For years, scientists and security analysts have warned that global warming looms as a potential source of war and unrest.

Storms, droughts, floods, and spells of extreme heat or exceptional cold: all can destroy wealth, ravage harvests, force people off land, exacerbate ancient rivalries and unleash a fight for resources, they say.

These factors are predicted to become more severe as carbon emissions interfere with Earth's climate system.

Yet some argue there is evidence that man-made warming is already a driver in some conflicts.

"In a number of African countries the increase in violent conflict is the most striking feature of the cumulative effects of climate change," South Africa's Institute for Security Studies (ISS) warned in 2012.

"In the Sahel region, desertification is causing clashes between herders and farmers because the availability of cultivated land is being reduced.

"Climate-related effects of this nature are already resulting in violent conflicts in northern Nigeria, Sudan and Kenya," it added.

The idea leapt to prominence in 2007, when UN chief Ban Ki-moon said violence in Sudan's Darfur region was sparked in part by a two-decade-long decline in rainfall that devastated cattle herds.

Arab nomads were pitched against settled farmers in a rivalry for grazing and water.

The tensions bloomed into full confrontation between rival militias -- an escalation due "to some degree, from man-made global warming," Ban argued.

Others have drawn a link between the 2011 Arab Spring uprisings and climate change-induced heatwaves in cereal-exporting countries.

Russia, Ukraine and Kazakhstan took their grain off the global market -- and within four months, global food prices hit their second record peak in three years.

This may have lit the fuse in powder-keg Arab countries burdened by poverty, youth unemployment and authoritarian rule, according to this view.

Former US vice president Al Gore, now a Nobel-honoured climate campaigner, believes climate change was a factor, among others, in the Syrian conflict.

"From 2006 to 2010, there was a climate-related historic drought that destroyed 60 percent of the farms in Syria, 80 percent of the livestock and drove a million refugees into the cities, where they collided with another million refugees from the Iraq war," Gore said in Davos last month.

- Caution -

Climate scientists are cautious about drawing a causal link between global warming and current conflicts -- as opposed to future ones.

"The example of Darfur is often put forward to illustrate the effect of climate on conflict between groups," French climatologist Jean Jouzel writes in a new book.

"But the reality is more complex, and most researchers acknowledge that the political and economic context was the prime factor."

Mark Cane, a professor of Earth and climate sciences at Columbia University in New York, said there was "a strong case" to link discontent in Syria to the drought which in 2007-2010 was the worst ever recorded there.

But he pointed to a problem: ascribing a role for climate change, usually discernible over decades, to a single weather event.

Furthermore, "it is impossible to look at any single conflict and argue conclusively that it wouldn't have happened but for a drought or some other climate anomaly," Cane told AFP by email.

Governance and other factors also weigh in, he noted. What magnified the impact of Syria's drought, for instance, was gross waste of water and a surge in population, other experts have said.

- Risk factor -

Scientists are cautious about declaring a link between conflict and climate change until the evidence is overwhelming.

In the military, though, it's different. Armed forces have to respond swiftly and cannot wait until the proof is all there, which is why climate is now a risk factor in their planning.

In many countries, military analysts already include climate change in risk management, Neil Morisetti, a former British admiral and climate advisor to the British government, now director of strategy at University College London, told AFP.

"Some will say it (the risk) is here already," he said.

"If you look at where climate change is going to have its greatest effect, and is already having an effect, it's that belt north and south of the equator... this is where a lot of raw materials are, where the world's supply chains and trade routes run, and where ultimately a lot of the number of the markets and emerging powers are."

And a volatile world, said Morisetti, "poses a risk to political geo-stability."

Whether or not they agree that the effects are evident, the experts are united in their heralding of worse to come.

"Human security will be progressively threatened as the climate changes," the UN's Intergovernmental Panel on Climate Change (IPCCC) warned in its overview report.

The Pentagon agrees.

"Rising global temperatures, changing precipitation patterns, climbing sea levels, and more extreme weather events will intensify the challenges of global instability, hunger, poverty and conflict," it said in a 2014 Global Climate Change Adaptation Roadmap.