"The cost of Guinea will cease to exist by the end of this century," said Stefan Cramer, a marine geologist and head of German green group Heinrich Boll Stiftung's operations in Nigeria.

"The countries most threatened by this looming environmental disaster are Gambia, Nigeria, Burkina Fasso and Ghana," he told AFP on the sidelines of a major UN climate conference in the Ghanaian capital Accra.

Cramer said sea levels were set to rise up to two centimetres (0.8 inches) per year, enough to devastate large swathes of fragile coastline, especially in low-lying and densely populated deltas.

Last year UN climate change experts initially predicted more modest rises of 18 to 59 centimetres (7.2 to 23.2 inches), but in a final version of their report left the upper limit open-ended due to mounting scientific evidence that levels might climb much higher.

The Intergovernmental Panel on Climate Change (IPCC) had not taken into account the potential impact of runoff from the 3,000-metre (1.9 mile) thick Greenland ice cap, which covers an areas three times the size of Nigeria.

Recent studies have suggested the continent-sized ice block could be melting far more quickly than once thought.

Among the cities worst hit would be the Gambian capital Banjul and Lagos, Nigeria's economic capital and home to 15 million. Some parts of Lagos lie below sea-level today and it is already subject to frequent flooding.

The Niger delta's income-generating oil fields are especially vulnerable, Cramer said.

In Ghana, "up to 1,000 kilometres of land may be lost in the Volta Delta owing to sea-level rise and inundation," Yvo de Boer, head of the UN Framework Convention on Climate Change, said at the meeting.

The devastation wrought by rising sea levels is amplified by increasingly violent tropical storms, which can create sea surges up to three metres (10 feet) high.

In August 2007 a storm 5,000 kilometres off the coast of Lagos destroyed protective beach barriers, highlighting the vulnerability of the entire African west coast.

Another serious threat is salty sea water intrusion into fertile agricultural land.

"This will make the ground water undrinkable and unsuitable for agricultural purposes. The result will be food and water insecurity," said George Awudi, Ghana Programme Coordinator for Friends of the Earth.

Environmental experts offer different solutions, but all agree on the futility -- and prohibitive cost -- of erecting massive sea barriers.

"The sensible option is moving to higher ground, which is a tough option especially for Nigeria as it means giving up its economic centres in Lagos and its oil installations in the Delta," Cramer said.

But Awudi insisted that relocation was an "unthinkable option due to its economic, social and cultural implications."

The solution, he said, is to focus on the source of the problem rather than on how to adapt to its consequences.

"The industrialised countries should take proactive steps in curtailing greenhouse gas emissions," he said.

But even if carbon dioxide emissions drop dramatically, experts say, sea levels would continue to rise for 50 to 100 years.

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