The Big Question: Are there more hurricanes, and are they the result of global warming?

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The Independent Online

Why are we asking this now?

Because hurricanes like the one which has careered across the Caribbean and was last night striking Mexico 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.

So is it happening already?

Some scientists have put forward fairly dramatic evidence that it may be, and 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 the dramatic evidence?

It came in two peer-reviewed scientific papers published within a short time of each other in the summer of 2005. They kicked off the whole hurricane-global warming argument. In fact, they caused a sensation. The first, in the journal Nature, was by Kerry Emanuel of the Massachusetts Institute of Technology, one of the world's leading hurricane researchers. Dr Emmanuel devised a new way of measuring hurricane intensity which he called the power dissipation index, and he said he could detect an increase in this which 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 tracked the increase in sea surface temperature related to climate change over the last century, and taking the conventional measure of hurricane strength, the Saffir-Simpson scale, they said that the number of storms that were reaching the top categories of 4 and 5 had doubled in recent decades.

And these papers caused a sensation?

They sure did. A worldwide one.

Not least because they were published in 2005, in the middle of the worst season of Atlantic hurricanes on record, which culminated in the disaster of Hurricane Katrina which hammered New Orleans so terribly in August. The 2005 season included a record 26 named storms, of which 13 grew big enough to be classified as hurricanes (so many that for the first time since 1953, when scientists started give tropical Atlantic storms names, letters of the Greek alphabet had to be used, as meteorologists had run through the original list of 21 alphabetically-ordered names. The final 2005 tropical storm was christened Epsilon.) For the environmental community the two papers were yet another devastating indictment of the lack of action on climate change, especially by the US 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-Sixties and early-Seventies, there was no global satellite coverage and measurement of tropical cyclones (which is 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 well have not been observed at all.

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 politicised?

'Fraid so. For example, the Bush administration put forward 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 conclusion 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 if the climate continues to warm (though not in hurricane frequency). 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?

We can give you chapter and verse on that. Last November, the World Meteorological Organisation held an International Workshop on Tropical Cyclones in Costa Rica, and at its conclusion, it issued a one-page document entitled "Summary Statement on Tropical Cyclones and Climate Change". Its first paragraph states: "Though there is evidence both 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 this year, 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."

So is climate change to blame?

Yes...

* The historical database shows a definite increase in frequency and intensity (one view)

* Supercomputer climate models unanimously predict that climate change will make hurricanes worse

* Warmer oceans contain more energy for storms

No...

* The historical database cannot be trusted to prove an increase in frequency and intensity (the other view)

* Any increase may be part of a natural cycle

* Even in a warming world, various climatic mechanisms may act to reduce increases

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