Once viewed as an issue of interest only to greens or academics, the threat posed by climate change to security is now eyed with deepening concern by politicians and defence chiefs.

Droughts and floods which devastate crops and rising seas which imperil coastal cities will become potent triggers for famine, disease and homelessness, in turn inflaming tensions and leading to unrest, say experts.

Indeed, some suspect that climate change is already an invisible driver of turbulence.

The conflict in Sudan's Darfur, caused by an exceptional drought that impoverished herding communities and forced them to migrate, has been cited as just such an illustration.

Another example may be this year's revolutions in Tunisia and Egypt, where food prices, propelled by devastating heatwaves in big grain-growing countries, fanned hunger, and then anger, among the poor.

"Extreme weather events continue to grow more frequent and intense in rich and poor countries alike, not only devastating lives but also infrastructure, institutions and budgets -- an unholy brew which can create dangerous security vacuums," UN Secretary General Ban Ki-moon said in July at a Security Council debate.

Climate change "not only exacerbates threats to international peace and security; it is a threat to international peace and security," he said.

In its 2010 Quadrennial Defense Review, the Pentagon said climate shift "could have significant geopolitical impacts around the world, contributing to poverty, environmental degradation and the further weakening of fragile governments."

"While climate change alone does not cause conflict, it may act as an accelerant of instability or conflict," it said.

Rear Admiral Neil Morisetti, an envoy for climate and energy security at Britain's ministry of defence, said climate migration was one of the hidden factors in this equation.

"What happens to those people who lose their land or who lose their livelihood?" Morisetti said at a conference in London last month.

"If they migrate, is it planned, coordinated, manageable migration in a country or between countries? Or is it unplanned mass migration that causes tension?

"If they lose their livelihood because of rising sea levels, rising temperatures, loss of crop yields, do they find a legal livelihood to replace that? Or are they susceptible to recruitment into crime, ultimately (becoming) a five-dollars-a-day AK-47 terrorist?"

Morisetti said the biggest risks were "in the equatorial belt, where we have seen conflict time and time again in the last 40 or 50 years, partly because the countries there and their governments do not have the capacity and the resilience to cope with those stresses and look after their population."

In a paper published last month by the US journal Science, an international team of researchers said "climate-related resettlement" was already underway in Vietnam's Mekong delta, along the Limpopo River of Mozambique, in China's Inner Mongolia, the coast of Alaska and the Carteret Islands in Papua New Guinea.

Calling for help to ensure fair and orderly migration, they urged changes to national and international law and the involvement of climate-threatened communities in deciding where they would be resettled.

Other factors in the murky interface between climate and security are health -- especially through the expansion of mosquito- and water-borne disease -- and the amplified risk of hunger and poverty from rising food prices.

Wheat, corn and sorghum have all seen global spikes in the past 18 months, but in the drought-hit Horn of Africa their prices have at times doubled or tripled compared to a five-year average.

Rice in flood-affected Thailand and Vietnam is some 25 percent more expensive than a year ago.

In February, the World Bank estimated 44 million people in developing economies had fallen into extreme poverty through spiralling food prices.

"For the poorest who spend up to 75 percent of their income on food, price rises on this scale can have consequences as families are forced into impossible trade-offs in a desperate bid to feed themselves," Oxfam said on Monday at the start of the UN climate talks in Durban, South Africa.