Can trees offset carbon footprint?

Global Warming
USA Today (Magazine)
6/1/2007

How effective are new trees in off-setting the carbon footprint? A study by Lawrence Livermore (Calif.) National Laboratory suggests that the location of the new growth is an important factor when considering such carbon offset projects. For instance, planting and preserving forests in the tropics is more likely to slow down global warming. However, the study also concludes that planting new trees in certain parts of the planet actually may warm the Earth.

New forests in mid- to high-latitude locations could create a net warming effect. Specifically, more trees in mid*latitude locations like the U.S. and most of Europe only would create marginal benefits from a climate perspective, but those extra trees in the boreal forests of Canada, Scandinavia, and Siberia could be counterproductive, maintains atmospheric scientist Govindasamy Bala.

Forests affect climate in three different ways: absorbing the greenhouse gas carbon dioxide from the atmosphere to help keep the planet cool; evaporating water to the atmosphere and increasing cloudiness, which also helps keep the planet cool; and absorbing sunlight (the albedo effect), thus warming the Earth. Previous climate change mitigation strategies that promote planting trees only have taken the first effect into account.

"Our study shows that only tropical rain forests are strongly beneficial in helping slow down global warming," Bala notes. "It is a win-win situation in the tropics because trees in the tropics, in addition to absorbing carbon dioxide, promote convective clouds that help to cool the planet. In other locations, the warming from the albedo effect either cancels or exceeds the net cooling from the other two effects"

The study concludes that, by the year 2100, forests in mid and high latitudes will make some places up to 10[degrees]F warmer than would have occurred if the forests did not exist. However, the authors caution that the cooling from deforestation outside the tropics should not be viewed as a strategy for mitigating climate change.

"Preservation of ecosystems is a primary goal of preventing global warming, and the destruction of ecosystems to prevent global warming would be a counterproductive and perverse strategy," concludes study co-author Ken Caldeira of the Carnegie Institution, Washington, D.C.

COPYRIGHT 2007 Society for the Advancement of Education
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