Katrina and the waves

[Tor Lor]

Katrina and the waves

August 29, 2006 Edition 1

Andrew Buncombe

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 the upper half of Florida.

This swirling mass is Alberto, the first tropical storm of the 2006 season, and it has already 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 in the centre of the room, Richard Pasch is on a conference call with colleagues across the region. It is clear that Alberto's power is falling and there is a discussion as to whether it should be downgraded from the status of tropical storm.

But they decide to maintain the warning for a few hours longer and issue the following statement: "There is a threat of flooding due to locally heavy rain over portions of the south-eastern United States."

After the 2005 hurricane season - the busiest on record - unprecedented attention is now given to warnings about tropical storms.

With more and 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.

[Tor Lor]

A year ago, Katrina made us think about hurricanes in a very different way. That was a category five storm that had weakened to a category three by the time it struck the Gulf coasts of Louisiana and Mississippi on the morning of August 29.

It led to the deaths of more than 1 800 people and caused damage estimated at around $80 billion.

New Orleans was brought to its knees, and it was not hard to find people voicing the opinion that something was out of kilter. This, after all, is a city that is used to hurricanes. In Johnny White's pub on Bourbon Street, drinkers would brag that the bar had never shut, not even when Hurricane Betsy tore into the city in 1965.

The question was soon being asked: could global warming be to blame? Among the science cited to back such a claim was a report in Nature magazine by Professor Kerry Emanuel, a climatologist from the Massachusetts Institute of Technology, which argued that the strength of hurricanes had increased in recent years and that this was linked to climate change.

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

[Tor Lor]

Emanuel had gathered 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.

He set these figures against data showing the average September sea-surface temperature for each of those years, and claimed to have discovered a remarkable link between the two.

Furthermore, he showed that in the past 30 years storms were lasting 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, and - taking into account an increasing coastal population - a substantial increase in hurricane-related losses in the 21st Century."

Emanuel was not the only person making such claims. In 2004 - which until last year was the busiest hurricane season on record - George Trenberth, of the National Centre for Atmospheric Research in Boulder, Colorado, argued: "Trends in human-influenced environment changes are now evident in hurricane regions."

[Tor Lor]

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 the past 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 past 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. It is a list of alternate male and female names running through the alphabet, missing out Q, U, X, Y and Z.

[Tor Lor]

Hurricanes that are especially destructive are "retired", never to be used again. The name Katrina was formally retired last April.

The season stretches from June 1 to November 30 and there are many more opportunities for chaos.

If, as predicted, there are 15 named storms this year, Oscar will be the last of the season.

There is no difference between a hurricane, typhoon or cyclone, other than where they occur.

In Heaven's Breath, a history of winds, Lyall Watson writes: "In the North Atlantic they are called hurricanes in memory of the Mayan storm-god Hunraken; in the Indian Ocean they have been known as cyclones from the Greek kuklos meaning circular, ever since the president of the Marine Courts at Calcutta first used this term in the middle of the 19th Century; in the China Sea they call them typhoon, from ty fung or "great wind"; in the Philippines it is baguoi; in Japan reppu; and asifat in the Persian Gulf."

They begin as areas of low pressure within a band of the tropical oceans known as the intertropical convergence zone or, more romantically, the doldrums.

Hot, damp air rises from the ocean, spirals, cools and then condenses. Rain falls, energy is released and the air is warmed, reinforcing the updraft.

[Tor Lor]

An area of low pressure develops and moisture-laden air rushes in at an angle determined by the rotation of the earth. The spiral spins faster and draws in air from the upper atmosphere. This air is warmed by compression and adds to the heat pool.

Soon the system has momentum and mass of its own, powered by heat from the sea and steered by winds and their own energy.

Around their core, winds grow with great velocity, generating violent seas. When they hit land they can sweep the ocean inwards with them while creating tornadoes and producing torrential rains and floods.

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 finally something called the accumulated cyclone energy index - a measure of the combined strength of all the storms of any season.

[Tor Lor]

The forecasters aim to be correct on at least three out of four of these measures.

Short-term forecasts are made once a storm is active and people need to know whether it is going to run through their town or pass them by.

In the North Atlantic, the area of tropical weather most studied, a tropical depression (which has no "eye") is a weather system with sustained winds of no more than 61km/h. A tropical storm is a system with maximum sustained winds of 62km/h to 117km/h.

Above this they become hurricanes, graded one to five on the Saffir-Simpson scale according to their strength and intensity; category five indicates sustained winds of more than 250km/h.

Researchers have found that a typical storm can release more than 10 million trillion joules of energy - the equivalent of around one million Hiroshima bombs - in a single day. Most of this energy is ultimately dissipated upwards.

[Tor Lor]

No one doubts that since the early '90s 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 the unanswered question.

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.

[Tor Lor]

The graph, including a line that follows the five-year average, shows an above average number of category three, four and five storms between the years 1948-1954; the number then gradually falls.

He says there is insufficient evidence to conclude that the increase in hurricane activity since the early 1990s is anything other than part of a natural cycle, and he strongly 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 1970s, 1980s and early 1990s.

But Landsea says it is now thought that the bias may in fact lie the other way, and that the strength of the storms of the 1970s, 1980s and early 1990s was underestimated.

"If you use the adjusted data there is a trend, if you use the original data it is just a cycle," said Landsea.

[Tor Lor]

Roger Piekle, of the University of Colorado's Centre for Science and Technology Research Policy, also questions Emanuel's conclusions.

Piekle has spent time investigating the damage caused by hurricanes, 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. Emanuel does not name names but asks why others looking at the same evidence reach different conclusions.

"The problem is that some of my colleagues have allowed their political and even their religious convictions to influence what they do."

Of Piekle's findings, Emanuel says his own way of showing the increase in the destructive power of storms involves measuring the entirety of the hurricane's lifespan - not simply its windspeed when it strikes land.

[Tor Lor]

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.

Yet, he says such smoothing did not affect his analysis of the western Pacific storms.

Furthermore, 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'll see a quiet decade like the '70s and '80s."

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