A new report by the Nature Conservancy demonstrates that trees are the only truly cost-effective solution addressing both deteriorating air quality and rising urban temperatures.
Can nature help address these twin problems of air that is too dirty or too hot? Trees and other vegetation, whether planted along a city street or growing in a park or residential yard, provide many benefits to people, such as aesthetic beauty, enhancement of property values, erosion prevention, stormwater management, and noise reduction. Trees also sequester carbon, helping mitigate climate change. Parks also provide space for urbanites to recreate, which brings real physical and mental health benefits.
It looks like trees may play an important role in making our air healthier, too. Dozens of studies now show that tree leaves filter out particulate matter from the atmosphere, along with many other air pollutants. Similarly, many scientific studies show that the shade trees cast, along with their transpiration of water during photosynthesis, can help reduce air temperatures while also reducing electricity use for residential cooling.
Some of the world’s largest cities could dramatically improve public health by those standards by investing just $4 per capita in their canopies, it finds. Crunching some numbers on how additional street trees (coniferous or leafy—palms don’t count here) could reduce pollution and heat inside the world’s 245 largest cities, the report shows that the residents of ultra-dense, ultra-populated, and ultra-polluted metropolises of Southeast Asia would see especially high ROIs, since the trees’ benefits would spread to so many people per square mile, and since material costs are comparatively affordable.
Instead, restoring and expanding urban tree cover should be thought of as a surprisingly powerful tool for cities that are dealing with climate-related health concerns.
The report’s analysis of trends over time suggests that the ecosystem services supplied by trees will be even more crucial in the future. There may be a 50 percent increase in the rate of mortality caused by PM2.5 by 2050, most of it in urban areas,4 and summer maximum temperatures in our sample of cities are forecast to increase by 2-5˚C (4-9° F) over the same time period. While these twin threats post a challenge to the health of those in cities, all else being equal, they will also increase the importance of the trees that are already there. There will also be a dramatic increase in urban population, which increases the number of people who might benefit from nature’s services.
Finally, all this urban development, or simply societal underinvestment in replacing trees lost, may reduce the amount of urban greening. For instance, we found that 26 percent of cities had a decline in forest cover over the period between 2000 and 2010, whereas only 16 percent of cities had an increase in forest cover over the time period.
We are at the beginning of the urban century. One of the preeminent tasks of cities will be making themselves vibrant, healthy, attractive places to live. This report has focused on just one small part of this task: the quest to make urban air healthier. Cities continue to strive to reduce concentrations of particulate matter and other atmospheric pollutants. And they are beginning to plan for the increased frequency and intensity of heat waves that climate change will likely bring. Succeeding against these twin challenges—air pollution and excess heat—will require an array of approaches. In this report, The Nature Conservancy – in coordination with C40 Cities Climate Leadership Group – has tried to understand whether nature can play a role in helping to solve these twin challenges.
The answer appears to be a qualified “yes.” Street trees can be a part of a cost-effective portfolio of interventions aimed at controlling particulate matter pollution and mitigating high temperatures in cities. While trees cannot and should not replace other strategies to make air healthier, trees can be used in conjunction with these other
strategies to help clean and cool the air. Moreover, trees provide a multitude of other benefits beyond healthier air.
In the right spot, trees can both help make our air healthier and our cities more verdant and livable.