PARIS, FRANCE - China has leapfrogged the United States as the world's biggest carbon emitter and India is heading for third place, scientists said on Friday in a report that warned global greenhouse-gas levels were scaling record peaks.

The report, by a research consortium called the Global Carbon Project (GCP), confirms an estimate that China has become the biggest producer of carbon dioxide (CO2), the principal gas that causes global warming.

Until 2005, rich countries emitted most of the world's man-made CO2. Today, developing countries now account for 53 percent of the total, the GCP said.

"The biggest increase in emissions has been taking place in developing countries, largely in China and India, while developed countries have been growing slowly," it said.

"The largest regional shift was that China passed the US in 2006 to become the latest CO2 emitter, and India will soon overtake Russia to become the third largest emitter."

The GCP said CO2 emissions last year were the equivalent to almost 10 billion tonnes of carbon. Fossil fuels accounted for 8.5 billion tonnes and changes to land use, mainly through deforestation, accounted for the rest.

Atmospheric concentrations of CO2 surged 2.2 parts per million (ppm) in 2007 to reach 383 ppm. The rise was 1.8 ppm in 2006.

At 383 ppm, CO2 levels are 37 percent above the benchmark of 1750 when the start of the Industrial Revolution unleashed voracious use of coal, oil and gas, it said.

"The present concentration is the highest during the last 650,000 years and probably during the last 20 million years," the report said.

It warned: "All of these changes characterise a carbon cycle that is generating stronger climate forcing, and sooner than expected."

The document, to be unveiled simultaneously at conferences in Paris and Washington on Friday, also made these points:

-- Emissions have risen starkly since the Millennium. From 2000-2007, the average annual hike has been 2.0 ppm. This compares with 1.3 ppm per year in the 1970s, 1.6 ppm in the 1980s and 1.5 ppm in the 1990s.

-- Fossil-fuel emissions this decade are running at four times those of the 1990s.

-- Tropical deforestation amounted to 1.5 billion tonnes of carbon last year, with Latin America and Asia each accounting for 600 million tonnes and Africa 300 million.

-- Natural "sinks" -- the ocean, forests and other land -- are "a huge subsidy" to the global economy, worth 500 billion dollars annually for soaking up more than half of the CO2 that would otherwise be released into the atmosphere and increase the greenhouse effect.

But these "sinks" are in a bad way. Their efficiency has fallen by five percent over the past 50 years "and will continue to do so in the future."

The GCP report, Carbon Budget 2007, is authored by eight scientists in a project sponsored by the International Geosphere-Biosphere Programme (IGBP), the International Human Dimensions Programme on Global Environmental Research (IHDP) and the World Climate Research Program.

It is based on UN data, statistical models and climate research published in major peer-reviewed journals and on energy data collected by the oil giant BP.

"Our numbers provide a reality check," said Corinne Le Quere, from the British Antarctic Survey (BAS) and the University of East Anglia in Eastern England.

"The scale of efforts (to tackle emissions) is not enough."

In 2007, China emitted 1.8 billion tonnes of carbon from fossil fuels, compared with 1.59 billion by the United States. Russia was third, with 432 million tonnes, followed by India, with 430 million.

In November 2006, the International Energy Agency (IEA) predicted that China would overtake the United States as No. 1 carbon polluter by 2010.

But in June 2007, the Netherlands Environmental Assessment Agency said China had already become the biggest emitter the previous year, thanks to soaring demand for coal and a surge in cement production.

Analysts say the question of top polluter is politically charged. It touches on a nerve point at UN talks for a new global deal to address climate change.

The United States has led the charge for China and India to sign up to tougher curbs on heat-trapping gases, arguing that a pact would be worthless without constraints by the big emitters of the future.