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GLOBAL WARMING & ATLANTIC HURRICANES
Backgrounder from WWF International, September 1999
1.
INTRODUCTION - GLOBAL WARMING AND THE OCEANS
In March 1999, the American Geophysical Union concluded that 1998 was the warmest year of this
millennium. Ten of the hottest years on record have occurred during the last 15 years. This is
happening at the same time as emissions of global warming pollution such as carbon dioxide from
burning coal, oil and gas have reached record levels. According to scientists reporting in the journal
Nature in June 1999, levels of global warming gases in the atmosphere are higher now than at any
time in the past 420,000 years. They called their findings, “probably the most convincing evidence to
date that humans are making some really large changes to Earth's climate system.”
Warming of the oceans provides some of the strongest evidence that global warming is beginning to
have serious impacts on nature:
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In March, the US State Department said coral reefs around the world had “suffered the most
extensive and severe bleaching and subsequent mortality in modern record” during 1998. Coinciding
with the highest recorded tropical sea surface temperatures, this was “likely a consequence of a
steadily rising baseline of marine temperatures, driven by anthropogenic global warming.”
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The disastrous collapse in salmon stocks on Alaska’s Yukon River in 1998 showed the lowest
number of fish returning in the State’s history. This coincided with water temperatures 5-6 degree
Celsius (9-11 degree F) above normal in 1997, remaining 2 degree C (3.6 degree F) above normal
through 1998.
This is only the tip of the iceberg, as WWF revealed in its first comprehensive overview of the topic,
“Turning up the Heat: How Global Warming Threatens Life in the Sea”, published in May 1999.
More seriously, global warming appears to be affecting ocean conditions themselves. One of the
consequences is likely to be the occurrence of more severe hurricanes.
The following section reviews the disturbing links between global warming and Atlantic hurricanes. It
points to global warming acting as the priming mechanism for stronger hurricanes in the future. The
destructive power of hurricanes destroys lives and property and forces large-scale evacuations of
coastal regions, causing immense economic devastation. What is often overlooked is the enormous
damage to important shoreline and marine habitats and fishing resources that re-establish themselves
only slowly, if at all.
The threat from hurricanes will always exist. But if governments and corporations take more decisive
action to reduce their carbon dioxide emissions they can minimise the risk of tomorrow’s hurricanes
becoming the super storms of the future.
2.
THE LINKS BETWEEN WARMING OCEANS AND ATLANTIC HURRICANES
Hurricane Mitch and 1998
Hurricane Mitch, which ripped through Central America in the final days of October 1998, was the
most destructive storm in the western hemisphere in 200 years. Its torrential rains brought havoc to
Honduras and northwest Nicaragua in particular, killing some 10 000 people in landslides and floods.
Its nearest historical rival was the hurricane that hit Galveston in Texas in the year 1900, which also
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killed around 10 000 and whose carnage was recently chronicled in Erik Larson's book “Isaac's
Storm”.
Disastrous hurricanes do not happen in climatic isolation. They normally occur when the summer's
climatic and oceanographic circumstances are right. So they tend to happen in clusters during years
primed for hurricanes. There is growing evidence that global warming may be starting to predispose
the Atlantic to hurricane formation.
Take last year. Mitch caught the headlines, but before that there had already been 14 tropical storms
strong enough for meteorologists to give them names. Of these, ten became full-blown hurricanes.
In what the US National Hurricane Center (NHC) in Miami called a "hyperactive" period of just over
a month, from mid-August to late September, no fewer than ten tropical storms formed in the
Caribbean. On 25 September 1998, for the first time in a hundred years of observations, there were
four Atlantic hurricanes in progress at one time.
A record era for hurricanes, thanks to La Niña
The late 1990s have been a period of unprecedented hurricane activity, according to the NHC. There
were 33 hurricanes between 1995 and 1998, an all-time record. Strong hurricane activity in the
Atlantic is generally associated with the La Niña phase of the El Niño weather cycle in the Pacific
Ocean, says Gerald Bell, research meteorologist for NOAA's Climate Prediction Center. El Niño is
responsible for droughts in southeast Asia and Australia, as well as storms and floods on the west
coast of the Americas. In the Pacific, its opposite, La Niña, often comes as a relief. But the global
reach of this phenomenon means that it impacts more violently on events in the Atlantic.
First it stimulates the formation of unusually warm waters in the tropical Atlantic, raising
temperatures above the 27 degrees C (81 degree F) "threshold" when hurricanes can form. Second it
produces more stable air in the upper atmosphere off West Africa, where the storms first form.
Normally wind "shear" in the upper air breaks up storm clouds as they form. But during La Niñas
the wind shear is low, conditions are more stable and storm clouds can reach their full height.
Forecasters are now confident enough about these links that they provide advanced warning of strong
hurricane seasons. In May this year Gerald Bell warned: "This year many of the most prominent
atmospheric and oceanic factors that can generate tropical storms and hurricane activity are already in
place. The expected continuation of these conditions is based on their strong link to La Niña [which
is] expected to persist through the remainder of the hurricane season." Even before Floyd, the
Atlantic had lived up to expectations. Four tropical storms formed during August: Bret, Cindy,
Dennis and Emily.
