Global Warming & Hurricane Intensity
Christian Science Monitor
August 01, 2005
As planet warms, storms grow stronger
Scientists see evidence that hurricanes and typhoons have
intensified. Are new responses needed?
By Peter N. Spotts | Staff writer of The Christian Science Monitor
For years, hurricanes and typhoons have served as poster children for
the hazards of global warming.
When simulated tropical storms churn inside the silicon universe of
researchers' computers, such cyclones grow in power, and sometimes in
number as well, as tropical temperatures increase. But when researchers
have looked for global warming's fingerprints on real tropical cyclones,
the evidence often has been inconclusive.
Now, one of the top researchers in the field reports that worldwide, these
storms are nearly twice as powerful today as they were 30 years ago.
Global warming has intensified the trend, exerting an influence stronger
than he would have believed even a few months ago, he says.
"I'd been thinking of a very modest response" of tropical cyclones to
climate change, "and what we're seeing is not so modest," says Kerry
Emanuel, a professor of atmospheric science at the Massachusetts
Institute of Technology in Cambridge, Mass.
The upshot: The 21st century could be a rough one for people who settle
in hurricane or typhoon-prone areas.
As a result, more communities should be drawing on the experience of
states such as Florida in devising building and zoning codes that can
reduce damage and fatalities, analysts say. For people who insist on
building on vulnerable barrier islands or along fragile coasts, insurance
companies should be given a freer hand in deciding who they will cover
and what they will charge for hurricane insurance, researchers and policy
Strong start to hurricane season
Dr. Emanuel's results are appearing at a time when residents along the
US Gulf Coast and throughout the Caribbean are still recovering from what
forecasters are calling the most active start to the hurricane season on
record. Since June 1, six storms grew strong enough to merit names --
from Arlene to Cindy to Franklin. Three became hurricanes. Two reached
a potent category four out of five. According to forecasters at the National
Hurricane Center in Miami, hurricane Dennis, which reached category four
on July 7, ranks as the earliest Caribbean storm on record to reach that
Some researchers argue that in practical terms, the allure to live near the
sea will do far more to boost society's risk from such storms over the next
several decades than any effect global warming could have on the storms
Until he concluded this study, Emanuel says he was among that group.
Now, he says, global warming's impact on the storms may play a bigger
role than previously believed in putting societies at risk, particularly in
less-developed countries. Dr. Emanuel's research, published Sunday on
the journal Nature's website, adds a fresh perspective to the discussion
about the effects of global warming on tropical cyclones, says Kevin
Trenberth, a senior scientist at the National Center for Atmospheric
Research in Boulder, Colo.
Early on, concerns about the future of these storms arose based on
computer forecasts and basic theory. "Given the information we had at the
time, the results were overhyped a bit," Dr. Trenberth acknowledges. He
notes that the study doesn't have much comment on the effects of storm
surges and torrential rainfall that accompany land-falling hurricanes --
factors far more destructive than winds.
Still, Emanuel's approach "adds a new element," says Trenberth. It shows
a strong real-world correlation between the oceans' current warming
trend - which scientists have linked to the heating- trapping effect of
industrial carbon dioxide and other "greenhouse gases" - and the
increasing power of tropical cyclones.
Global warming vs. natural cycle
Other researchers have noted that this is more likely a natural period
of intense activity for Atlantic hurricanes. For example, William Gray,
a specialist in tropical meteorology at Colorado State University who
pioneered seasonal hurricane forecasts, notes that the region goes
through swings in activity that can span decades. He and his colleagues
have noted that the US and its southern neighbors have faced above-
average hurricane seasons for the past decade and is likely to do so for
some time to come.
Emanuel acknowledges that such cycles are important. Depending on the
region under scrutiny, the impact of natural cycles such as El Niño, or the
multidecade cycles Dr. Gray observes, can swamp any global-warming
signal the storms may carry. But viewed worldwide, the signal starts to
His latest finding, he says, grew out of attempts to answer a broader
question: Do hurricanes help drive large-scale ocean currents? These
currents carry tropical waters toward the poles, bringing warmth to middle
and high latitudes.
Measuring a typhoon's punch
Initial calculations suggested that hurricane activity could account for
up to half or more of the driving force behind these currents. If so, a
significant long-term rise in tropical cyclones could push warmer water
toward higher latitudes. This could lead to warmer average temperatures
at middle and high latitudes than climate models currently project.
To answer the question, however, Emanuel needed to gauge a hurricane's
or typhoon's punch. So he built a measure based on sustained wind
speeds over the life of each storm and on each storm's duration.
Combined they reflect a storm's total power output. Since the mid-70s,
storm power fluctuated with well-known natural cycles. But through this
natural "noise," global warming's signal emerged as an increase in
strength that tracked rising temperatures in the tropical oceans' surface
The work certainly will not be the last word on the subject. Some
researchers are already raising questions about Emanuel's approach.
In one sense, however, there is broad agreement, notes Roger Pielky Jr.,
director of the Center for Science and Technology Policy Research at
the University of Colorado at Boulder. Whether scientists attribute the
increased tropical cyclone intensity to global warming or natural cycles,
the trend is likely to hold for at least a decade.
Looking at the costs to society from these storms, for every dollar in
damage from tropical cyclones the Intergovernmental Panel on Climate
Change anticipates by 2050, the IPCC's demographic numbers suggest
that societal changes will add another $22 to $60 in impact. "If you're a
planner, you're saying: We'd better get ready," Dr. Pielke observes.