A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Nature, fossil fuels, and global warming: How many trees should I plant to balance my yearly CO2 output?

January 3, 2003

Dear Cecil:

In my high school biology class we were studying biogeochemical cycles, including the carbon cycle. One paragraph detailed how humans, by burning fossil fuels, are putting more carbon dioxide into the air than is being removed, causing global warming, etc. According to my textbook, transportation accounts for most of the carbon dioxide being added to the air since our cars use petroleum-based fuels. The book also notes that trees and other green plants remove carbon dioxide from the atmosphere. How many houseplants, acres of grass, and trees should I have if I want to take as much carbon dioxide out of the atmosphere as I'm putting in?

Cecil replies:

Scoffers may say: This kid's on crack. But if he is, so is the United Nations. A controversial trees-for-pollution trading scheme, hereafter referred to as "T4P" (houseplants and grasses are too short-lived to be useful), was inserted into the UN's 1997 Kyoto Protocol on climate change. The idea: Polluting countries can earn "forest credits" by planting or conserving trees to remove from the atmosphere the carbon dioxide their cars and factories pump in. Under Bush the U.S. has disavowed the Kyoto agreement, but even so a brisk business in "certified tradable offsets" and such has sprung up — meaning, among other things, that first world polluters can pay third world nations to set aside forests as "carbon sinks."

Critics, however, say that while T4P addresses a serious question, it's sure a stupid answer. Their main objections:

(1) It's not fair. Polluters in wealthy industrialized nations will be able to buy their way out of their problems by establishing huge tree farms in developing countries. Poor people in those countries won't have enough land to grow food. True? Who knows? But appeals to justice generally have zero impact on global environmental policy, so it's not worth debating whether it's moral for the developed world to pollute (or consume) more than its share.

(2) It won't work. This argument is more telling. Unlike carbon buried in coal, oil, or sedimentary rock, carbon in trees is unstable — one giant forest fire and much of it is released back into the atmosphere, leaving you back where you started, and even without a fire carbon is eventually liberated by decay. There are yawning gaps in our knowledge of global carbon exchange at the most basic level: Each year humans release about 6.7 billion tons of carbon into the atmosphere, and natural processes take about 3.5 billion tons out. We know the oceans absorb roughly 2 billion tons. The remaining 1.5 billion . . . well, we're not quite sure where it goes. Evidence suggests there are large natural carbon sinks on land in the middle latitudes of both hemispheres, as well as natural carbon sources in the tropics. But we've still got a lot to learn. Even in well-understood processes such as plant respiration, many variables are involved — if the climate became permanently warmer, for instance, many assumptions about how trees process carbon dioxide would go out the window.

(3) It diverts attention from the real issue. Vast though the amount of carbon in the atmosphere is, there's more than five times that much locked in fossil fuels. (For comparison, the quantity of carbon tied up in living things is a bit less than what's in the air.) Now we're energetically extracting and burning coal and oil, undoing the work of aeons in a few hundred years. We can't possibly park all that liberated carbon in trees, no matter how many we plant. In the long term, the only real solution is conservation. But Americans, who in 1990 accounted for 36 percent of world greenhouse gas emissions, don't conserve for squat —  witness our love affair with the SUV. While T4P makes it seem like we're doing something useful, what we really need to do is bite the bullet and use less fossil fuel. For more on the anti-T4P argument, see www.wrm.org.uy/plantations/material/carbon.html.

Lest you get the wrong impression, critics of T4P are currently in the minority. Many respected environmental groups have endorsed the concept. Despite the uncertainties, most experts agree that having more trees is bound to help, and of course saving the rain forests would have benefits other than sequestering more carbon.

Still, it's fair to say that claims that planting x trees offsets y tons of pollution are simplistic. Consider two of the carbon calculators I found on the Web. Both agree that driving a medium-size car 10,000 miles a year will generate roughly four tons of carbon dioxide annually. But though American Forests (www.americanforests.org/resources/ccc/index.php) figures you'll need to plant twelve trees to offset that (which for a fee they'll happily do), Future Forests (http://www.futureforests.org.uk/#!buy-a-tree/h1ujr) says you can get by with four. Sure, maybe American Forests is talking strictly about temperate-zone trees and Future Forests is including tropical ones, which have a greater impact on the carbon balance sheet. The point is, getting a handle on greenhouse gases is complicated, and we'd be foolish to think we've got it all figured out.

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