Green Alert

June 18, 2002 Vol. 1, No. 32

Urban trees, according American Forests’ Gary Moll, are the ultimate urban multi-taskers. They provide an array of benefits. High on the list is absorption of large quantities of air pollutants. D.J. Nowak, who is with the U.S. Forest Service’s Urban Forest Ecosystem Research Unit, estimates that trees in New York save Gotham’s taxpayers some ten million dollars a year because of the improvements they make in the city’s air quality. Urban trees also serve as homes for resident bird populations and rest-stops for many migratory species. They provide recreational opportunities along with good old shade. Just the sight of a tree is a comfort to urban dwellers. Now we can add to the list. Trees are front-line warriors in the effort to mitigate global warming. They sequester atmospheric carbon – with a vengeance – right where much of it originates, among fossil fuel users in cities.

The first U.S. assessment of carbon storage by urban trees was carried out by Nowak (1993). Based on an extrapolation of carbon storage data from Oakland, California coupled with tree-cover data from a number of other cities, Nowak’s research yielded an estimated national carbon storage range of between 350 and 750 million tons. A later assessment, which included carbon storage data from a second city (Chicago) raised the numbers to 600 and 900 million tons. Now, Nowak and Crane (2002) provide an updated, much more robust assessment of the phenomenon. Based on carbon storage data from ten cities (Atlanta, Baltimore, Boston, Chicago, Jersey City, New York, Oakland, Philadelphia, Sacramento, and Syracuse) Nowak and Crane’s most analysis puts total carbon storage by urban trees in the coterminous United States at approximately 700 million tons.

Now that’s still a big number, but it represents only 4.4% of the estimated 15,900 million tons of carbon stored in the country’s non-urban forests. It’s what human activities release in five-and-a-half months. In terms of annual impact, the numbers are even less impressive. The total amount of carbon extracted from the air each year by all urban trees in the U.S. (22.8 million tons) is equivalent to the amount of carbon emitted by the country’s human population over five days.

Even though we’ve made a big number seem woefully small, it still is significant for a couple of reasons. First, individual urban trees typically sequester considerably more carbon than do individual trees in forests. This is because cities contain more large trees and because in an urban environment individual trees intercept more light, they grow faster than do their country cousins. Nowak and Crane note that "individual urban trees, on average, contain approximately four times more carbon than individual trees in forest stands."

Another important consideration that long has been recognized by urban utility companies is that planting trees in strategic locations near buildings can reduce energy use because of enhanced shading and evaporative cooling in summer, and wind speed reduction in winter. Both lower the demand for electricity to heat and cool indoor environments. As a consequence, less coal and natural gas gets burned. This is a very significant effect according to the researchers.

Nowak and Crane write how such atmospheric CO2 "avoidance" from strategically planted trees is approximately four times greater than the amount of CO2 the trees otherwise remove from the air. So it would appear that the average urban tree, which is four times more effective in removing carbon from the atmosphere than are trees in forests are some sixteen times more effective in mitigating the potential for global warming when they are planted in a way that reduces cooling and heating of buildings.

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This is part of a series of "greening alerts" that Greening Earth Society periodically publishes online in response to research concerning the impact of carbon dioxide emissions on earth’s biosphere, especially as it relates to plant life’s ability to sequester carbon. The alerts are prepared by Drs. Sherwood B. Idso and Keith E. Idso of the Center for the Study of Carbon Dioxide and Global Change in Tempe, Arizona (www.co2science.org).