The vast majority of lower-income urban neighborhoods across the U.S. have fewer trees than higher-income neighborhoods, an inequity that contributes to higher temperatures, poor health and, in some cases, premature death.

According to a new paper published Wednesday in PLOS ONE, low-income blocks have 15.2% less tree cover, on average, than high-income blocks, and summer temperatures are 1.5 degrees Celsius hotter, on average, in places with less-moneyed residents. 

Overall, low-income neighborhoods have 62 million fewer trees than high-income neighborhoods.

"It was systematic disparity everywhere," said Tanushree Biswas, a co-author of the paper and a data scientist at The Nature Conservancy. "We have seen studies which have observed these trends on smaller scales — individual small cities [and regions] — but I was really surprised to see that this is consistent across the entire U.S."

Urban trees provide a number of quantifiable benefits: They reduce air pollution, sequester carbon, provide shade and lower buildings' energy use. Trees also make neighborhoods more pleasant, encourage outdoor activities and improve people's mental health. Prior research has shown that every 0.6 degree Celsius increase in air temperature increases the heat wave mortality risk by 2.5% in urbanized areas, and this new study identified more than a dozen cities with surface temperatures that are over 3 degrees Celsius higher in low-income neighborhoods than high-income neighborhoods. Thus, wealthier city- and suburb-dwellers are seeing most of the benefits from trees, which can cool air temperatures by between 0.5 and 2 degrees Celsius in the summer.

This is "a climate justice issue," said Robert I. McDonald, a researcher at The Nature Conservancy's Center for Sustainability Science and the lead author of the paper. He referenced a finding from an earlier paper of his: 1,200 lives are saved in the U.S. each year by current tree cover. As the new paper reveals, "those benefits are really inequitably distributed," he said. "That protective capacity of trees is primarily playing out in richer neighborhoods, mostly white neighborhoods, and more in the suburbs."

The current study looked at the 100 largest urban areas in the United States, where about 167 million people — about 55% of the population — live. The urban areas, each over 500 square kilometers, contained 3,520 municipalities and 2,291 Census-designated places. The researchers designed an algorithm that analyzed satellite images with 2-meter resolution to detect trees. They cross-referenced tree data with summer land surface temperatures from satellite images and Census demographic data at the block and block group level.

The differences they found were stark: A neighborhood with a 5% higher income level — for example, the 20th percentile compared to the 15th percentile — has 1.2% more tree cover. The researchers found disparities between low-income and high-income blocks in 92% of the areas they analyzed. Accounting for population density and how built-up the urban landscape was, 78% of areas have significant income differences in tree coverage, while 67% have significant racial disparities, leaving whiter blocks with markedly more trees. The worst disparities are in the Northeast.

The researchers divided the study areas to account for differences between biomes and found inequality persisted almost everywhere. The Northeast has the gravest problems, particularly between Washington, D.C., and Boston. The urbanized area including Stamford and Bridgeport, Connecticut, is the most unequal place in the sample; richer Stamford had nearly twice the tree cover of Bridgeport. Providence, Rhode Island, has the biggest temperature difference: 5.4 degrees Celsius. Worcester, Massachusetts, and Philadelphia also have high- and low-income neighborhoods with temperature differences of over 4 degrees.

Prior to this study, Biswas told The Academic Times, "There isn't really anything at a resolution that is effective that urban planners or city planners can really use" at the national level. Biswas worked with her co-authors to harness Google Earth Engine; she said other researchers could use the work to look at the places they live. "It's all open-source," she said, and she is "happy to share the algorithm."

McDonald said, "Tree-planting has to be done in concert with local communities," and that residents and politicians could use the information from their cities to decide what they want to do. 

"I certainly hope the work motivates policymakers at municipal or state or federal levels to think about this environmental-justice issue and plan for ways to rectify it," he said.

The paper, "The tree cover and temperature disparity in US urbanized areas: Quantifying the association with income across 5,723 communities," published April 28 in PLOS ONE, was authored by Robert I. McDonald, Tanushree Biswas, Timothy M. Boucher and Charlotte K. Stanley, The Nature Conservancy; Cedilla Sachar and Deborah Balk, City University of New York; Ian Housman, independent researcher; David Nowak, U.S. Department of Agriculture Forest Service; Erica Spotswood, San Francisco Estuary Institute; and Stefan Leyk, University of Colorado-Boulder.