The link between hurricanes and global warming

This year's hurricane season shattered records for frequency and ferocity. And it's going to get worse, some scientists say. But others aren't so sure. Steve Connor reports

It was the year of the hurricane. It was also the year when scientists said that global warming can increase the intensity of hurricanes. But it was not the year when everyone could agree that hurricane Katrina, which devastated New Orleans in August, was at least in part the result of global warming brought on by emissions of man-made greenhouse gases.

The 2005 hurricane season is now officially rated as the busiest on record by the US National Oceanic and Atmospheric Administration (NOAA). The 2005 season included 26 named storms, of which 13 grew big enough to be classified as hurricanes. Of these, seven turned out to be major hurricanes of category three or higher on the five-point Saffir-Simpson scale. This was five more major hurricanes than average for a typical Atlantic season. Hurricane Wilma in October became the strongest Atlantic storm ever recorded, breaking the previous record set in 1988.

It wasn't just strength, it was frequency. For the first time since 1953, when scientists started to name tropical Atlantic storms, letters of the Greek alphabet had to be used because meteorologists had exhausted the original list of 21 alphabetically ordered names. The final tropical storm, Epsilon, formed over the central Atlantic Ocean on the penultimate day of the hurricane season.

"This hurricane season shattered records that have stood for decades - most named storms, most hurricanes and most category-five storms," says NOAA administrator Conrad Lautenbacher. "Arguably, it was the most devastating hurricane season the country has experienced in modern times. I'd like to foretell that next year will be calmer, but I can't. Historical trends say the atmosphere patterns and water temperatures are likely to force another active season upon us."

The real record-breaker was Katrina, the most destructive hurricane to strike the United States in living memory. More than 1,000 people died and some estimates put the cost of repairing New Orleans and its flood defences as high as $200bn (£117bn).

Some environmentalists were quick to point the finger of guilt at global warming, but not all scientists were convinced - not until some of them began to look at the data more carefully, that was. Until recently there was a fairly broad consensus among meteorologists that global warming could not explain the general upsurge in hurricanes recorded over the past couple of decades. People thought this current increase was just part of the normal long-term cycle driven by changes to the Atlantic circulation system. Few believed that it was directly related to an increase in sea-surface temperatures caused by climate change.

But then came a study by Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge who, in a letter published in the journal Nature, described how he had formulated a new measure of hurricane destructiveness. He compared changes in this measure of destructiveness, called "total dissipation of power", with the rise in sea-surface temperatures recorded in the Atlantic over the past 30 years.

Emanuel found that he could link an increase in hurricane activity with rises in ocean temperature caused by global warming and climate change. "My results suggest that future warming 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," he says.

The study concluded that tropical storms are lasting 60 per cent longer and their wind speeds are up to 15 per cent higher. "This work implies that global tropical cyclone activity is responding in a rather large way to global warming," Emanuel says.

It is not unreasonable to pose a link between hurricanes and global warming given that sea-surface temperatures play a critical role in their formation. A hurricane is a tropical cyclone, an area of intense low pressure in the tropics surrounded by a violent, rotating storm. In the Atlantic, such phenomena are called hurricanes, in the eastern Pacific they are known as typhoons, and in the Indian Ocean they are known as cyclones.

"On the face of it, global warming can only make things worse," says science author Fred Pearce in the latest issue of New Scientist. "The initial pillar of humid air generally forms only when the temperature of the sea surface exceeds 26C. As the oceans warm, larger areas will exceed the threshold. And every degree above the threshold seems to encourage stronger hurricanes," he says.

When Katrina formed, the temperature of the Gulf of Mexico was about 30C, which encouraged the view that in a warmer world we can expect more violent hurricanes. Real life is more complex, however. A warmer world could also lead to a warmer atmosphere and this could diminish the critical difference in the temperatures between air and ocean that helps hurricanes to form.

