Urban Tree Canopy Affects How Heat Stress Impacts Residents

Mark Altaweel

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With climate change, heat waves have become more severe and prolonged. This has led to a rise in heat-related fatalities, especially in urban regions that often become heat islands. In poorer areas, heat-related deaths are higher than more affluent regions. Even when cooling is available, energy consumption greatly increases in neighborhoods lacking the cooling benefit of mature trees and vegetation.

One natural strategy to reduce heat-related deaths is to increase tree coverage in urban areas, which helps lower temperatures. A recent study highlights the disparities in health and energy consumption across the United States, stemming from insufficient urban tree coverage. It also details the potential health and economic advantages that increased tree presence can offer to urban environments.

A recent study looked at 2020 US census data and the effects of trees on neighborhoods across the United States.[1] In this study, mortality, morbidity, and electricity consumption across 5,723 US municipalities and regions were examined, which includes 180 million people. The research also included a high-resolution (2  meter) tree cover map for areas studied.

Lower income and People of Color (POC) dominant neighborhoods tend to have fewer urban trees

What the research from this study indicates is that areas with demographics made up of mostly minorities and low income individuals generally have 11% less trees, are 1.5 °C hotter than majority white and higher income areas, and have 14% more impervious surface.


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A series of maps measuring heat stress and heat risk reduction in a neighborhood in Washington D.C. and the relationship to urban tree cover.
The upper left map shows a predominately persons of color (POC) neighborhood in Washington D.C. with significantly less urban tree canopy than the predominately white neighborhood to the west. This POC neighborhood experiences relatively more heat stress (map B) and a higher potential for heat risk reduction by planting more trees (map C). Maps: McDonald et al., 2024.

Benefits of urban trees

Trees have been found to have great benefits in urban environments. Mature trees provide shade and absorb heat which helps to lower electricity consumption. Urban areas with fewer trees experience a 0.45–4.6% increase in electricity demand for every 1°C increase in temperature. In mostly white and affluent areas, trees reduce air temperate by an average 1.01 ± 0.03 °C. In these mostly white neighborhoods, there are also annual reductions in 190 ± 139 deaths, 30,131 ± 10,406 doctor visits, and 1.4 ± 0.5 terawatt-hours (Twhr) of electricity consumption relative to non-white neighborhoods.

Where there is good news is that there is a lot of potential to improve these statistics and make neighborhoods more equal in terms of tree coverage. In fact, most health and economic benefits would be seen in areas that have a higher population density and are generally non-white and poorer.[2]

Using regression analysis, researchers drew on current census estimates and map data to demonstrate the effects of tree planting. This analysis allowed the authors of the study to project various scenarios in which tree planting could yield diverse benefits, impacting mortality, morbidity, energy consumption, and economic gains.

A heat map with gradients of green to show tree density in Manhattan.
A heat map with gradients of green to show tree density in Manhattan using publicly available data. Map: Caitlin Dempsey.

Large urban areas such as Chicago, New York, and Los Angeles have significant neighborhoods that would benefit from more urban tree planting. In fact, just looking at New York and Chicago, these two cities combined could sequester 1 MtCO2e yr-1 under urban reforestation plans.

Suburban areas, with their wide extent across the United States, also have great potential for reforestation. However, there would be diminished returns if tree planting mostly focused on these affluent areas. If one looks at tree planting as an investment in avoiding deaths, hospitalization, and overall energy consumption, then dense neighborhoods located in city centers should be targeted for tree planting.

Planting trees in areas with sparse urban tree canopy can reduce heat stress and lower cooling costs

In the most ambitious tree planting scenario for urban areas, researchers estimate about $9.6 billion dollars in annual benefits accrued. A modest 5% nominal target increase in tree planting in poorer neighborhoods could result in about $32 million in benefits but cost around $29 million to implement. In wealthier neighborhoods, planting trees could yield additional economic benefits but may lead to greater economic costs than benefits because one would need to also maintain trees in areas that are more dispersed.

There would be a greater net improvement in poorer and more densely urban neighborhoods. These benefits would be seen in reduced overall energy consumption and improved health that leads to economic gains. The most ambitious scenario, which includes 1.2 billion trees planted, shows that morbidity benefits would include an additional 80,785 people benefiting, with increase carbon sequestration by 23.7 MtCO2e yr−1, and decrease electricity greenhouse gas (GHG) emissions by 2.1 ± 0.1 MtCO2e yr−1. Potentially, this would save about 464 lives per year.

What this latest research demonstrates is that tree planting can have a great overall positive impact on health and economic potential in urban areas across the United States. Other studies have, in fact, also attempted to demonstrate that every year thousands of lives can be saved in places such as Europe if urban tree planting enabled cities to have a high level of tree coverage.[3] 

This is particularly important as urban areas heat up due to climate change. However, this impact is not equal. Currently, poorer and minority areas are more negatively impacted by increased temperatures due to less tree coverage in their neighborhoods. Nevertheless, many of these areas would more greatly benefit, in terms of health, energy consumption, and economic impact, relative to affluent areas if tree planting focused on poorer urban neighborhoods. 

References

[1]    The study analyzing current deaths, morbidity, and energy consumption due to tree cover in urbanized areas across the United States as well as potential future impacts under different tree planting scenarios can be found here: McDonald, Robert I., Tanushree Biswas, T. C. Chakraborty, Timm Kroeger, Susan C. Cook-Patton, and Joseph E. Fargione. 2024. “Current Inequality and Future Potential of US Urban Tree Cover for Reducing Heat-Related Health Impacts.” Npj Urban Sustainability 4 (1): 18. https://doi.org/10.1038/s42949-024-00150-3.

[2]    A popular story on urban heat waves and trees can be found here:  https://www.sciencenews.org/article/heat-waves-illness-death-cities-fewer-trees.

[3]    This study uses NDVI for urban areas to estimate how many lives could be saved by urban tree planting across Europe:  Barboza, E. P. et al. 2021. Green space and mortality in European cities: a health impact assessment study. Lancet Planet. Health 5, e718–e730.

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.