John Roach
for National Geographic News
Is Global Warming
Making Hurricanes
Worse?
August 4, 2005
Hurricanes bring winds and slashing rains
that flood streets, flatten homes, and leave
survivors struggling to pick up the pieces.
But has global warming given the storms an
added punch, making the aftereffects more
dreadful?
temperatures may be increasing more
quickly than atmospheric temperatures.
According to hurricane historian Jay Barnes
of Pine Knoll Shores, North Carolina, ocean
heat is the key ingredient for hurricane
formation. More heat could "generate more
storms and more intense hurricanes," he
said.
Anatomy of a Hurricane
Numerous studies in recent years have
found no evidence that the number of
hurricanes and their northwest Pacific
Ocean cousins, typhoons, is increasing
because of the rise in global temperatures.
But a new study in the journal Nature found
that hurricanes and typhoons have become
stronger and longer-lasting over the past 30
years. These upswings correlate with a rise
in sea surface temperatures.
The duration and strength of hurricanes
have increased by about 50 percent over
the last three decades, according to study
author Kerry Emanuel, a professor of
atmospheric science at the Massachusetts
Institute of Technology in Cambridge.
Emanuel's finding defies existing models for
measuring storm strength. Current models
suggest that the intensity of hurricanes and
typhoons should increase by 5 percent for
every 1ºC (1.8ºF) rise in sea surface
temperature.
"When that happens we've shown
theoretically you get an increase in the
intensity of hurricanes," he said.
According to Barnes, who has authored
several books on U.S. hurricane history, the
physics of hurricanes are complex and full
of variables. "But the sun beating down on
Earth is the primary thing that gets it going,"
he said.
Barnes explains in his book North Carolina's
Hurricane History that the summer heat
warms the ocean's surface and spurs
evaporation. As heat and moisture rise into
the atmosphere, billowing clouds, scattered
showers, and thunderstorms form.
As the thunderstorms multiply, they can get
picked up by low-pressure systems drifting
through equatorial waters, forming a tropical
depression.
The spin of the Earth causes the winds
within the storm to whirl around the center of
the low pressure. This spinning can form an
eye of a storm.
The strongest part of a hurricane is the eye
wall, on the edge of the calm center. "The
size of the eye wall can vary, and the
intensity of the storm can vary depending on
how much heat is available" and other
factors such as high altitude winds, Barnes
said.
"We've had half a degree [Celsius] of
warming, so that should have led to a 2.5
percent increase [in intensity], which is
probably not detectable," Emanuel said.
"What we've seen is somewhat bigger than
that, and we don't really know why."
According to Emanuel, if global
temperatures continue to rise, it is
reasonable to assume that hurricane activity
will increase, as there is more heat to drive
the storms.
One possibility, Emanuel said, is that ocean
Global Observation
Previous studies have tried to measure
whether typhoons and hurricanes were
becoming more frequent. Emanuel's
research, however, focused on the total
energy generated by the storms over their
duration.
"They can have the same frequency, but if
they get stronger or last longer this metric
will show an increase," he said.
When Emanuel looked at the hurricane
record in the North Atlantic, where the
storms of most interest to U.S. residents
form, he found that intensity fluctuated from
decade to decade.
This fluctuation roughly corresponded with
factors such as the El Niño weather
phenomenon, which has been shown to
influence hurricane formation.
However, North Atlantic hurricanes account
for only 12 percent of the total number of
hurricanes and typhoons that form globally
each year, Emanuel said.
"If you look at a more global measure of this
metric, you don't see these strong
interdecadal swings. They cancel each
other out between one ocean and the
other," he said. "You see instead a large
upward trend."
According to Emanuel, on a global scale,
the strength of storms corresponds with
ocean temperatures: It goes up when
temperatures go up, down when
temperatures goes down.
Most scientists say the rise in sea surface
temperature in the last 30 to 50 years is a
signal of global warming.
"That's their conclusion, not mine," Emanuel
said. "[But] it would follow reasonably well
from this metric that the upswing [in
intensity] … is a result of global warming."
Questions:
1) According to hurricane historian Jay Barnes what is the key ingredient to
hurricane formation?
2) How much have the duration and strength of hurricanes increased over the past 3
decades?
3) According to Barnes what is the primary variable in what gets hurricanes going?
4) Describe how hurricanes form.