Dear Science,

I try to do my part to prevent global warming, but it's recently been pointed out to me that my global jet-setting is something of a problem, increasing my carbon footprint well beyond that of the worst SUV driver. I know there are a bunch of websites that allow you to purchase carbon credits to mitigate your damage, but I can't help but feel that I'm wasting my money. Does this purchasing actually do anything positive, or is it just a conscience saver?

Guilty Flier

Dear Guilty Flier,

Take solace in this fact: SUV drivers use more carbon per mile (about one and half pounds per mile) than you do flying (about one pound per mile). Even a typical car does worse per mile than flying. So, hurray!

Well, maybe. Let's figure out just how guilty you should feel by examining how much carbon you emit each year. According to the Kyoto accord, your carbon-dioxide emissions should be about 16 metric tons, or about six round-trip flights from Seattle to New York (that is, if in the rest of your life you live in a tent, walk to work, and eat only local berries). In truth, the accord targets are pretty arbitrary—why could we safely add 16 tons this year, but not 24? What we really should care about is the total amount of carbon in the atmosphere—indirectly determined by the balance between the carbon we put in and the carbon plants take out.

So, how much carbon can the atmosphere handle? Preindustrial carbon concentrations were about 278ppm. One reasonable estimate says we should probably keep concentrations below about 550ppm, or roughly double the amount before industrialization. Others claim the number should be much lower, more like 410ppm or 480ppm. The most conservative groups point out that even the current levels, about 370ppm, might be unsustainable. Science enjoys being conservative when it comes to the climate. If current levels might be too high, each of us should plan to add no net carbon dioxide to the atmosphere—we should balance how much we emit by paying to have an equal amount of carbon removed.

Finally getting to your question, calculate your total carbon impact—airline flights are only the start of the bad news. (Feeling really terrible yet?) Try one of the excellent carbon footprint calculators on the web. Beyond flying less, try other ways to reduce your impact—eat local and seasonal foods, walk to the store, turn down the heat in the winter, and so on. What indulgence should you buy for the rest? Rather than bribing several others to use less carbon, spend your money to remove carbon from the atmosphere. A planted acre of trees removes about a metric ton of carbon from the atmosphere each year for 90 years or so, and has many other environmental benefits. Twenty acres or so should cover you for life.

Environmentally yours,


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