Dec. 27, 2007 For the second year in a row, the National Football League plans to offset 100% of the carbon emissions from its biggest game. But unlike last year, Super Bowl 2008 will not be billed as "carbon neutral," as the NFL takes criticism for its carbon calculation methods, the Arizona Republic reports.

The NFL is buying substantial renewable energy certificates and replanting 42 acres worth of Arizona forest devastated by wildfires more than enough to offset the 350 tons of greenhouse gas produced by its 3,000-vehicle ground-transportation fleet. Yet the offset program does not, for example, take into account the air travel of NFL teams and staff during Super Bowl week, a decision that has drawn criticism from environmental groups.

The controversy highlights the difficulty in calculating the carbon impact of a particular event. For example, are event organizers responsible for the emissions created by attendee air travel?

"If creating a mess is part of our business plan, then cleaning it up needs to be part of the model as well," says Jack Groh, director of the NFL Environmental Program.

In making its carbon calculations, the NFL received guidance from Princeton University researchers and federal environmental agencies. Energy use from hotels, convention centers, and media operations were not considered because the facilities would likely have been in use anyway, according to Groh.

The NFL's carbon-footprint figure will continue to be adjusted as the league refines its methodology, Groh says.

Click here for more about the NFL's green plans for Super Bowl 2008.