Case Study : Urban Tree Planting and Greenhouse Gas Reductions…

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 2 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).





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