On Friday, Dr. Nadine Unger wrote an op-ed in the New York Times entitled, “To Save the Planet, Don’t Plant Trees.” A group of 31 scientists took issue with her logic and have responded in a one-page letter.
In Warsaw last year, several western nations, including the U.S., donated millions of dollars to helping developing countries reduce their tropical deforestation. In the lead-up to the September 23 climate summit, Dr. Unger writes that nations will likely call for a reduction of deforestation and a program of replanting in exploited areas.
“The assumption is that planting trees and avoiding further deforestation provides a convenient carbon capture and storage facility on land,” Unger writes. “That is conventional wisdom. But the conventional wisdom is wrong.”
Unger, who is an assistant professor of atmospheric chemistry at Yale, claims that the dark color of trees causes them to absorb more of the sun’s energy and raise the planet’s surface temperature. Trees also, when threatened by extreme weather or insect infestations, emit volatile organic compounds (VOCs) that “contribute to air pollution and are hazardous to human health.”
In their response to Unger’s claims, the scientists begin by stating that forests exercise a net cooling effect on the planet’s climate. They do this by storing vast amounts of carbon in their trunks, leaves and soil – carbon that is released whenever forests are cut down or degraded. For this reason, “[s]lowing forest clearing and degradation is precisely the focus of the UN’s mechanism for encouraging tropical nations to reduce emissions fore deforestation and forest degradation.”
“Unger’s statement,” they write, “confuses this fundamental aspect of forest carbon dynamics.”
Moreover, forests cool the atmosphere by converting solar energy into water vapor. And though Unger is correct in pointing out that – unlike snow, rocks, grassland and soil – trees reflect less solar energy, they increase sky albedo (reflectivity) by forming clouds. This effect is strongest in tropical rainforests, the most vulnerable regions to deforestation.
Finally, on the subject of VOCs, the scientists say that Unger’s op-ed confuses the findings of her own study on biogenic volatile organic compounds, which states that they can have a cooling or warming effect. In fact, “any potential cooling effect from reducing BVOC emissions through tree removal is outpaced by the larger warming effect of carbon emissions from deforestation.”
In closing, the 31 scientists write that, “We strongly disagree with Professor Unger’s core message. We agree, however, with her statement that the protection of forests offers ‘unambiguous benefits to biodiversity.’”