By Elisabeth Rosenthal

…

Offsets have played a growing role in the greening of travel because carbon dioxide emissions from airplanes are growing so quickly and there is currently no technological fix that would drastically lower them.

In the United States, dozens of hotels and airlines have embraced such programs in the last year or two. United Airlines became the latest American airline to offer one this summer. Globally, offset programs have grown into a multimillion-dollar industry.

But it has proved difficult to monitor or quantify the emissions-reducing potential of the thousands of green projects financed by customers’ payments, and there are no industrywide standards.

Responsible Travel is not the only organization that has changed its mind about the usefulness of offsets: Yahoo and the United States House of Representatives both ended trial offset-purchase programs this year, concluding that the money was better spent on improving their buildings’ energy efficiency.

Some of the world’s leading experts on the emissions issue have reviewed and rejected purchasing offsets for air travel.