover more equitable across the city’s communities.
Cities are in general hotter than areas around them, and some neighborhoods swelter more than others. A New York City analysis found, for example, that temperature can vary block by block.
Lower-income communities and communities of color generally have fewer trees, which contributes to making residents more vulnerable to harm caused by heat, according to Maxwell. “The more we can understand the interplay between trees and their benefits, the more that can help us prioritize our investments in the places that need them most,” she says.
Crossing the threshold
When heat overwhelms the body’s ability to shed it, core temperature rises and organs begin to falter. In the most severe cases, heat, on its own, can cause death. Each year, New York City records about five deaths caused directly by heat stroke.
Much more often, high temperatures exacerbate chronic medical conditions. For instance, people with diabetes are more likely to experience dangerous dehydration, as are those with kidney disease. Heat strains the heart and blood vessels, an additional threat for those with cardiovascular disease, and it exacerbates breathing problems like emphysema and asthma.
Meanwhile, many common medications, including those for blood pressure, heart disease, diabetes, depression and other common psychiatric illnesses, can make heat exposure more dangerous. Interactions like these cause almost all of the roughly 500 annual heat-related deaths, and many more ER visits and hospitalizations, according to Dr. Ghosh.
Medical charts rarely capture heat exposure, he notes. Without these direct accounts, the Cool Trees team is looking for statistical evidence of heat-aggravated illness from within state records of hospitalizations and emergency department visits. Because the state data links back to patients’ neighborhoods, they can determine how the risk to residents’ health changes in that area as temperatures rise — and, further down the line, how new trees could tip the odds in their favor.
Tree by tree
To understand how much difference trees can make, the team has created, and is now refining, a highly detailed picture of what’s already growing throughout New York City.
Dr. Katz and his team start with existing information from the city, then augment it with data collected by satellites and an airplane-borne sensor. With this information, they determine the rough identity of every tree in the city and its size. These details matter because they affect the cooling a tree provides. In the long run, Dr. Katz says Cool Trees can provide guidance to tree planters wondering, “If I plant this species of oak versus a dogwood, how much will it affect temperatures and people’s health?”
Once the team has assessed each tree, including estimating its leaf area, collaborators led by Dr. Chenghao Wang, an assistant professor at The University of Oklahoma, calculate how much cooling each tree provides. The climate modeling approach he uses also factors in a tree’s immediate surroundings.
It takes a community
To cool a community, trees need people to want them and care for them. But neither is a given. In neighborhoods with sparse canopies — often places that have seen less investment in general — residents may worry that new greenery can lead to “green gentrification” that could price them out of their homes.
To account for concerns like these, the Cool Trees team has published peer-reviewed studies with community leaders and professionals, such as city arborists, who plant, study and maintain trees. Through this and other research, they have sought to understand barriers to increasing tree cover and how best to give communities ownership of the process.
Dr. Ghosh points out that the stakes continue to rise: New Yorkers, like Americans in general, are aging, extreme weather is increasing and planet-warming carbon dioxide levels continue to climb.
“We have to be ready,” he says.
Sources
- “More than 500 New Yorkers”
- “Modeling studies of Paris and 10 U.S. cities”
- Population Living Within The Hottest Cities: data from climatecentral.org
