As international leaders gather in New York next week for a United Nations climate summit, they will be preoccupied with how to tackle the rising rate of carbon emissions. To mitigate the crisis, one measure they are likely to promote is reducing deforestation and planting trees.
A landmark deal to support sustainable forestry was a heralded success story of the last international climate talks, in Warsaw last year. Western nations, including the United States, Britain and Norway, handed over millions of dollars to developing countries to kick-start programs to reduce tropical deforestation. More funds are promised.
Deforestation accounts for about 20 percent of global emissions of carbon dioxide. The assumption is that planting trees and avoiding further deforestation provides a convenient carbon capture and storage facility on the land.
That is the conventional wisdom. But the conventional wisdom is wrong.
In reality, the cycling of carbon, energy and water between the land and the atmosphere is much more complex. Considering all the interactions, large-scale increases in forest cover can actually make global warming worse.
Of course, this is counterintuitive. We all learn in school how trees effortlessly perform the marvel of photosynthesis: They take up carbon dioxide from the air and make oxygen. This process provides us with life, food, water, shelter, fiber and soil. The earth’s forests generously mop up about a quarter of the world’s fossil-fuel carbon emissions every year.
So it’s understandable that we’d expect trees to save us from rising temperatures, but climate science tells a different story. Besides the amount of greenhouse gases in the air, another important switch on the planetary thermostat is how much of the sun’s energy is taken up by the earth’s surface, compared to how much is reflected back to space. The dark color of trees means that they absorb more of the sun’s energy and raise the planet’s surface temperature.
Climate scientists have calculated the effect of increasing forest cover on surface temperature. Their conclusion is that planting trees in the tropics would lead to cooling, but in colder regions, it would cause warming.
In order to grow food, humans have changed about 50 percent of the earth’s surface area from native forests and grasslands to crops, pasture and wood harvest. Unfortunately, there is no scientific consensus on whether this land use has caused overall global warming or cooling. Since we don’t know that, we can’t reliably predict whether large-scale forestation would help to control the earth’s rising temperatures.
Worse, trees emit reactive volatile gases that contribute to air pollution and are hazardous to human health. These emissions are crucial to trees — to protect themselves from environmental stresses like sweltering heat and bug infestations. In summer, the eastern United States is the world’s major hot spot for volatile organic compounds (V.O.C.s) from trees.
As these compounds mix with fossil-fuel pollution from cars and industry, an even more harmful cocktail of airborne toxic chemicals is created. President Ronald Reagan was widely ridiculed in 1981 when he said, “Trees cause more pollution than automobiles do.” He was wrong on the science — but less wrong than many assumed.
Chemical reactions involving tree V.O.C.s produce methane and ozone, two powerful greenhouse gases, and form particles that can affect the condensation of clouds. Research by my group at the Yale School of Forestry and Environmental Studies, and by other laboratories, suggests that changes in tree V.O.C.s affect the climate on a scale similar to changes in the earth’s surface color and carbon storage capacity.
While trees provide carbon storage, forestry is not a permanent solution because trees and soil also “breathe” — that is, burn oxygen and release carbon dioxide back into the air. Eventually, all of the carbon finds its way back into the atmosphere when trees die or burn.
Moreover, it is a myth that photosynthesis controls the amount of oxygen in the atmosphere. Even if all photosynthesis on the planet were shut down, the atmosphere’s oxygen content would change by less than 1 percent.
The Amazon rain forest is often perceived as the lungs of the planet. In fact, almost all the oxygen the Amazon produces during the day remains there and is reabsorbed by the forest at night. In other words, the Amazon rain forest is a closed system that uses all its own oxygen and carbon dioxide.
Planting trees and avoiding deforestation do offer unambiguous benefits to biodiversity and many forms of life. But relying on forestry to slow or reverse global warming is another matter entirely.
The science says that spending precious dollars for climate change mitigation on forestry is high-risk: We don’t know that it would cool the planet, and we have good reason to fear it might have precisely the opposite effect. More funding for forestry might seem like a tempting easy win for the world leaders at the United Nations, but it’s a bad bet.
Nadine Unger is an assistant professor of atmospheric chemistry at Yale.