The downside is this, though: What was thought to an alarming blip in the number of hurricanes since 1995 could well turn out to be a return to the norm.

Over the past 12 years, there has been an annual average of 4.1 major hurricanes -- storms rating at least three on a five-point severity scale -- compared with only 1.5 such storms per year between 1971 and 1994.

This sudden rise has caused many experts to ponder if seas, progressively warmed by the greenhouse effect, are to blame.

But hurricanes are known to occur in cycles. The task for climatologists is to get a long-term view, enabling them to filter out man-made interference with the climate system from natural factors.

The problem, though, is that the reliable observational record for these storms dates back only to 1944, so scientists have to look to other sources to reconstruct the past.

Johan Nyberg of Sweden's Geological Survey believes he has found such a source -- in Caribbean corals (whose growth is affected by temperature and nutrients stirred up by hurricanes) and in cores of shore sediment, whose layers are built up by wind-whipped storm waves.

Nyberg's team has used these indicators, called proxies, to build up a picture of hurricane frequency since 1730.

"The phase of enhanced hurricane activity since 1995 is not unusual compared to other periods of high hurricane activity in the record," they report on Thursday in Nature, the weekly British science journal.

"(It) appears to represent a recovery to normal hurricane activity, rather than a direct response to increasing sea-surface temperature."

In fact, the 1971-94 period was abnormally calm and this, ironically, coincides with a buildup in global warming. During this period, the atmospheric temperature, stoked by greenhouse gases, warmed faster than the sea and that differential may have discouraged hurricanes from forming.

Most hurricanes are born in the so-called Main Development Region (MDR), a zone of the Atlantic between the Caribbean and West Africa, between 10 and 20 degrees latitude north.

The raw fuel for a hurricane is a warm sea. A temperature of at least 27 degrees Celsius (80 degrees Fahrenheit) is the threshold at which Atlantic cyclones start to form.

But the Nyberg study sheds light on another factor, called wind shear. When a hurricane starts to form, its nascent, circular action can be torn apart by winds that blow into the MDR at different altitudes.

What the paper found was the vertical wind shear may be even more important than ocean temperature in dictating whether the hurricanes get going -- in other words, warmer water alone is not enough.

Still unclear, though, is what impact global warming will have on vertical wind shear. A theoretical combination of lower wind shear and higher sea-surface temperatures in the MDR could result in storms that last longer, are more vicious and more frequent, too, warns Nyberg.

It is the latest investigation into cyclones (a term that also applies to typhoons in the Pacific and storms in the Indian Ocean) and climate change.

Previous work in this field in recent weeks suggests that hurricane activity is linked to periods of fewer El Nino events, and that cyclones may be a powerful contributor to the circulation of warm water around the oceans.

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