trees in New York

New York City wants to plant a million new trees. (http://news.yahoo.com/s/ap/20070422/ap_on_re_us/green_nyc)

They say it will reduce air pollution, cool temperatures and promote "long-term sustainability." All of these things are true, as least in terms of its local impacts. Will it reduce global warming? This is something that people might assume, especially since the article mentions the new trees will reduce pollution. One million trees in NYC may not reduce global warming, they may even increase temperatures on Earth. How can that be?

Trees have two impacts on global warming - they store carbon when they grow and they absorb/reflect sunlight. All trees absorb carbon as they grow, which reduces atmospheric carbon and reduces the impact of the greenhouse effect. However, trees reflect and absorb different amounts of sunlight. Light colored trees refelct a lot of light - sending it back to the sky and out of the atmosphere (some of it atleast). Dark trees absorb sunlight, capturing its heat and energy near the Earth's surface. Most of the trees planted in the US are dark, which means they absorb a lot of heat. The impact of this heat absorption may balance out the impact of soaking up carbon (trees are only a temperary carbon store, since they will slowly release all the carbon when they die). The net impact is unclear - it may increase global warming or may decrease it. If NYC decides to plant tropical trees in the Amazon, the trees will clearly reduce global warming. Tropical trees are lighter in color than US trees, meaning they reflect more sunlight. They also grow more densely, so they absorb more carbon per square meter.

Judged solely based on local impacts, the claims in the article are pretty accurate. Local city temps should drop (will this have an impact on reducing evaporation? This might increase global warming by decreasing cloud cover... ) and non-greenhouse gas pollutants (sulfur, etc) should be absorbed, as well as some GHG pollutants, like carbon and NOx. Horray for NYC!