PARIS, France (AFP) – Weather events that occur in the distant Pacific and West Africa help determine the frequency of major hurricanes that strike the Caribbean and West Atlantic, according to research published Wednesday.

The study touches on one of the big puzzles about climate change: Will global warming, by heating the seas that give hurricanes their power, also make these storms more frequent?

Experts poring over this question face a problem in getting a long-term perspective so that they can compare the effects of recent man-made global warming with past, naturally-caused bouts of climate change.

Jeffrey Donnelly and Jonathan Woodruff of Woods Hole Oceanographic Institution in Massachusetts found a way around this problem by drilling sedimentary cores in a lagoon in Puerto Rico, one of the most hurricane-exposed spots on the Caribbean.

Whenever big hurricanes whacked the island of Vieques, their waves also churned the seabed, depositing a layer of coarse sand on the bottom of the lagoon.

Using these layers as telltales, Donnelly and Woodruff were able to construct a history of local hurricanes going back 5,000 years.

They found a remarkable link between these storms and records elsewhere for the frequency of El Nino – the build-up of warm water in the East Pacific that has a knock-on effect on the weather systems around the world – and the intensity of monsoons in West Africa.

The finding is important, the authors argue in their paper, which appears in the weekly British journal Nature.

Many climate specialists believe that warmer water in the West Atlantic, by pumping up atmospheric convection, will make hurricanes more powerful as well as more frequent. Some suspect this process may already be underway.

In April, the UN’s Intergovernmental Panel on Climate Change (IPCC), gathering the most authoritative opinion on global warming and its effects, declared it was “likely” that intense tropical storms, a category that includes typhoons as well as hurricanes, would increase in activity this century.

The new research, though, says heat alone is not the only factor in hurricanes, for the atmospheric dynamics caused by El Nino and the African jetstream also play a big role.

In the latter half of the so-called Little Ice Age – a mysterious period of sudden cooling which ran from the 16th to the mid-19th centuries – there was a flurry of hurricanes, noted Donnelly and Woodruff.

“To accurately predict changes in intense hurricane activity, it is therefore important to understand how the El Nino/Southern Oscillation and the West African monsoon will respond to future climate change,” they say.