US scientists say they have found a method of combining carbon dioxide with ordinary minerals that could be a way of helping to tackle climate change.

Carbon dioxide (CO2) is the most abundant gas released by human activities that is implicated in atmospheric warming.

It is given off in the burning of forests and of fossil fuels like oil, coal and gas. Humans added more than six billion tonnes of CO2 to the atmosphere in 1998.

Technology 'within reach'

But scientists at the US Department of Energy's Los Alamos National Laboratory say they have found a way of binding carbon to common magnesium- or calcium-bearing minerals.

The resulting mixture, a mineral carbonate, would trap the CO2 and prevent it reaching the atmosphere and warming the earth.

Living trees store carbon, burning ones emit it

The Los Alamos team leader, Klaus Lackner, said the method could prove practicable, and affordable.

"Although large volumes will be needed, the required minerals are extremely abundant and can be readily mined.

"We have shown that a reasonable goal for this approach is a disposal cost on the order of 0.8 to two cents per kilowatt hour."

The technology of removing excess atmospheric CO2 is known as carbon sequestration.

The DoE has formed two study centres to look at how it can be done in soils, plants, and the oceans.

One, led by the DoE's Oak Ridge, Pacific Northwest and Argonne laboratories, will concentrate on land-based sequestration.

Too much to absorb

The other, led by the Lawrence Berkeley and Lawrence Livermore labs, will study ocean sequestration.

Although some tree species can sequester large amounts of CO2, the sheer volume of gas that humanity is adding rules out any quick fix.

The answer could lie beneath the waves

A study by scientists at the Woods Hole Research Center in Massachusetts found that fossil fuel burning and deforestation in the US had contributed about 27 bn tonnes of CO2 between 1700 and 1945.

Since then, replanted forests and cropland have removed only about 2 bn tonnes.

The oceans already sequester more than 50% of CO2 of human origin, and there are hopes that they could take far more.

An experiment at the Natural Energy Laboratory of Hawaii is trying to see whether it could be feasible to pump liquid CO2 deep below the surface.

Scientists believe it would stay there in liquid form because of the high pressures involved. But there are fears that it might damage marine life, or possibly escape as a gas bubble.