Hey Mr. Green,
I've heard that planting one tree is better than buying a Prius or even not driving for a year. What do you think? If one continues to drive an outdated vehicle, could one make up for it by planting a tree every year?
There is a teensy-weensy grain of truth in the proposition that planting a tree every year is "better" than buying a Prius, but only if you are thinking exclusively about offsetting carbon dioxide emissions a hundred years or so from now. But sophists do have a way of ramping up a grain into a veritable asteroid, so this sounds like another perverse argument from Prius haters. As noted elsewhere in this column, lots of people have it in for this car. And unfortunately some of them have a lot of power: The EPA under President George W. Bush has ruled that individual states may not propose stricter vehicular emission standards than those set by the federal government. Watch as the lawsuit against this decision unfolds.
Back to your question. Let's grant that a tree's ability to absorb carbon dioxide could match a Prius's ability to reduce CO2 emissions. That's nice. But offsetting carbon dioxide, although it's now getting the lion's share of our attention, is not the only good reason to reduce fossil-fuel consumption. Driving a Prius instead of a regular car also reduces the environmental damage caused by fossil-fuel extraction, refining, and shipping; cuts the unhealthy air pollution caused by burning fossil fuels; and conserves the finite supply of fossil fuels, which common sense tells us to use as efficiently as possible. (Hey, I dislike global warming as much as Al Gore does, but let's be clear: There are plenty of other serious threats to the environment that also merit our attention--like the expanding dead zone in the Gulf of Mexico that will result from stepped-up ethanol production when all the fertilizers used to grow more corn wash down the Mississippi River from the Corn Belt.)
Now let's compare the Prius with an older car. The Prius is rated to get 46 miles per gallon. Let's say your old car gets half as much, or 23 miles per gallon. The average car is now driven about 12,500 miles a year--sad but true, or truly sad. So if you bought a Prius instead of keeping your old car, over that 12,500-mile year you'd save 272 gallons of gas. Since burning a gallon of gas generates 19.5 pounds of carbon dioxide, the Prius would emit 5,300 pounds less CO2 per year than the old car. But because it requires the energy equivalent of about 1,000 gallons of gas to manufacture a Prius--which results in 19,400 pounds of carbon emissions--it would take four years before the Prius started to save that <i>net</i> amount of 5,300 pounds of CO2. In ten years you would have planted ten trees, which together would only absorb about 1,320 pounds of carbon, way less than the almost 32,000 pounds the Prius would've kept out of the air in that period. In the real world, as opposed to the imaginary enchanted forest of Prius haters, it would take 30 or 40 years for the trees to match the Prius's savings on CO2, assuming they all survive--which just might be too long to wait. Seems like we should be buying hybrids and planting trees or, better yet, not driving and planting trees.
You should remember that when a tree dies and decays or is burned, a lot of the carbon it originally stored gets kicked back into the atmosphere anyway. (We've observed this during all the recent fires in the West, the intensity of which some blame on global warming.) So sequestering is not forever, and maybe a much shorter period.
Finally, I don't see the point in arguing that planting a tree is "better" than not driving. Planting a tree is a classy thing to do in addition to not driving. I mean, you do breathe harder walking or biking, but you're hardly emitting enough CO2 to aggravate global warming, and if you take efficient mass transit, you're reducing emissions a lot more than if you keep driving the old car. But if you have to have a car, a hybrid or a very efficient, small conventional model are still the best choices. To explore the possibilities, go to fueleconomy.gov.