Plenty of people concerned about global warming have replaced their household light bulbs with compact fluorescent ones and bought hybrid cars. But even the most politically correct Americans are loath to cut back on their biggest contribution to climate change: airline travel.

Commercial aviation worldwide ranks among the biggest single emitters of greenhouse gases linked to global warming, contributing nearly as much carbon dioxide to the atmosphere every year as the United Kingdom or Canada, according to the advocacy group Environmental Defense. So a few entrepreneurs and environmental groups have devised a simple way to compensate for this damage: Plant a tree.

Trees serve as what scientists call a "carbon sink," each absorbing more than a ton of carbon dioxide from the air over the course of roughly a century. At least two companies, the Welsh-based Treeflights and the U.S.-based travel booking site Travelocity, now offer a way for travelers to plant trees for every trip they take. While this will address only a fraction of the pollutants linked to climate change, the gesture has become increasingly popular among U.S. and British consumers.

"The whole world is looking for a solution to the biggest problem we've ever faced," said Ru Hartwell, the Welsh professional tree planter who runs Treeflights. "I seem to have stumbled across something that could help and is easy to understand."

For about $18.70, Hartwell will plant the tree of your choice in his Welsh forest. He estimates that the average tree absorbs 1.25 tons of carbon dioxide over the course of its lifetime, which is equivalent to the amount emitted by a commercial airline during a round-trip flight between New York and London. (The amount of carbon dioxide absorbed by a given tree varies according to species and region: The Conservation Fund, a Virginia-based environmental group, estimates that the hardwoods it plants in the U.S. Southeast absorb 1.33 tons of carbon each over 100 years.)

Hartwell, who has planted 1,000 trees since starting his operation in July (www.treeflights.com), estimates that 40 percent of his online customers come from the United States. He also has formed partnerships with British travel agents and tour operators, who offer their clients the option of paying a bit extra to help mitigate their trips' environmental damage.

Hartwell has three planting sites with room for 40,000 to 50,000 trees. The Forest Stewardship Council, a Washington-based nonprofit group, is in the process of certifying his operation. He has yet to turn a profit on his venture.

While Treeflights is a small operation -- Hartwell fears that demand will outstrip his ability to plant trees on the 60 acres he owns -- Travelocity has launched an ambitious partnership aimed at soaking up its customers' greenhouse gas emissions.

In concert with the Conservation Fund, the online booking agency offers customers three levels of environmental commitment. A $10 contribution offsets emissions stemming from one person's cross-country air travel, a one-night hotel stay and rental car. Compensating for a couple's cross-country flight, a four-night hotel stay and rental car costs $25. And for $40, a Travelocity client can atone for four individuals' flights, a four-night hotel stay and rental car.