The Power of Urban Trees

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KEY CONCEPTS

  • America’s growing urban population is particularly vulnerable to extreme heat, heavy rainfall, and poor air quality – all compounded in a warming world.

  • Climate Central’s new report, The Power of Urban Trees, explains how trees can mitigate the effects of urbanization and help communities adapt to climate change.

  • By cooling air and surface temperatures, trees reduce health hazards from extreme heat.

  • Trees also slow down and soak up rain; filter the air; and absorb harmful carbon pollution.

  • Climate Central’s analysis using the US Forest Service’s i-Tree data shows the estimated annual benefits of urban trees in 242 US cities (defined by county or counties).

Download local data
Read the full report

CM: Local Power of Trees 2023 (DE)Click on the downloadable graphic: Local Power of Trees

Around 80% of Americans live in urban areas, and that percentage could rise to nearly 90% by 2050. As urban populations grow, so too does concern about climate risks in cities. Built environments can increase risks associated with extreme heat and heavy rainfall, which become more likely in a warming world.

Cities are generally warmer than the surrounding landscapes due to the urban heat island effect, which occurs as sidewalks and buildings trap heat and return it to the surrounding air. Paved surfaces also do not absorb rainwater and are therefore prone to flooding from runoff stormwater.

Concentrated human activity (particularly traffic) in urban areas contributes to poor air quality. Burning fossil fuels releases greenhouse gases and other tiny particles into the air that can cause serious health risks. Cities contribute up to 60% of total greenhouse gas emissions worldwide.

The power of city trees

Cities integrate nature into their infrastructure to mitigate the effects of urbanization and build resilient communities. A growing body of research shows that urban forests — the trees in cities and surrounding communities — and other green spaces can help cities adapt to climate change.

Trees offer key benefits in an increasingly warm and urbanized world. Trees help communities:

  • Reduce heat. Trees reduce the risk of extreme heat, the leading cause of weather-related death. Tree canopies help lower surface temperatures and cool the air by shading heat-absorbing buildings and walkways. Shade trees also offer instant relief to people who work, play, or exercise outdoors.

  • Avoid flooding from rainwater runoff. Tree canopies slow down rain, while roots and soil filter and absorb water – minimizing the amount of rainwater washing over the sidewalk and contributing to runoff and flooding.

  • improve air quality. Leaves absorb harmful gases like ozone and nitrogen dioxide and trap dangerous fine dust particles that pollute the air.

  • Remove and store greenhouse gases. Trees remove CO2 and other greenhouse gases from the atmosphere. They absorb CO2 during photosynthesis and store the carbon in their biomass (including their leaves, trunks, branches and roots).

  • Protect health and establish a connection to nature. Studies have linked trees and time spent in nature to a range of physical and mental health benefits.

Climate Central’s new report further explains “The Power of Urban Trees” and discusses how communities can protect and support urban forests to continue benefiting from them.

CM: Rainwater runoff avoided by trees 2023 (DE)Click on the downloadable graphic: Rainwater runoff prevented by treesCM: Air Pollution Reduced by Trees 2023 (EN)Click on the downloadable graphic: Air Pollution Reduced by TreesCM: Carbon Pollution Removed by Trees 2023 (EN)Click on the downloadable graphic: Carbon Pollution Removed by Trees

Measuring the utility of urban trees

The US Forest Service and partner organizations developed i-Tree – a set of free web tools that map and measure urban forests and their benefits. Climate Central combined the i-Tree data to estimate annual tree benefits for 242 cities (defined by counties or counties) in the continental US

  • Each year, trees in these counties help prevent about 110 billion gallons of stormwater runoff (equivalent to nearly 167,000 Olympic-size swimming pools).

  • These urban trees also reduce nearly 4 billion pounds of air pollution and absorb about 150 million tonnes of carbon dioxide equivalent (or CO2e, including CO2 and other greenhouse gases) annually.

  • The three most populous cities in the analysis — Los Angeles (Los Angeles County), New York City (in five counties), and Chicago (Cook County) — are among the top contributors to global greenhouse gas emissions. Together, trees in these counties help absorb about 1 million metric tons of CO2e each year (equivalent to the annual greenhouse gas emissions of about 192,000 cars).

