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Eyes on the hurricane debate
Saturday August 26, 2006
By Andrew Buncombe
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
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