CARBON DIOXIDE is heating up the Earth. Ice caps are melting, ocean levels
are rising, hurricanes are intensifying, tropical diseases are spreading and the
threat of droughts, floods and famines looms large. Can planting a tree help
stop all this from happening?
To some, it’s a no-brainer: We add carbon
dioxide to the atmosphere every time we use energy from coal, oil or gas; but
each tree can remove more than a ton of carbon dioxide from the atmosphere over
its lifetime. Based on this logic, it might seem a good idea to go out and plant
a tree to slow global heating.
And if you don’t have the time, projects
have sprung up throughout the world claiming to help cool the earth, ready to
accept your money and plant a tree in your name. The computer company Dell will
now donate $2 from every laptop sale to planting trees in an effort to offset
the carbon dioxide emissions that result from powering their computers. For a 2
percent to 4 percent surcharge on bills, Pacific Gas and Electric will offer to
offset its customers’ carbon emissions by helping to preserve California’s
carbon-storing forests.
While preserving and restoring forests is
unquestionably good for the natural environment, new scientific studies are
concluding that preservation and restoration of forests outside the tropics will
do little or nothing to help slow climate change. And some projects intended to
slow the heating of the planet may be accelerating it instead.
Trees
don’t just absorb carbon dioxide — they soak up the sun’s heating rays, too.
Forests tend to be darker than farms and pastures and therefore tend to absorb
more sunlight. This has a warming influence that appears to cancel, on average,
the cooling influence of the forest’s carbon storage. This effect is most
pronounced in snowy areas — snow on bare ground reflects far more sunlight back
to space than does a snowed-in forest — so forests in areas with seasonal snow
cover can be strongly warming.
In contrast, tropical forests appear to
be doubly valuable to the earth’s climate system. Not only do they store copious
amounts of carbon, the roots of tropical trees reach down deep, drawing up water
that they evaporate through their leaves. In the atmosphere, this water may form
clouds that reflect sunlight back to space, helping to cool the
earth.
These findings have important policy implications. It has been
suggested that agreements to limit climate change should consider carbon stored
in forests. If so, they would need to consider the direct climate effects of
forests so as to avoid perverse incentives to plant warming forests in places
like the United States, Canada, Europe and the former Soviet Union. However,
tropical forests, which are generally found in developing countries, may be due
a double climate credit — one for their carbon storage and another for their
cooling clouds.
What does this mean for local reforestation efforts?
Consider Pacific Gas and Electric’s surcharge plan. While the carbon soaked up
by California’s forests reduces atmospheric carbon dioxide concentrations
everywhere, cooling Crete, Cancún and Calcutta, the sunlight they absorb warms
the state and the surrounding region. So, it might even cool us if we were to
cut down those dark forests. Lumber interests might look gleefully upon the
prospect.
Clear-cutting mountains to slow climate change is, of course,
nuts. The broadest goal is neither to slow the growth of carbon dioxide in the
atmosphere nor to slow climate change, but rather to preserve the irreplaceable
natural balance that sustains life as we know it on this planet. We want to
avoid climate change so that we might pass these diverse natural riches on to
future generations. In this light, preserving and restoring forests is a
valuable activity, regardless of its impact on climate — we need more trees, not
fewer.
But the notion that we can save the planet just by planting trees
is a dangerous illusion. To preserve our environment, we must drastically reduce
carbon dioxide emissions, and this will require a major transformation of our
energy system. A primary goal for the next half-century should be to transform
our energy system to one based on clean, safe and environmentally acceptable
energy sources like wind, solar and perhaps nuclear. This means solving the real
problems involved with storing and distributing power, providing energy for
transportation, and using nuclear plants.
We cannot afford to indulge
ourselves with well-intentioned activities that do little to solve the
underlying problem. Instead, we must demand that our political leaders do more
to revolutionize our energy system and preserve our environmental inheritance
for future generations.
And then we can plant a tree.
Ken
Caldeira is a scientist at the Carnegie Institution’s department of global
ecology.