Paris (AFP) May 6, 2008
In 2005, Hurricane Katrina laid waste to parts of the US Gulf Coast.
Last year, the Arabian peninsula was hit by a super-cyclone, Gonu.
Now, an unusual early-season storm, Nargis, has slammed into Myanmar, brutally changing gear from a Category One to a Category Four cyclone just before it made landfall.
Are these events -- massively costly in lives and treasure -- all linked?
Could they be part of an alarming trend of weird, more powerful storms stoked by global warming?
That's a question that causes fierce jousting among climate scientists.
Experts agree that a single weather event cannot be pinned to climate change, which is part of a long-term pattern spanning decades or centuries.
"It's impossible to say," Adam Lea of the Benfield UCL Hazard Research Centre at University College London told AFP.
"It's only in the long term that you get the perspective that lets you say whether an extreme event is part of a wider trend," said French researcher Herve Le Treut, who contributed to last year's landmark report by the UN's Intergovernmental Panel on Climate Change (IPCC).
But that's where the scientific consensus ends.
Some experts argue the evidence is already hard enough to identify a probable trend: storms are becoming more powerful as global warming heats up the oceans.
One of the most respected voices in the field is that of Massachusetts Institute of Technology (MIT) meteorology professor Kerry Emanuel, who calculates that the power of tropical cyclones has roughly doubled since the 1950s.
The massive increase has especially occurred over the last three decades, mirroring a rise in man-made global warming, he notes. And the trend stepped up a couple of gears from the mid-1990s, when global mean temperatures began to scale ever-higher annual peaks.
Others, though, say these judgements are premature.
They argue that we still need long-term historical data -- in which big weather oscillations and cycles in hurricane activity are filtered out -- in order to get a clear picture.
Far more is known about storm activity in the Atlantic, for instance, than in the Indian Ocean, and the Atlantic records themselves go back only 30 years or so, to the advent of satellite monitoring, Le Treut noted.
A study published last year by Johan Nyberg of Sweden's Geological Survey used Caribbean corals, whose growth is affected by temperature and nutrients stirred up by storms, to get a view spanning two and a half centuries.
Nyberg concluded that 1971-94 was abnormally calm for hurricane activity and that the big increase in storm numbers since 1995 was "not unusual" when compared to the longer record.
Tropical storms are called hurricanes when they occur in the Atlantic, typhoons when they happen in the Pacific and cyclones when they brew in the Indian Ocean.
The basic cause is the same -- heat and moisture provided by seas warmed to at least 26 or 27 degrees Celsius (78.8-80.6 degrees Fahrenheit).
But another factor is vertical wind shear, or the angle of prevailing winds.
This determine whether the nascent storm develops into the notorious wheeling "eye" of a cyclone or is torn into harmless shreds.
It's still unclear what impact global warming will have on vertical wind shear, say some experts. A theoretical combination of lower wind shear and warmer seas could result in storms that last longer, are more vicious and more frequent, too.
The IPCC's 2007 report said tropical cyclones were "likely" to become more intense, packing higher winds and rain, by 2100. But it also highlighted the fact that human settlement in vulnerable areas increased the toll from when the storms strike.
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