Hey Mr. Green,
I'd like to grow my own carbon credits by planting Douglas firs, which can live for more than 500 years, on my woodlot. How many do I need to plant to offset my carbon footprint?
--Larry in Vancouver, Washington
Planting trees is a splendid activity, but it's hard to plant your way out of global warming unless you have lots of land. And 500 years in the future is too remote for even Mr. Green to render a plausible prophecy. By then, your prospective Dougs might be thriving brilliantly, or driven to extinction, or exist only as DNA spirals in a spacecraft carrying your distant progeny on a galactic quest to find a planet to replace the one we wrecked.
So let's look a modest 50 years ahead instead. If you're an average American, you now emit 22 tons of carbon dioxide per year. In 50 years, you'll have pumped out 1,100 tons. To soak that up, you'd need to plant some 270 Douglas firs, assuming all of them make it to age 50. However, only 35 percent are likely to live that long, so you'd need to plant about 800 trees--requiring some 11 acres--to compensate for the firs that are bound to fall.
However, you can get by with planting fewer trees if you emit a lot less carbon dioxide than the average person. To find out where you stand, try the EPA's household emissions calculator at tinyurl.com/epacalc.