"Carbon neutral" programs typically invest in non-polluting wind, solar or hydropower projects in Africa, Asia and Latin America to offset emissions of heat-trapping carbon dioxide released by burning fossil fuels such as coal, oil or gas.
Or they pay to plant trees, which soak up carbon, or invest in renewable energy or energy-efficiency projects.
The aim is for governments, individuals or companies to offset the carbon emissions they produce.
Once limited to a hard core of green activists, carbon neutral projects are winning wider favor. "We're talking about millions of tons a year (of carbon dioxide offsets) instead of perhaps 100,000 a couple of years ago," Shopley said.
Still, the amounts are a pinprick in world emissions from human activities of more than 25 billion tons a year. Many scientists say global warming, widely blamed on greenhouse gases, could trigger more floods, droughts and heatwaves and drive up world sea levels.
For one passenger flying from Paris to Sydney, for instance, Germany's Atmosfair reckons the emissions total 5.99 tons, due to a "high altitude surcharge."