La Niña may not be the only factor involved in hurricanes. There are also suspected links between
hurricane formation and the strength of ocean currents in the North Atlantic, especially the warm Gulf
Stream, which since the mid-1990s has been at its strongest for 30 years. A strong Gulf Stream
brings more hurricanes. Some scientists link the Gulf Stream and hurricanes to another weather
phenomenon known as the North Atlantic Oscillation (often called the Atlantic version of El Niño).
Still others say the North Atlantic Oscillation is itself just an offshoot of El Niño, La Niña and the
events of the Pacific.
But, amid the confusion, there is a growing belief that all these phenomena are linked to global
warming, because they are all driven by warmer ocean waters. And the one certainty about global
warming is that it will warm the oceans, which cover two-thirds of the planet.
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El Niño, La Niña and global warming: finding the link
El Niño is one of the most talked about weather phenomenon of the decade. Its rising destructive
power has coincided closely with growing evidence that the world is warming. And the link is not a
chance event.
Though often thought of as distinct, El Niño and La Niña are part of a constant cycling of events in
the Pacific Ocean of fundamental importance to the world's climate system. For climatologists the El
Niño/La Niña system is the flywheel of the climate system, an occasional redistributor of heat and
energy that kicks in when the regular circulation systems cannot cope. Its unparalleled activity in
recent years suggests that the regular system is under severe strain.
It works like this. In normal mode, driven by the Earth's rotation, winds and waters flow across the
tropical Pacific from the Americas in the east to Indonesia in the west. In the tropical heat, the water
warms as it goes. The result is the gradual accumulation of a pool of warm water around Indonesia.
This cannot last and, typically every three to seven years, this warm water breaks out and flows back
across the surface of the ocean. As it moves, so the wind and air pressure systems associated with it
move too, and with them the weather. So the wet rainforest climate of Indonesia rages through
normally arid Pacific islands, often reaching the deserts of the coastal Americas. Meanwhile,
Indonesia and much of Australia dry out. The ripples from this huge wave of water are felt round
the world as sea temperatures change, weather systems move, bring droughts and storms to Africa and
Europe and, it seems, storms to the Caribbean and the Atlantic seaboard of the US.
El Niño and La Niña have both been operating with greater force since about 1976. The question is:
why? Many oceanographers point out that El Niño has always had decades when it is unusually
quite or busy or plain weird. And that has had a knock-on effect on Atlantic hurricanes. Some of
these variabilities seem to have their own rhythm and have been given their own names. Most
prominent is the Pacific Decadal Oscillation. Michael McPhaden of the National Oceanic and
Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, Washington,
argued in Nature earlier this year (15 April, v 398, pp 559-62) that there had been a warm phase in the
PDO over the past two decades. But it may not be that simple.
A recent analysis of 130 years of sea surface temperatures recorded by ships, conducted by David
Enfield at NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami, suggests that
the PDO has a cycle of between 10 and 20 years. The current warm phase has now been going on for
much longer than that. So something else seems to be happening. Many believe that something is
global warming. The timing is certainly suggestive. The mid-1970s is exactly the time when the
speed of global warming began to intensify, resulting in a series of record warm years through the
1980s and 1990s.
Dan Schrag, of the department of earth and planetary science at Harvard University in Cambridge,
Massachusetts says what has happened since 1976 is not simply an unusual crop of El Niños and La
Niñas, nor even a generalised ocean warming. The warming seen since the mid-1970s is in fact
largely a result of a very specific warming of ocean surface waters in the cold season in the eastern
Pacific. This area of ocean is a constant battle ground between warm surface waters and cold waters
that upwell from the deep. Most of the time the upwelling is dominant. But during El Niños, when
warm waters wash across the Pacific from the west, the upwelling is shut off. What seems to have
happened is that this shut-off has become near permanent. (Science 10 July 1998, vol 281 pp 240-43).
Upwelling normally keeps the eastern Pacific cool, maintains the normal trade winds and so
suppresses the outbreak of El Niños. Reduce the upwelling and the system is permanently primed for
an El Niño. So Schrag concludes that the post-1976 change "may be responsible for the increase in
frequency and intensity of El Niño events since 1976".
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Why global warming will mean more El Niños - and more Atlantic hurricanes
Hurricanes are normally triggered by ocean temperatures above 27 degree C (81 degree F). Once
this temperature is reached, so much water evaporates from the sea surface that, as it condenses,
sufficient energy is released to create a "vortex" around which the hurricane forms. Every degree
above that temperature produces an exponential increase in the propensity for storms.
It stands to reason that a warmer world will probably have more intense and more frequent tropical
storms. Oceanographers have been making the point since at least 1987, when Kerry Emanuel of the
Massachusetts Institute of Technology reported in the journal Nature her calculations of what climate
model predictions of global warming would mean. Emanuel predicted that by the mid-21st century
there would be "a 40-50 per cent increase in the destructive potential of hurricanes." (Nature, vol
326, p 483-5).
