"The market has ... exploded in the past 12 months," said Jonathan Shopley, managing director of The Carbon Neutral Co. in London whose clients include carmaker Honda and British broadcaster BSkyB.

"Carbon neutral" schemes 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 by growing, or invest in renewable energy or energy-efficiency projects.

The aim is for governments, individuals or companies to prevent as much carbon emissions as they produce.

Once limited to a hard core of green activists, carbon neutral projects are winning wider favour.

"We're talking about millions of tonnes 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 tonnes 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.

REGULATIONS

The voluntary carbon trade lacks global rules or a central registry -- emissions could in theory be sold more than once. Prices for a tonne of non-toxic carbon dioxide vary widely.

"The market is growing fast. We're hoping for commonly accepted standards in coming months," said Renate Heuberger, managing director of Swiss-based myclimate.org which is helping soccer's governing body FIFA at the World Cup finals in Germany.

FIFA plans to offset 100,000 tonnes of carbon dioxide linked to travel by World Cup spectators. Under one project, it and the German tournament organisers will invest in replacing coal-fired boilers at a South African factory with boilers run on sawdust.

Some environmentalists are sceptical of the offset projects, particularly forest plantings since they say many saplings die.

They want the world to focus on axing fossil fuel use.

"'Plant a tree and fly guilt-free' is obvious nonsense," said Steve Sawyer of Greenpeace.

Yet with growing public concern about global warming, some businesses find that it pays to go green.

HSBC says it became the world's first carbon neutral bank in 2005 by investing in clean energy projects such as a wind farm in New Zealand to offset emissions from its core operations.

HSBC bought 170,000 tonnes of carbon dioxide for US$4.4 per tonne for the fourth quarter of 2005, a total of about US$750,000. Some environmentalists say HSBC should extend its scheme, which excludes the carbon impact of its loans.

Gore's movie calls itself the "first carbon neutral documentary". But Gore was criticised after he was spotted driving 500 metres to a screening at the Cannes film festival.

The World Bank said this month it would offset 148,000 tonnes of emissions to become "carbon neutral".

The projects are springing up in parallel to the United Nations' Kyoto Protocol, which obliges 35 industrial nations to cut emissions by 5.2 percent below 1990 levels by 2008-12.

President George W. Bush pulled the United States out of Kyoto in 2001, saying it would cost jobs and wrongly excludes developing nations. The United States is the world's number one source of carbon dioxide, emitting about 24 tonnes per person.

"If you only look at US politics you miss how much the average person has a concern for these issues," said Jack Groh, director of the National Football League's environment programme.

Organisers of the Super Bowl, won by the Pittsburgh Steelers in Detroit in February, planted enough tree seedlings to cover 3.5 acres (1.42 hectares) and soak up 260 tonnes of carbon.

BEWILDERING VARIETY

World Cup organisers are buying "certified emission reductions" (CERs) created by a Kyoto scheme for promoting investments in clean energy in developing countries, with an additional quality stamp known as the "gold standard".

"This is a market with very different currencies, we believe that CERs will be the benchmark," Heuberger at myclimate said.

He said that CERs bought for the World Cup cost almost 10 euros (US$12.58) a tonne. In a European Union market set up under Kyoto, prices are around 15 euros a tonne.

Shopley's company -- founded before Kyoto started -- relies mostly on "verified emissions reductions" (VERs), checked by external auditors but often cheaper than CERs. VERs are often energy projects too small to qualify for Kyoto mechanisms.

For travellers, Web sites can be bewildering.

For one passenger flying from Paris to Sydney, for instance, Carbon Neutral reckons emissions are 1.9 tonnes, but Germany's Atmosfair reckons the emissions total 5.99 tonnes, due to a "high altitude surcharge".