Eyes on the hurricane debate

UNITED STATES - It is late afternoon inside the low-ceilinged room on the edge of Miami and a bank of computer monitors is showing a mass of throbbing colours - green and blue and yellow - steadily marching north-east across Florida.

This swirling mass is Alberto, the first tropical storm of this year's season, and it has moved across the western Caribbean and the Gulf of Mexico, deluging Cuba and Grand Cayman and blasting them with winds of 112km/h, tracked by the experts from the National Weather Service's Tropical Prediction Centre.

Hunched over a telephone, Richard Pasch is on a conference call with colleagues. It is clear that Alberto's power is falling and they discuss whether it should be downgraded from the status of tropical storm. They decide to maintain the warning for a few hours longer.

After last year's hurricane season, the busiest on record, unprecedented attention is now given to warnings about tropical storms. With more people moving to coastal communities, never have so many lives, and so much money, been at stake.

And yet never before has the science of tropical weather prediction been riddled by such disagreement. The debate is part of a broader discussion about the extent and implications of climate change and whether storms are getting stronger as a result of man-made global warming. Some say there is no convincing evidence, others that the evidence is obvious.

A year ago, Katrina made people question if global warming could be to blame.

Among the science cited to back such a claim was a report in Nature magazine by Professor Kerry Emanuel, a Massachusetts climatologist, which argued that the strength of hurricanes had increased in recent years and that this was linked to climate change.

Although sea temperatures had increased only by around half a degree over 30 years, the destructive power of hurricanes had doubled in that period, he said.

Emanuel had statistics dating back to 1930 relating to the power of hurricanes and - having made adjustments to counter what was widely considered an inaccuracy in some earlier measurements - worked out a figure to measure their annual destructive power, which he called the Power Dissipation Index (PDI). He set these figures against data showing the average September sea-surface temperature for each of those years, and claimed a remarkable link between the two.

He also showed that storms lasted longer and were more intense. He wrote: "My results suggest that future warming [of the oceans] may lead to an upward trend in tropical cyclone destructive potential."

In 2004 George Trenberth, of the National Centre for Atmospheric Research in Colorado, also argued that "trends in human-influenced environment changes are now evident in hurricane regions".

A third piece of evidence came from Peter Webster and Greg Holland, who said their research suggested the number of category-four and category-five hurricanes worldwide had nearly doubled over 35 years. "Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity," said Webster.

The British Government's chief scientist, Sir David King, also entered the debate. "We have known since 1987 that the intensity of hurricanes is related to surface sea temperature and we know that, over the last 15 to 20 years, surface sea temperatures in these regions have increased by half-a-degree Centigrade," he said. "So it is easy to conclude that the increased intensity of hurricanes is associated with global warming."

In the prediction centre in Miami, the names of future storms are spelt out on a large map of the Atlantic using magnetic letters. The list of alternate male and female names runs through the alphabet, missing out Q, U, X, Y and Z. Hurricanes that are especially destructive are "retired". The season stretches from June 1 to November 30 with August to October the busiest time. If, as predicted, there are 15 named storms this year, Oscar will be the last of the season.

Experts say they are getting better at predicting hurricanes. Twice-yearly seasonal forecasts predict the number of named storms, the number of hurricanes, the number of major hurricanes and something called the accumulated cyclone energy index - a measure of the combined strength of all the storms of any season. Short-term forecasts are made once a storm is active and people need to know whether it is going to run through their town.

No one doubts that since the early 1990s storms have increased in their intensity and no one doubts that average sea temperatures have increased slightly over the past 30 years. Whether there is a link between these two phenomena remains unanswered. In October 2004, when Trenberth first claimed a connection, a fellow scientist, Chris Landsea, with whom he was collaborating on a chapter for the report of the Intergovernmental Panel on Climate Change, resigned in outrage.

Landsea is a climatologist at the Tropical Prediction Centre, administered by the US National Oceanic and Atmospheric Administration. As his colleagues plot the course of Alberto, Landsea hands me a graph that plots the major Atlantic hurricanes from 1944-2005, spikes marking the years of the most powerful storms.

The graph shows an above average number of category-three, four and five storms between 1948-1954. The number then gradually falls. He says there is insufficient evidence to conclude that the increase in hurricane activity since the early 90s is anything other than part of a natural cycle, and he questions Emanuel's methodology.

He understands why Emanuel made those adjustments - because of research Landsea carried out, the scientific community believed that hurricane measurements from the 1930s overstated the strength of storms compared to those of the 70s, 80s and early 90s.

But Landsea says it is now thought that the bias may lie the other way, and that the strength of the storms of the 70s, 80s and early 90s was underestimated.

Roger Piekle, of the University of Colorado's Centre for Science and Technology Research Policy, also questions Emanuel's conclusions. Piekle has investigated hurricane damage, arguing that if hurricanes have increased in intensity and potential destructive power, one should be able to quantify that damage. Yet, he argues, no such conclusion can be drawn.

Each side accuses the other of a narrow-minded fundamentalism. In response to Landsea's criticisms, Emanuel accepts that his application of "smoothing" some of the data may have exaggerated the intensity of recent storms in the Atlantic. But he argues that there remains a high correlation between storm intensity and sea temperatures. In an interview, he warned: "We probably won't see a quiet decade again in the Atlantic. We may see some quiet years - this year may be quiet - but I don't think we will see a quiet decade like the 70s and 80."

- INDEPENDENT