KEY POINTS:
Are hurricanes now more frequent and are they the result of global warming?
Hurricanes like Dean, the one which has careered across the Caribbean to Mexico's Yucatan peninsula, are only formed when the surface temperature of the ocean exceeds a specific point, which is 26C.
As the oceans warm globally with climate change, much larger areas of water will exceed the threshold and more energy will be available to power a given storm. On the face of it, therefore, the connection might seem a reasonable, even a natural, one.
Is it happening already?
Some scientists have put forward fairly dramatic evidence that it may be. This has been seized on by the environmental community as another piece of the global warming jigsaw, to impress on governments the need to act to cut back on the carbon emissions causing the climate to heat up. But other scientists resolutely dispute the proposition and say it cannot be proved.
What is this "dramatic" evidence?
It came in two peer reviewed scientific papers published within a short time of each other in the summer of 2005 that effectively began the hurricane-global warming argument. The first, in the journal Nature, was by Kerry Emanuel, of the Massachusetts Institute of Technology, one of the world's leading hurricane researchers. Emanuel devised a new way of measuring hurricane intensity which he called the power dissipation index.
A detectable increase in this index, he said, could be related to increases in sea surface temperatures over recent decades.
The second paper was by Greg Holland, of the National Centre for Atmospheric Research in Boulder, Colorado, and Peter Webster, of the Georgia Institute of Technology in Atlanta, (published in Philosophical Transactions of the Royal Society).
Holland and Webster said they had discovered a rise in the number of Atlantic hurricanes that paralleled the increase in sea surface temperature related to climate change over the last century. Taking the conventional measure of hurricane strength, the Saffir-Simpson scale, they said the number of storms reaching the top categories of 4 and 5 had doubled in recent decades.
These papers caused a sensation, not least because they were published in the middle of the worst season of Atlantic hurricanes on record.
For the environmental community, the two papers were yet another devastating indictment of the lack of action on climate change, especially by the United States Government of George W. Bush.
So is the connection proved?
Not at all. It is hotly disputed. The difficulty lies in how we use and interpret the database of records of previous storms. Before the late '60s and early '70s, there was no global satellite coverage or measurement of tropical cyclones (the generic term for circular tropical storms - they're hurricanes in the Atlantic, typhoons in the west Pacific and cyclones in the Indian Ocean).
So the strength of some early recorded storms may have been misinterpreted. They may actually have been much stronger than we think and, thus, a general increase in intensity may be an illusion. Some storms may have gone unrecorded.
Furthermore, an increase may be part of a natural cycle, rather than being caused by human activities.
The leading proponent of the no-link theory, Christopher Landsea, a senior American hurricane researcher and forecaster based at the National Hurricane Centre in Miami, has published research contending that the historical hurricane database simply cannot support the claims made by Emanuel, and by Holland and Webster, in their respective papers.
Has the argument become political?
Yes. The Bush administration proffered Landsea to assert that there was no connection between Hurricane Katrina and climate change and he is often attacked by environmentalists.
But he is a serious and respected scientist and he is by no means alone in his concern that the record does not show an increase in hurricane power and strength. One of Britain's leading experts on tropical cyclones, Julian Heming, of the UK Met Office, says: "I am of the view that this issue of the historical database is a significant one and I think we need to be cautious about deriving too many definitive conclusions from the historical records."
Is there no consensus?
Well, there is much more of a consensus between scientists about what is likely to happen in future, than about what has happened in the past, or what is happening now. The supercomputer models used for climate change prediction tend to show an increase in future hurricane wind speed and rainfall (though not in frequency) if the climate continues to warm.
This is not generally disputed. However, it is a smaller increase than that which the two papers from 2005 claim to have detected already.
Where is the argument now?
Last November, the World Meteorological Organisation held an international workshop on tropical cyclones in Costa Rica. It ended by issuing a one-page document Summary Statement on Tropical Cyclones and Climate Change. Its first paragraph states: "Though there is evidence for and against the existence of a detectable anthropogenic signal [signs of a human cause such as man-made global warming] in the tropical cyclone climate record to date, no firm conclusion can be made on this point."
So the jury's out?
Not quite. The fourth assessment report of the UN's Intergovernmental Panel on Climate Change, published in February, gives a table showing recent climatic trends. It suggests that intense tropical cyclone activity has probably increased in some regions since 1970 and, under the heading "Likelihood of a human contribution to observed trend", it observes succinctly: "More likely than not."
- INDEPENDENT