Another way global warming could kill hurricanes is by generating stronger winds, which could stir up the sea, thereby cooling surface temperatures. Stronger winds could also disrupt the tall pillar of humid air needed for hurricane formation. "With all these uncertainties and contradictions, it is not surprising that different computer models have predicted everything from fewer hurricanes to more hurricanes as global warming kicks in," Pearce says.

More fuel was added to the fire of the debate in September when meteorologist Peter Webster of the Georgia Institute of Technology in Atlanta and his colleagues published data showing that the frequency of category four and five cyclones - the two strongest types of hurricanes - had increased significantly over the past 35 years. Globally, category four and five storms climbed 57 per cent from the first half of the period to the second, they found. With the characteristic understatement of scientists treading in a controversial area, they conclude: "This trend is not inconsistent with recent climate model simulations that a doubling of carbon dioxide may increase the frequency of the most intense cyclones, although attribution of the 30-year trend to global warming would require a longer global data record and, especially, a deeper understanding of the role of hurricanes in the general circulation of the atmosphere and ocean, even in the present climate state."

In other words, the evidence is mounting that global warming can indeed affect the formation of hurricanes but as yet no one can say for sure whether this is happening now. And equally no one can say for sure that Katrina was made worse by greenhouse emissions. Previous hurricanes have after all killed far more people - the East Pakistan hurricane of 1970 probably killed half a million.

Most climate scientists believe that it will take far longer to assess whether global warming is having a real impact on the frequency or intensity of hurricanes. Emanuel, for example, estimates that it could take a further 50 years to detect an unambiguous trend in the intensity of hurricanes hitting the US. But whatever the long-term trend, it is clear that more hurricanes than usual are expected next year. In the words of one NOAA scientist: "We are in an active hurricane era."

How hurricanes form

All types of hurricanes start out as a cluster of thunderstorms which form as columns of warm, moist air rise above the surface of a tropical ocean where the temperature of the sea surface is above a critical threshold of 26C. If the thunderstorms become concentrated they can form a tall pillar of humid air extending from the sea surface to the high-altitude boundary of the stratosphere.

The low pressure at the base of the pillar of air sucks in more humid air. As the air rises, it cools and condenses, releasing heat that drives the engine of the tropical storm. Wind speeds pick up, and the entire weather system begins to rotate in a violent vortex centred around the eye of the storm.

Arts and Entertainment
The first film introduced Daniel Radcliffe to our screens, pictured here as he prepares to board the train to Hogwarts for the first time.
booksHow reading Harry Potter helps children grow up to be gay-friendly
Sport
Frank Lampard will pass Billy Wright and equal Bobby Charton’s caps tally of 106 caps against
sportFormer Chelsea midfielder in Etihad stopgap before New York contract
Arts and Entertainment
A scene from Aladdin is performed at the Tony Awards in New York in June
theatreBrit producer Lythgoe makes kids' musical comedy a Los Angeles hit
Sport
Usain Bolt of Jamaica smiles and shakes hands with a competitor after Jamaica won their first heat in the men's 4x100m relay
sport
PROMOTED VIDEO
News
ebookA unique anthology of reporting and analysis of a crucial period of history
News
Chancellor George Osborne, along with the Prime Minister, have been 'complacently claiming the economy is now fixed', according to shadow Chancellor Ed Balls
i100... which is awkward, because he is their boss, after all
News
i100
Arts and Entertainment
Adèle Exarchopoulos and Léa Seydoux play teeneage lovers in the French erotic drama 'Blue Is The Warmest Colour' - The survey found four times as many women admitting to same-sex experiences than 20 years ago
filmBlue Is The Warmest Colour, Bojack Horseman and Hobbit on the way
News
Kenny Ireland, pictured in 2010.
peopleBenidorm actor was just 68
Arts and Entertainment
Preparations begin for Edinburgh Festival 2014
Edinburgh festivalAll the best shows to see at Edinburgh this year
Independent
Travel Shop
the manor
Up to 70% off luxury travel
on city breaks Find out more
santorini
Up to 70% off luxury travel
on chic beach resorts Find out more
sardina foodie
Up to 70% off luxury travel
on country retreats Find out more
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

(Senior) IT Support Engineer - 1st-3rd Line Support

£40000 per annum: Ashdown Group: A successful IT service provider that has bee...

