Do Trees Make It OK to Drive an SUV?

A forest in the Adirondack Mountains is seen from the Tongue Mountain Range in the Town of Bolton, N.Y., Tuesday, May 15, 2007. People worried about global warming increasingly are trying to "offset" the carbon dioxide they spew into the atmosphere when they drive, fly or flick on a light. One popular way with the eco-conscious is to have trees planted on their behalf. (AP Photo/Mike Groll) (Mike Groll - AP)

ALBANY, N.Y. -- If you plant some trees, is it OK to drive an Escalade? The question isn't as silly as it sounds. People worried about global warming increasingly are trying to "offset" the carbon dioxide _ the leading greenhouse gas _ they spew into the atmosphere when they drive, fly or flick on a light. One idea popular with the eco-conscious is to have trees planted for them. You get to keep driving and flying, but those trees are supposed to suck in your trail of carbon.

Whole forests have been funded by tree-loving celebrities like Leonardo DiCaprio and Coldplay, and more modest packages tailored to typical consumers are proliferating.

But some researchers say planting trees _ while a good thing _ is at best a marginal solution to global warming. Still others decry tree planters who continue to jet off to Cannes, drive their SUVs or generally fail to reduce their fuel-hungry lifestyle. To those critics, plantings and other carbon offsets are like the medieval practice of selling indulgences to wash away sins: It may feel good, but it doesn't solve much.

"The sale of offset indulgences is a dead-end detour off the path of action required in the face of climate change," says a report by the Transnational Institute's Carbon Trade Watch.

Groups that offer tree offsets typically rely on Web calculators requiring users to type in how many miles they drive, how much electricity they use and how far they fly. Figure out how much CO2 someone is responsible for (output), compare it to the work average trees can do (input), and you have a formula for neutralizing a person's "carbon footprint."

While the band Coldplay famously funded 10,000 mango trees in India to soak up emissions related to the production of a CD, the average consumer can get off far easier. For $40, Trees for the Future will plant 400 trees in a developing country to handle your car emissions. In June, Delta Air Lines will allow online ticket buyers to help offset emissions of their flights through tree plantings in the U.S. and abroad: $5.50 for domestic round trips, $11 for international.

"It's easy to do and it makes a big difference," said Jena Thompson of the Conservation Fund, Delta's partner and one of many groups that will plant trees on your behalf.

The science is sound: Trees take in carbon dioxide as part of photosynthesis and store the carbon. But even conservationists caution it's not as simple as planting a sapling so you can crank up the air conditioning without guilt.

Offset groups use averages to estimate how much carbon a given tree or forested acre can capture. For instance, the nonprofit Conservation Fund figures that each tree planted captures less than 1 1/2 tons over 100 years.

To put that in perspective, consider that about 7.3 billion metric tons of carbon dioxide was produced from the burning of fossil fuels worldwide in 2003, the most recent estimate available.

And how much carbon dioxide a tree can soak up varies, said John Kadyszewski of Winrock International, a nonprofit that works on environmental projects. A huge California redwood might have 30 tons of carbon stored while a 100-year-old pine might have less than a ton.

"Trees are all different," said Kadyszewski, coordinator for ecosystem services for Winrock, "and the amount of carbon in the tree depends on how old it is and where it's growing and what kind of tree it is."