by Robin Pagnamenta, Energy Editor and Ben Webster, Environment Editor

A global deal to cap surging emissions of carbon dioxide from soil will form a critical part of any successful agreement to tackle climate change in Copenhagen later this year, Al Gore said today.

The former US Vice President and environmental campaigner urged world leaders who are set to gather for a UN meeting in the Danish capital in December to recognise the critical importance of soil carbon: an often overlooked part of the debate on global warming.

"There is three times as much carbon in the first two meters of soil than there is in all of the world's vegetation," he told an environmental conference at the Smith School in Oxford.

Current estimates indicate that changing land use - including the burning of peatland, the conversion of degraded former forest land to agriculture and desertification through over-farming - is responsible for as much as 30 per cent of the world's carbon emissions, more than either deforestation, power generation or transport.

Mr Gore cited the example of Indonesia, the world's third largest emitter of carbon dioxide after China and the US.

He said Indonesia's high level of emissions were chiefly the result of soil degradation rather than the linked but distinct problem of deforestation.

"Brazil cuts down twice as many trees as Indonesia but Indonesia emits twice as much carbon dioxide as Brazil."

Much of Indonesia's tropical rainforest lies on peat soil. After the trees have been cut, the peat is often burnt before the land can be reused - mostly for the creation of palm oil plantations.

The practice releases vast amounts of carbon into the atmosphere and each burning season leaves plumes of smoke hanging over much of southeast Asia for months at a time.

The greenhouse effect is compounding the problem of soil degradation because rising temperatures add to the drying and destruction of carbon-rich soils.

Mr Gore said China was leading the way in trying to "recarbonise" degraded soil through tree planting - an effort which he said needed to intensify globally.

"China now plants two and a half times more trees than the rest of the world put together," he said.

"Every Chinese citizen between the ages of 6 and 60 has to plant three trees a year." Soil is the third-largest natural store of carbon in the world after the oceans and fossil fuels such as coal, oil and gas.

Mr Gore expressed optimism that the vast challenge of cutting emissions remained feasible.

"Time is short. Today we will put 70 million tonnes of carbon dioxide into the thin shell of the atmosphere surrounding the planet [but] there is no question that we can solve this crisis."