  • Counties that benefit the most are scattered across the country, but some places appear at the top of the rankings more than once, including: Aroostook and Penobscot Counties in Maine; Lane County, Oregon; and Humboldt County, California

district Avoided Stormwater Runoff (Million Gallons)*
Harris County, Texas (Houston) 5.9
Allegheny County, PA (Pittsburgh) 4.6
Cook County, Illinois (Chicago) 4.6
Multnomah County, OR (Portland) 4.0
Jefferson County, AL (Birmingham) 3.3
district Reduced air pollution (million pounds)*
Aroostook County, ME (Presque Island) 185
Lane County, OR (Eugene) 150
Humboldt County, California (Eureka) 125
Penobscot County, ME (Bangor) 106
St Louis County, Minnesota (Duluth) 71
district CO2 equivalent absorbed (millions of tons)*
Aroostook County, ME (Presque Island) 6.2
Humboldt County, California (Eureka) 5.5
Lane County, OR (Eugene) 4.1
Penobscot County, ME (Bangor) 3.8
Jackson County, OR (Medford) 2.4

*Based on annual estimates of the utility of the US Forest Service’s i-Tree tools.

Download local data for all 242 counties.

LOCAL STORY ANGLE

Discover tree canopy and its associated benefits in your neighborhood, city, county or state.

The data-rich i-Tree web tools allow users to measure local tree canopies at various scales, from individual trees to urban forests in an area. Examine numerous variables ranging from health outcomes to economic benefits. Users can review support resources or attend regular training courses hosted by the i-Tree support team to learn more about the tools’ capabilities.

Evaluate the fairness of trees and access to nature in your area.

The non-profit organization American Forests developed Tree Equity Score cards to quantify tree equity in cities across the United States and inform community planning. The Trust for Public Land provides a ParkScore for the 100 most populous cities, ranking locations based on key variables related to access to green space.

CONTACT EXPERTS

dr Vivek Shandas, urban forestry expert from Portland State University, is being hosted by SciLine and will be available for interviews Thursday, May 4 from 2:00-3:30pm ET. Click here for more info.

Jason Grabosky, PhD
professor
Department of Ecology, Evolution and Natural Resources
Rutgers University
Relevant expertise: urban forests
Contact: grabosky@sebs.rutgers.edu

Jason Henning, PhD (he/him)
Research City Ranger
The Davey Institute
Relevant expertise: i-Tree tools, urban forest assessment
Contact: jason.henning@davey.com

David J. Nowak, PhD
Senior Scientist Emeritus (retired)
US Forest Service, Forest Inventory and Analysis
Relevant expertise: Urban forest inventories and ecosystem services
Media Contact: Sharon Hobrla, sharon.hobrla@usda.gov

FIND EXPERTS

Contact the federal urban forest program manager for your area or see your state coordinator.

Submit a request to SciLine at the American Association for the Advancement of Science or to Columbia University’s Climate Data Concierge. These free services quickly connect journalists with relevant academic experts.

Browse maps from climate experts and services at regional offices of NOAA, USDA, and the Department of the Interior.

Search databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs and Diverse Sources to find and amplify diverse expert voices.

Contact your state climate agency or nearest land-grant university to connect with scientists, educators, and counselors in your area.

METHODOLOGY

In 2010, county-level annual estimates for avoided runoff (gallons), absorbed CO2-equivalent (tonnes), and reduced air pollution (lbs) were obtained for 242 locations using the US Forest Service’s i-Tree County tool. Detailed information on the methodology can be found here. Estimates are not available for Alaska, Hawaii, and Puerto Rico. Local graphs highlight data for the county in which each city is located. For cities spread across multiple counties (Amarillo, Texas; Huntington, W.Va.; Joplin and Kansas City, Mo.; New York City, NY; and Shreveport, La.), estimates were aggregated for all counties. County names can be found in the footnote of each local chart.

www.climatecentral.org

https://www.climatecentral.org/climate-matters/the-power-of-urban-trees-2023