World emissions of the greenhouse gas carbon dioxide increased three times faster after 2000 than in the 1990s, putting them at the high end of a range of forecasts by an international climate change panel, scientists reported.

At the same time, a trend towards cutting Earth's energy intensity - the ratio of how much energy is needed to produce a unit of gross domestic product - appears to have stalled or even reversed in recent years, the researchers reported.

"This paper should be a rallying cry," said Chris Field, a co-author of the study published in the Proceedings of the National Academy of Sciences.

Noting recent media reports about countries and companies making serious commitments to combat climate change, Mr Field said: "This basically says what the challenge is, how serious they need to be."

Mr Field, of the Carnegie Institution's Department of Global Ecology in Stanford, California, said the study found that between 2000 and 2004, worldwide carbon dioxide emissions increased by 3.1 per cent a year, about three times as fast as the 1.1 per cent rate of increase in the 1990s.

In addition to energy intensity, the speed-up is also due to a rise in how much carbon it takes to make the energy people use. Other factors include growth in world population and individual gross domestic product, the study said.

Mr Field noted the scientific consensus that carbon emissions contribute to global climate change.

Much of the accelerated carbon dioxide emissions come from China, where a fast-growing economy is powered largely by coal-fired energy. The developing world, including India and China, and some of the least-developed countries accounted for 73 per cent of the growth in global emissions in 2004 and contain about 80 per cent of world population, the study found. By contrast, the study said the world's richest countries contributed about 60 per cent of total emissions in 2004 and account for 77 per cent of cumulative emissions since the start of the Industrial Revolution.

The research showed global emissions since 2000 grew faster than in the most extreme scenarios developed by the United Nations Intergovernmental Panel on Climate Change.

The panel has said that global carbon dioxide emissions must fall 50 per cent to 85 per cent by 2050 to stop the Earth from heating up more than two degrees Celsius. Higher temperature rises could prompt more deadly floods, droughts and heatwaves.

The Bush administration has pointed to recent declines in US carbon intensity and has set the goal of cutting this measurement by 18 per cent over 10 years.

Mr Field called the US government's goal "very modest". "Historically, since 1980, without doing anything, the carbon intensity of the US economy has gone down about 1.5 per cent per year, so when they talk about a goal of getting 1.8 per cent per year, it's not much change from where we are now," Mr Field said by telephone.

Another co-author, Gregg Marland of the Oak Ridge National Laboratory in Tennessee, said part of the reason for the decline in US carbon intensity was that many high-carbon manufacturing processes have been moved from the United States to other countries, including China, which sells many of the products made this way to the US.

Solving all the pieces of this problem will be difficult, Mr Marland said: "This is at the core of how we live. We're just using more energy and being more consumptive. ...Putting everybody in hybrid cars isn't going to solve this one."