The first project under the scheme was approved only in late 2004.

By giving rich nations incentives to invest in green energy ranging from hydro to solar power, the programme aims to brake a build-up of heat-trapping carbon dioxide in the atmosphere from burning fuels such as coal or oil.

"The known project potential ... is presently estimated to generate around a billion tonnes of emission reductions by the end of 2012," the Bonn-based secretariat said in a statement.

That is the estimated reduction between now and end-2012.

Annual world greenhouse gas emissions from human activities -- mainly from fossil fuels burnt in power plants, vehicles and factories -- exceed 25 billion tonnes. About a quarter is from the United States.

"The one billion tonne mark in emission reductions corresponds to the present (annual) emissions of Spain and the United Kingdom combined," the secretariat said. Britain emits about 650 million tonnes of carbon dioxide, Spain 350 million.

The secretariat said more than 200 green energy projects had now been approved under the programme, known as the Clean Development Mechanism (CDM), with about 600 others in the pipeline.

Under the CDM, rich nations can invest in renewable energy projects in developing nations -- such as hydroelectric power plants in Guatemala or a methane capture scheme in China -- and then claim credits back home for the emissions they save.

$100 BILLION

Those credits can in theory then be sold -- giving the rich nations the incentive to invest. Some experts say that the CDM could eventually channel more than $100 billion to renewable energy schemes from Africa to Latin America.

The Kyoto Protocol obliges 35 industrial nations to cut emissions of greenhouse gases by 5.2 percent below 1990 levels by 2008-12. The United States pulled out in 2001, saying Kyoto would cost US jobs and wrongly excluded developing nations from targets under the first round.

Kyoto is meant as a first step to slow a rise in world temperatures that many scientists say could wreak havoc by causing more heatwaves, floods and droughts and drive up world sea levels by up to a metre by 2100.

But the Climate Secretariat said that the growth in the CDM had been lopsided.

"Whilst the mechanism is seeing exponential growth, the growth is still too unevenly distributed," said Richard Kinley, officer in charge of the secretariat.