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60-Second Earth

Trees Write Air Pollution Record In Wood

The red cedar trees of West Virginia reveal just how effective U.S. clean air laws have been. David Biello reports

The red cedar trees of Grant County, West Virginia can tell a fascinating tale. Because locked inside them is the history of U.S. air pollution in the 20th century.

Researchers began pulling cores from five randomly selected red cedar trees in 2008. The ratio of heavy and light isotopes of carbon found in the tree rings told them many secrets about the trees and their environment. 

From the 1940s through the 1970s, the trees closed the tiny holes in their leaves known as stomata to protect themselves from acid rain. That acid rain was coming from the sulfur dioxide pollution spewed by coal-fired power plants upwind in states like Ohio. Then, in 1982 or so, about 10 years after passage of the Clean Air Act that cut back on such pollution, the stomata began to reopen. And they've been opening wider ever since, boosting growth.

The red cedars also show that the same thing happened back in the 1930s—because the Great Depression’s effect on industry actually helped clean up the air. The research appeared in Proceedings of the National Academy of Sciences. [Richard B. Thomas et al., Evidence of recovery of Juniperus virginiana trees from sulfur pollution after the Clean Air Act]

The question now is: what will the red cedars tell us about increasing levels of CO2 from all our fossil fuel burning a few decades from now?

—David Biello

[The above text is a transcript of this podcast.]

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