Trees, Cities and CO2

Implications:  New York City has pledged to plant 1 Million new trees by 2017 to reduce pollution and cool temperatures in the city. The target article which was analyzed ( http://abcnews.go.com/US/wireStory?id=3064822 ) also mentions that NYC has a goal of reducing CO2 emissions by 30 % over the next two decades, but (quite properly) does not link this to the tree planting.

Analysis: Trees can play a great role in improving the livibility of a city. Benefits include reduction of peak summer temperatures (as much as 3.5deg. F at street level), filtering and absorption of pollutants such as ozone and NOX, acting as noise "filters", softening the harsh angular impact of city views, andsequestering (for a relatively short period) CO2.
Planting of trees, especially in a large city, for the primary purpose of long term storage of CO2 is for the most part simply a "feel good" action. Yes, trees take up CO2 and give off O2 (though the process is reversed at night) , and yes they convert CO2 to O2 - storing the Carbon in biomass, for a net short term reduction of CO2 emissions.  However, much of the carbon storage is in the leaves which simply release that carbon back to the atmosphere each year when they are burned or rot down. Additionally, urban trees have an average life of only a few decades, and the carbon stored in the trees themselves is likewise returned to the atmosphere when they are burned or rot down.  Thus, unless NYC is willing to utilize a very large percentage of its precious landfill space to deeply bury its tree residue, urban trees are not a long term solution to reducing its net CO2 emissions.

Although long term sequestration of carbon is not a good justification for planting urban trees, the other benefits of trees in the city make planting and maintaining them a good idea.