As Mark Maslin of the Environmental Change Research Centre at University College, London, told
the Guardian newspaper while Hurricane Georges lashed the Gulf of Mexico last autumn: "Global
warming is constantly putting more energy into the climate system. This energy must be dissipated
both by speeding up the whole system and by increasing the number and intensity of storms. Rapid
warming increases global storminess, creating perfect conditions for hurricanes. The Caribbean and
US will be hit more often, by bigger, meaner hurricanes."
But some scientists have disagreed. They point out that other things are going on in the atmosphere.
We now know that the surges of Atlantic warm water associated with the La Niña phase of the El
Niño cycle are vital to Atlantic storms. And some people predict near-permanent El Niño conditions
in the Pacific. La Niñas could fizzle out, they say, which could actually reduce hurricanes in the
Atlantic.
But that seems much less likely following new research by a German group from the Max-Planck
Institute for Meteorology in Hamburg earlier this year (Nature, vol 398, p 694-6). Majib Latif took
an existing successful model of the world's climate that forecast the 1997 El Niño and "pre-cast" most
of the El Niños of the past 50 years. He plugged in some numbers to simulate a world hit by global
warming. Then he looked to see what happened to the frequency of El Niños and La Niñas. He
found two things. First that the world did slip towards a semi-permanent El Niño state, which
sounded bad news for the Pacific nations but perhaps good news for those fearing more Hurricane
Floyds. But second, he found that "year-to-year variations may become more extreme" and the most
dangerous types of La Niña will "become more frequent" and more violent. So the Atlantic
hurricanes, when they do occur, are likely to be worse than ever.
"Models are not proof of what will happen," says Latif, "but for the past 50 years our model shows
well what has happened." And his prediction does coincide eerily well with the real weather record
of the 1990s. After a series of major hurricanes in the late 1980s, the first half of the 1990s were
quiet along the Atlantic coast of America, with the major exception of Hurricane Andrew in 1992.
As William Gray of Colorado State University has pointed out, it was just about the quietest period
for hurricanes this century. It led some people to suggest that fears of super-hurricanes in a future
greenhouse world were misplaced.
But that quiet period also coincided with a long period of El Niño-like conditions out in the Pacific.
But when the El Niño finally broke and La Niñas hit the road in 1996 and again in 1998 and 1999, all
hell broke loose. The Pacific seems to be moving into a period of long El Niños broken by sharp and
very intense La Niñas. The Atlantic may finds itself given several years unmolested by hurricanes,
only to be ripped apart by a barrage of super-hurricanes during La Niña years. Floyd, like Mitch last
year, looks like a sign of things to come.
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3.
HURRICANE INTENSITY
Meteorologists classify hurricanes into five categories, according to wind intensity. Category One
causes minimal damage. Typical was Hurricane Allison which passed over the gulf coast of Florida
in 1995. According to the US agency NOAA, the 1992 killer Hurricane Andrew, which caused $25
billion worth of damage, killed 35 people and left a quarter of a million homeless, was rated Category
Four. Wind speeds in this category range from 131 to 155 miles per hour (210-250 km/h). Floyd, as
it has approached US shores, has been on the very margin of Category Five with wind speeds put at
155 miles per hour. Only two Category Five hurricanes have hit the US this century. The most
recent, Hurricane Camille, blasted the Gulf coast in 1969.
4.
HURRICANE LOSSES
The cost of damage caused by hurricanes has escalated dramatically in recent years. Modern society
is increasingly vulnerable, as expanding coastal development places more people and property at risk.
NOAA (US National Oceanic and Atmospheric Administration) concludes that US hurricane losses
each decade never exceeded $5 million (at 1990 prices) in the first five decades of this century. But
in the 1960s, 70s and 80s, they exceeded $15 million. And the 1990s have already far exceeded any
other decade. Indeed before it is out, it could exceed all the other decades of the century combined.
Hurricane Andrew alone cost $25 million. It left seven insurance companies insolvent. The total US
damage in 1998 from Atlantic storms was over $7 billion. Of this, Hurricane Georges was the
biggest, requiring $3.3 billion in payouts from the insurance industry, according to research by the
world reinsurance giant MunichRe. Research by NOAA released in January this year concluded that
the US had seen 31 "billion-dollar storms" since 1988, with total damage of $160 billion.
MunichRe has slightly lower figures, because its figures only relate to actual insurance payouts.
Even so, it believes the global insurance payout for weather-related disasters so far in the 1990s is $92
billion, close to four times the total payout during the 1980s, with 1992 and 1998 the most expensive
years, reflecting the ravages of hurricanes such as Andrew, Mitch and Georges. The company
calculates that some $2 trillion in insured property now lies within 30 kilometres of coasts exposed to
Atlantic hurricanes.
According to a computer simulation prepared by the Arkwright Mutual Insurance Company for last
year's annual meeting of the American Meteorological Society, a Category Five hurricane on the US
east coast could cause damage up to $100 billion, placing insurers and reinsurers at risk of insolvency.
"We are... an industry with a disaster waiting to happen," it said.
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