Quality of urban forest carbon credits
- Urban Greening, Neelam C. Poudyala, Jacek P. Sirya, J. M. Bowkerb
Several stories have appeared recently in popular news outlets suggesting that trees are not a solution in the fight against global warming. While these pop-media pieces represent the views of a few researchers, an overwhelming body of peer-reviewed research from forest scientists around the world points to the importance of forests in reducing carbon dioxide in our atmosphere and slowing the buildup of that greenhouse gas. The pop-media pieces include a report from Reuters (Gardner 2007) in which Ken Caldeira, a Carnegie Institute climate scientist, was reported to say, “It’s probably a nice thing to do, but planting trees is not a quantitative solution to the real problem.” In the same article, Philip Duffy of Lawrence Livermore National Laboratory said, “If you plant a tree [CO2 reductions are] only temporary for the life of the tree. If you don’t emit in the first place, then that permanently reduces CO2.” Dr. Caldeira had made similar arguments previously in an op-ed in the New York Times (Caldeira 2007). A New Scientist article (Brahic 2006) reports results from a study by ecologist Govindasamy Bala of Lawrence Livermore National Laboratory. The model developed by Bala and colleagues indicates that, while trees planted in tropical regions have a clear net cooling effect, trees planted in mid-latitudes may absorb so much heat from the sun that they actually contribute to warming. Because these reports fail to capture the complexity and the potential of the role that trees play in fighting global climate change, they have motivated rebuttals from the scientific community. I wrote this column to assure the public that trees do indeed reduce carbon dioxide in the air, thereby reducing the warming “greenhouse” effect of the gas, and to explain that urban trees in particular are valuable because they provide that benefit in more than one way. First, as they grow, trees take carbon dioxide out of the air and transform it into roots, leaves, bark, flowers, and wood. Over the lifetime of a tree, several tons of carbon dioxide are taken up (McPherson and Simpson 1999). Second, by providing shade and transpiring water, trees lower air temperature and, therefore, cut energy use, which reduces the production of carbon dioxide at the power plant. Two-thirds of the electricity produced in the United States is created by burning a fuel (coal, oil, or natural gas) that produces carbon dioxide. On average, for every kilowatt hour of electricity created, about 1.39 pounds of carbon dioxide are released (eGRID 2002). It is certainly true, as Dr. Duffy states, that not emitting carbon dioxide in the first place is a good strategy. Lowering summertime temperatures by planting trees in cities is one way to reduce energy use and thereby reduce carbon dioxide emissions. And planting trees is an immediate solution. Even if we were able to switch immediately to fuel sources that do not emit carbon dioxide, the levels in the air will remain high for decades or even centuries because of the long “lifetime” of carbon dioxide. Urban forestry doesn’t require the development of new technologies or massive investment in alternative energy sources. Planting a tree to shade a building is something all of us can do now. The following sections address the other claims previously made.