The Opinion Pages | Op-Ed
Contributor
To Save the
Planet, Don’t Plant Trees
A landmark deal to support sustainable forestry was a heralded success
story of the last international climate talks, in Warsaw United States , Britain and Norway
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
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.
No comments:
Post a Comment