When 16-year-old climate activist Greta Thunberg sailed across the Atlantic Ocean on a zero-emissions boat, she did so to make a point about the millions of tons of carbon dioxide emitted into the atmosphere every year as a result of air travel. Spend about 10 hours flying this holiday season and your travel could add as much as a metric ton, or 2,000 pounds, of carbon to the atmosphere.

If you feel guilty about those climate-warming carbon emissions, you might be tempted to purchase a carbon offset. In the past year, Google searches for “carbon offsets” have been on the rise, and those who sell them say their sales are up.

Carbon offset vendor Cool Effect says individual purchases