In their book on the history of the Wissahickon, “Metropolitan Paradise,” authors David Contosta and Carol Franklin included a photo taken from the air over the Wissahickon Valley. The image, looking southeast at the distant skyscrapers of Center City is striking for the sea of trees between the camera and the city.

One of northwest Philadelphia’s most significant features is captured perfectly in that image. It is a series of city neighborhoods – East Falls, Germantown, Roxborough, Mt. Airy and Chestnut Hill – that remain well covered by canopies of green. People are definitely attracted to the area because of the history, but the proximity to a genuine forest is just as important.

Planting even more trees in the city has long been a goal of neighborhood activists, including the Chestnut Hill Tree Tenders, who have worked to plant trees in almost any location they can find. Trees, these groups say, look good, add shade to keep us cool and make life in our neighborhoods all the more pleasant.

It turns out the efforts of tree planters could be even more important than that.

A study published in the journal “Science” last week argues that a global effort to plant a trillion trees could be the most effective way to fight global climate change.

Scientists studied satellite photo data and found that 900 million hectares (that’s almost 2.25 billion acres) of earth that can support more trees. If properly forested with the right species of trees, the researchers calculated the new growth would absorb 205 gigatons of carbon dioxide in the next 40 to 100 years. To put those gigatons in perspective, that’s two thirds of all the carbon dioxide generated by man since the start of the industrial revolution.

“We all knew restoring forests could play a part in tackling climate change, but we had no scientific understanding of what impact this could make,” said one of the study’s authors, Thomas Crowther, an assistant professor of ecology at the Swiss Federal Institute of Technology.

Reforestation, Crowther said, is the best global climate solution available today.

While planting a trillion trees seems easier than other potential efforts, it might not be as easy as it sounds.

Robin Chazdon, an ecologist and evolutionary biologist at the University of Connecticut who did not participate in the study, told The Scientific American that it would be difficult to combat competing interests. Forests are still actively cleared for agriculture. They also require water.

Still, Chazdon was optimistic about reforestation. If scientists could work with local governments, such programs could be successful.

“I hope there will be more interaction between scientists and politicians, realizing that the tools we now have can guide reforestation that is the most cost-effective, and has multiple benefits and fewer tradeoffs,” she told the Scientific American.

While planting a single tree in your yard is a far cry from a global reforestation program, it’s better than not planting any. It’s one tree closer to making a real contribution to combating climate change.

Pete Mazzaccaro

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