Hey Mr. Green,
How many trees must I plant to offset a typical auto's carbon dioxide emissions?
--Robert in Hampton, West Virginia
There are lots of optimists out there who think we can plant our way out of global warming by growing more trees and/or biofuels, but it just isn't the case. Sure, it would help to plant more trees, as long as they weren't some invasive species that wrecked the ecosytem. And biofuels are useful tools, as long as the crops they come from are raised with enough restraint to prevent erosion and pollution of land and water, and we don't wind up starving people to feed vehicles. But all this horticultural activity can't provide a simple solution to global warming.
The amount of carbon offset a tree can achieve depends on the species, climate, soil type, density of planting, and other variables, so the range can be fairly wide. The faster a tree grows, the more carbon it will sequester, and a fast-growing hardwood has a higher sequestration rate than a fast-growing softwood.
So let's take a fast-growing hardwood, like a red oak. Over 50 years, it can sequester about 10,500 pounds of CO2. A fast-growing softwood, like a red pine, can stow away about 7,700 pounds.
A car that gets the all-too-typical 25 miles per gallon and is driven 10,000 miles a year will use 400 gallons of gas annually. Every gallon of gas burned produces about 19.5 pounds of CO2. So in a year the car would emit about 7,800 pounds of CO2. To offset that carbon, theoretically you'd have to plant one every year for every 10,000 miles driven. Of course, it's a rough world out there, and your tree won't necessarily survive to 50--only about 20 percent do--so you'd better sock in two or three more saplings every year to be on the safe side.
Also, remember that we're talking about a 50-year period of growth, and these trees aren't going to remove nearly as much CO<sub>2</sub> in their youth as they do when they reach maturity. In the first ten years, your tree would only sequester about 315 pounds of CO<sub>2</sub>. So if you think we're running out of time with global warming, you should probably plant, oh, another dozen or so trees to be on the safe side.
If you're mathematically inclined and want to play with the numbers, check out the document "Method for Calculating Carbon Sequestration by Trees in Urban and Suburban Settings" (U.S. Department of Energy, Energy Information Administration, 1998) at ftp://ftp.eia.doe.gov/pub/oiaf/1605/cdrom/pdf/sequester.pdf.The Wisconsin Department of Natural Resources even provides an Excel spreadsheet to do sequestration calculations at www.dnr.state.wi.us/org/aw/air/registry/quantexamples/example10.html. So besides planting trees, we need to drive much more efficient cars and drive them a whole lot less (or not at all) not only to reduce global warming but also to conserve our precious yet finite supply of fossil fuel.