The main green effort by online travel ? indeed, for travel in general, and for many other industries ? involves so-called carbon offsets. These are fees customers can pay to subsidize alternative energy projects. In the U.S., it's almost always voluntary, at least for now

The fee is calculated to equal the amount of carbons that, say, an airline passenger is responsible for releasing into the air. So, in theory, the fee offsets the bad stuff.

The offsets are passed onto green investment firms, which then invest the money in solar, wind and other alternative energy projects. Some of the money also finds its way to tree-planting projects and other such efforts.

Offset programs could even have the extra benefit of sparking more travel by eco-conscious consumers and businesses. The offsets process, however, is complex.

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