Wind Farm Civil Design Engineer

£55000 - £65000 Per Annum: The Green Recruitment Company: The Green Recruitmen...

Principal Marine Mechanical Engineer

£60000 - £70000 Per Annum: The Green Recruitment Company: The Green Recruitmen...

Principle Geotechnical Engineer

£55000 - £65000 Per Annum: The Green Recruitment Company: The Green Recruitmen...

Day In a Page

Dress the Gaza situation up all you like, but the truth hurts

Robert Fisk on Gaza conflict

Dress the situation up all you like, but the truth hurts
Save the tiger: Tiger, tiger burning less brightly as numbers plummet

Tiger, tiger burning less brightly

When William Blake wrote his famous poem there were probably more than 100,000 tigers in the wild. These days they probably number around 3,200
5 News's Andy Bell retraces his grandfather's steps on the First World War battlefields

In grandfather's footsteps

5 News's political editor Andy Bell only knows his grandfather from the compelling diary he kept during WWI. But when he returned to the killing fields where Edwin Vaughan suffered so much, his ancestor came to life
Lifestyle guru Martha Stewart reveals she has flying robot ... to take photos of her farm

Martha Stewart has flying robot

The lifestyle guru used the drone to get a bird's eye view her 153-acre farm in Bedford, New York
Former Labour minister Meg Hillier has demanded 'pootling lanes' for women cyclists

Do women cyclists need 'pootling lanes'?

Simon Usborne (who's more of a hurtler) explains why winning the space race is key to happy riding
A tale of two presidents: George W Bush downs his paintbrush to pen father’s life story

A tale of two presidents

George W Bush downs his paintbrush to pen father’s life story
Restaurateur Mitch Tonks has given the Great Western Pullman dining car a makeover

The dining car makes a comeback

Restaurateur Mitch Tonks has given the Great Western Pullman dining car a makeover
Gallery rage: How are institutions tackling the discomfort of overcrowding this summer?

Gallery rage

How are institutions tackling the discomfort of overcrowding this summer?
Louis van Gaal has £500,000 video surveillance system installed to monitor Manchester United players

Eye on the prize

Louis van Gaal has £500,000 video surveillance system installed to monitor Manchester United players
Women's rugby: Tamara Taylor adds fuel to the ire in quest to land World Cup

Women's rugby

Tamara Taylor adds fuel to the ire in quest to land World Cup
Save the tiger: The day America’s love of backyard tigers led to a horrific bloodbath

The day America’s love of backyard tigers led to a horrific bloodbath

With only six per cent of the US population of these amazing big cats held in zoos, the Zanesville incident in 2011 was inevitable
Samuel Beckett's biographer reveals secrets of the writer's time as a French Resistance spy

How Samuel Beckett became a French Resistance spy

As this year's Samuel Beckett festival opens in Enniskillen, James Knowlson, recalls how the Irish writer risked his life for liberty and narrowly escaped capture by the Gestapo
We will remember them: relatives still honour those who fought in the Great War

We will remember them

Relatives still honour those who fought in the Great War
Star Wars Episode VII is being shot on film - and now Kodak is launching a last-ditch bid to keep celluloid alive

Kodak's last-ditch bid to keep celluloid alive

Director J J Abrams and a few digital refuseniks shoot movies on film. Simon Usborne wonders what the fuss is about
Once stilted and melodramatic, Hollywood is giving acting in video games a makeover

Acting in video games gets a makeover

David Crookes meets two of the genre's most popular voices