PARIS (AFP) - Climate shift has at times been fingered as a culprit in triggering conflict, fuelling for instance the 1789 French Revolution by wrecking harvests and driving hungry peasants to the city.

Evidence to back the theory has often been contested as sketchy or anecdotal -- but the case has been boosted by the first scientific study to declare an unmistakeable link between climate fluctuations and violence.

It says tropical countries affected by the notorious El Nino weather event are twice as likely to be hit by internal unrest compared to the phenomenon's cooler, wetter counterpart, La Nina.

The civil war and famine gripping the Horn of Africa is a typical example of what happens when a climate swing causes drought and overstresses an already fragile society, say its authors.

The inquiry, appearing in Thursday's issue of the journal Nature, focusses on a naturally occurring pattern of climate change.

But its authors say there is a disturbing lesson about violence driven by man-made warming, which is expected to bite deep in coming decades.

"What it does show and show beyond any doubt is that even in this modern world, climate variations have an impact on the propensity of people to fight,"

said Mark Cane, a climate scientist at the Lamont-Doherty Earth Observatory at Columbia University in New York.

"It's difficult to see why that won't carry over to a world that's disrupted by global warming."

Formally known as the El Nino-Southern Oscillation (ENSO), the cycle occurs every two to seven years and last from nine months to two years, often inflicting massive losses on agriculture, forestry and fishing.

It starts when warm water builds on the western side of the tropical Pacific and shifts across the ocean.

This part of the cycle, El Nino, can cause dramatic changes in rainfall patterns and temperatures, unleashing scorching heat or drying winds in much of Africa, South and Southeast Asia and Australia.

When the cycle goes into reverse, a phase called La Nina, the water in the eastern Pacific cools, often bringing heavy rain to those regions.

The study looked at ENSOs from 1950 to 2004 and overlaid this data with civil conflicts -- violence that had taken place within national borders, as opposed to cross-border wars -- that had killed more than 25 people in a given year.

The data included 175 countries and 234 conflicts, more than half of which caused more than 1,000 battle-related deaths.

In countries whose weather cycles are determined by ENSO, the risk of civil conflict occurring during La Nina was about three percent; during El Nino, this doubled to six percent, the paper says.

Countries not affected by ENSO remained at two percent regardless.

Overall, according to the study, El Nino may have played a role in 21 percent of civil wars worldwide, and nearly 30 percent in those countries that are specifically affected by El Nino.

Lead author Solomon Hsiang of Columbia's Earth Institute said El Nino was an invisible factor -- but not the only one -- in driving intra-border conflict.

By causing crop losses, hurricane damage or helping to spread epidemics of water-borne disease, it amplified hunger, loss, unemployment and inequality, which in turn fuelled resentment and division.

Other factors that could affect risk and the outcome are the country's population growth and prosperity and whether its government is able to manage El Nino events properly.

"Even though we control for all of these factors simultaneously, we still find that there's a large and pervasive El Nino effect on civil conflicts," Hsiang said in a teleconference.

Although the current crisis in the Horn of Africa occurred beyond the parameters of the study, it was a "perfect example" of the hidden destruction of an El Nino.

"Forecasters two years ago predicted that there would be a famine in Somalia this year, but donors in the international aid community did not take that forecast seriously," said Hsiang.

"We hope our study can provide the international community and governments and aid organisations with additional information that might in the future help avert humanitarian crises that are associated with conflict."