Paris (AFP) - A swampy forest in central Africa the size of England covers previously unknown carbon stocks equivalent to three years' worth of global CO2 emissions, scientists revealed Wednesday.

Draining these peatlands for agriculture, or reduced rainfall due to climate change, would release massive amounts of planet-warming greenhouse gases, they warned in a study published in Nature magazine.

"We found 30 billion tonnes of carbon that nobody knew was there," said Simon Lewis, co-lead author of the study and a professor at the University of Leeds.

"If the Congo Basin peatlands were to be destroyed, it would release billions of tonnes of CO2 into the atmosphere," he told AFP.

"Keeping that carbon locked up" should be a priority, he added.

Peatlands are carbon-rich ecosystems that cover three percent of Earth's land surface, but store about a third of all soil carbon.

Most peat -- dense, dark-brown muck composed of decaying plants -- is located in Canada, Scandinavia and Siberia, but the tropics hold large stores as well.

Until the mid-20th century, it was often cut into bricks, dried and burned as a fuel.

More recently, however, scientists have understood that peatlands, which are at least 30 centimetres (a foot) thick, harbour vast stores of carbon in the form of the greenhouse gases that are driving global warming.

The Congo Basin peatland average about 2 metres (six feet) in thickness.

Most climate change in caused by burning oil, gas and coal to power our economies, but a tenth of global emissions come from land use, mainly deforestation and agriculture.

In Southeast Asia -- notably in Indonesia -- vast expanses of peatland have been stripped of wetland forests and drained to make way for commercial crops, especially palm oil.

That process not only releases CO2 and nitrous oxide -- another potent greenhouse gas -- into the atmosphere, it also creates health-wrecking pollution when forests are burned.

- Haven for gorillas -

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The Congo Basin's Cuvette Centrale peatlands -- astride the Republic of Congo and the Democratic Republic of Congo -- "are currently relatively undisturbed," said Emma Stokes, director of the Wildlife Conservation Society's Central Africa programme.

"But palm oil is starting to happen in Africa," she said by phone, referring to the possibility of peatlands being sacrificed to make the way for plantations.

"It is not an immediate risk, but we can't all sit back and not worry about it," she said by phone.

And while there is some concern among climate scientists that global warming many be decreasing the area's rainfall, there is not enough evidence so far to know.

Simon, who discovered the massive peatlands and helped map its contours, explained how a 145,500 square kilometre (56,000 square mile) patch could escape notice for so long.

To start with, the buried organic matter does not form near rivers, which are often the only transport arteries in sparsely populated tropical forests.

"You have to trek deep into the swamp to find it," Simon said.

"Also, you can see peat from space."

But you can distinguish ground cover. So once Simon and his colleagues realised that certain vegetation only grew on top of peatlands, they used satellite images to help map the Cuvette Centrale's contours.

A small portion of the peatlands -- some 4,500 square kilometres (1,750 square miles) -- is already protected as part of the Republic of Congo's Lac Tele Community Reserve.

Stokes, who lives nearby, is in discussions with the government about extending its boundaries, she said.

"The area is home to some of the highest densities of gorillas in the world," she said.

Forest elephants -- also threatened -- and many water birds also find haven there, partly because it is so remote from human population centres, she added.