Gabriela Bhaskar/The New York Times

October 25, 2023

Cornell Prof. Mitigates Extreme Urban Heat Island Effect With Trees 

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Researchers predict that by 2050, the number of days over 90 degrees Fahrenheit in New York City will triple. The world is expected to see a climb in heat-related hospitalizations and emergency department visits — especially in urbanized areas. Prof. Daniel Katz, integrative plant science, has conducted extensive research on how to mitigate extreme heat in urban environments, specifically through increasing tree coverage in cities like New York.

Due to a process called the urban heat island effect, cities are often significantly warmer than more rural areas year-round, on average up to seven degrees Celsius. This phenomenon poses dangerous health risks to people residing in these urbanized locations all over the country — national heat-related deaths and emergency hospitalizations are both projected to increase by over 50 percent in the next 20 years compared to data from 1971 to 2000, according to non-profit conservation organization American Forests.  

And there’s a reason for this. Due to cities being composed of a far higher proportion of buildings and roads compared to natural areas, they have a greater number of dark surfaces. Unlike lighter surfaces that reflect light energy from the sun, darker surfaces absorb it. 

The absorbed wavelengths of light then get converted into heat, which is why an asphalt road is much hotter to the touch than a wooden deck. Lighter surfaces — which includes natural structures like trees, plants and certain types of rocks — converts less light to heat.

“[Cities are] often a few degrees Celsius warmer,” Katz said. “That could be the difference between getting heat stroke or heat exhaustion and not getting it.”

According to Katz, solutions to the urban heat island effect exist at a variety of scales: painting roofs white so that they reflect more sunlight, and more consistently distributing air conditioning units to reduce chances of heat stroke and heat exhaustion are just a few. 

Katz, however, focused his research on tree coverage.

Trees not only cool ecosystems by reflecting back more light, but also through a mechanism called transpiration: a process in which a plant ‘sweats’ and releases water vapor through its leaves, stems and flowers. Katz’s project revolves around maximizing these different cooling benefits from trees.

He additionally found that there were notable disparities in tree coverage within individual cities in the United States. Poorer communities and communities of color often had fewer trees and urban forests than the more affluent areas. Katz noted that these areas also tend to be the ones where people are particularly vulnerable to extreme heat as they may not have the proper resources to cool down their living space.

A likely reason behind this difference traces back to the 1940s and 1950s when redlining — a practice from the homeowners’ loan corporation which denied people living in poorer communities home loans — was especially prevalent. This led to people in these specific areas — often people of color — being much less likely to have ownership over their home.

The term itself was created because, at the time, the federal government and lenders would mark areas on a map in red that they considered high risk or hazardous. Colors like yellow, blue and green were used to indicate safer and more low risk areas, respectively. The areas that were marked in red also were the ones with the highest percentage of Black residents. 

“We see clear associations that the areas that were redlined and faced this systematic discrimination have fewer trees than the neighborhoods where loans were more likely approved,” Katz said.

However, the answer isn’t simply to plant more trees. Katz described various social angles to the issue, which included communities being reluctant to have tree planting campaigns in their neighborhoods. This mixed reception mainly stemmed from a fear of gentrification. People worried that with an increase in trees, house prices would also go up and residents would ultimately be financially forced out.

Katz noted the importance of his team connecting with the community and stakeholders. Through building long-standing relationships with local residents, his team communicates how potential forestry solutions will impact their lives. By allowing residents to feel their concerns are being heard, Katz aimed to create solutions that work for everybody and benefit the environment at the same time.

There are also multiple bills at play helping Katz and his team in both direct and indirect ways. New York City just passed bills 1065 and 1066 this year, one of which mandates that NYC include tree cover in its sustainability plans and the other of which mandates that the city increase tree cover from 22 percent to 30 percent. 

The Inflation Reduction Act of 2022 has provided around $1.5 billion for community and urban forestry, and through a joint grant from the Cornell Atkinson Center for Sustainability and the Environmental Defense Fund, Katz and his team have received additional funding and opportunities to collaborate with scientists and staff members from the EDF. 

According to Katz, by working with the EDF, his team has gained significant insight on how to effectively work with communities on environmental issues. They are also working to establish better connections with stakeholders of local areas. 

“We have a fantastic team member who is going to be doing interviews with some of these community stakeholders and taking a more social science and anthropological approach,” Katz said. “We hope to really understand what the concerns of folks are so that we don’t recommend something that would cause more harm than good.” 

Katz mentioned that his research on trees and urban heat has been greatly supported by many researchers who are approaching the problem from a diversity of backgrounds. His team is composed of physicians, climate modelers, plant ecologists like himself and people carrying out community engaged research, all of whom have expertise in either urban forestry, social science, public health and more.

Katz hopes that his current work will soon expand to other areas in the country as well. He sees his research as a proof-of-concept type of study that could be used to choose different species of trees and the locations that make most sense to plant them. 

“The whole framework could be applied to other cities in the United States and around the world where we have similar issues and similar data sets,” Katz said. “And so, this is — we hope — just the first of many.”

Madison Kim can be reached at [email protected].