Global Warming: The Culprit?
- Global Warming: The Culprit?
Time Magazine, 3 October 2005 - Nature doesn't always know when to
quit--and nothing says that quite like a hurricane. The atmospheric
convulsion that was Hurricane Katrina had barely left the Gulf Coast
before its sister Rita was spinning to life out in the Atlantic. In
the three weeks between them, five other named storms had lived and
died in the warm Atlantic waters without making the same headlines
their ferocious sisters did. With more than two months left in the
official hurricane season, only Stan, Tammy, Vince and Wilma are
still available on the National Hurricane Center's annual list of 21
storm names. If the next few weeks go like the past few, those names
will be used up too, and the storms that follow will be identified
simply by Greek letters. Never in the 52 years we have been naming
storms has there been a Hurricane Alpha.
If 2005 goes down as the worst hurricane season on record in the
North Atlantic, it will join 2004 as one of the most violent ever.
And these two seasons are part of a trend of increasingly powerful
and deadly hurricanes that has been playing out for more than 10
years. Says climatologist Judy Curry, chair of the School of Earth
and Atmospheric Sciences at the Georgia Institute of Technology: "The
so-called once-in-a-lifetime storm isn't even once in a season
Head-snapping changes in the weather like this inevitably raise the
question, Is global warming to blame? For years, environmentalists
have warned that one of the first and most reliable signs of a
climatological crash would be an upsurge in the most violent
hurricanes, the kind that thrive in a suddenly warmer world.
Scientists are quick to point out that changes in the weather and
climate change are two different things. But now, after watching two
Gulf Coast hurricanes reach Category 5 in the space of four weeks,
even skeptical scientists are starting to wonder whether something
serious might be going on.
"There is no doubt that climate is changing and humans are partly
responsible," says Kevin Trenberth, head of the climate-analysis
section at the National Center for Atmospheric Research (NCAR) in
Boulder, Colo. "The odds have changed in favor of more intense storms
and heavier rainfalls." Says NCAR meteorologist Greg Holland: "These
are not small changes. We're talking about a very large change."
But do scientists really know for sure? Can man-made greenhouse gases
really be blamed for the intensity of storms like Rita and Katrina?
Or are there, as other experts insist, too many additional variables
to say one way or the other?
That global warming ought to, in theory, exacerbate the problem of
hurricanes is an easy conclusion to reach. Few scientists doubt that
carbon dioxide and other greenhouse gases raise the temperature of
Earth's atmosphere. Warmer air can easily translate into warmer
oceans--and warm oceans are the jet fuel that drives the hurricane's
turbine. When Katrina hit at the end of August, the Gulf of Mexico
was a veritable hurricane refueling station, with water up to 5thF
higher than normal. Rita too drew its killer strength from the Gulf,
making its way past southern Florida as a Category 1 storm, then
exploding into a Category 5 as it moved westward. "The Gulf is really
warm this year, and it's just cooking those tropical storms," says
Local hot spots like this are not the same as global climate change,
but they do appear to be part of a larger trend. Since 1970, mean
ocean surface temperatures worldwide have risen about 1thF. Those
numbers have moved in lockstep with global air temperatures, which
have also inched up a degree. The warmest year ever recorded was
1998, with 2002, 2003 and 2004 close behind it.
So that ought to mean a lot more hurricanes, right? Actually, no--
which is one of the reasons it's so hard to pin these trends down.
The past 10 stormy years in the North Atlantic were preceded by many
very quiet ones--all occurring at the same time that global
temperatures were marching upward. Worldwide, there's a sort of
equilibrium. When the number of storms in the North Atlantic
increases, there is usually a corresponding fall in the number of
storms in, say, the North Pacific. Over the course of a year, the
variations tend to cancel one another out. "Globally," says
atmospheric scientist Kerry Emanuel of the Massachusetts Institute of
Technology, "we do not see any increase at all in the frequency of
But frequency is not the same as intensity, and two recent studies
demonstrate that difference. Two weeks ago, a team of scientists that
included Curry and Holland published a study in the journal Science
that surveyed global hurricane frequency and intensity over the past
35 years. On the whole, they found, the number of Category 1, 2 and 3
storms has fallen slightly, while the number of Categories 4 and 5
storms--the most powerful ones--has climbed dramatically. In the
1970s, there were an average of 10 Category 4 and 5 hurricanes a year
worldwide. Since 1990, the annual number has nearly doubled, to 18.
Overall, the big storms have grown from just 20% of the global total
to 35%. "We have a sustained increase [in hurricane intensity] over
30 years all over the globe," says Holland.
Emanuel came at the same question differently but got the same
results. In a study published in the journal Nature last month, he
surveyed roughly 4,800 hurricanes in the North Atlantic and North
Pacific over the past 56 years. While he too found no increase in the
total number of hurricanes, he found that their power--measured by
wind speed and duration--had jumped 50% since the mid-1970s. "The
storms are getting stronger," Emanuel says, "and they're lasting
Several factors help feed the trend. For example, when ocean
temperatures rise, so does the amount of water vapor in the air. A
moister atmosphere helps fuel storms by giving them more to spit out
in the form of rain and by helping drive the convection that gives
them their lethal spin. Warm oceans produce higher levels of vapor
than cool oceans--at a rate of about 1.3% more per decade since 1988,
according to one study--and nothing gets that process going better
than greenhouse-heated air. "Water vapor increases the rainfall
intensity," says Trenberth. "During Katrina, rainfall exceeded 12
inches near New Orleans."
It's not just warmer water on the surface that's powering the
hurricanes; deeper warm water is too--at least in the Gulf of Mexico.
Extending from the surface to a depth of 2,000 ft. or more is
something scientists call the Loop Current, a U-shaped stream of warm
water that flows from the Yucatan Straits to the Florida Straits and
sometimes reaches as far north as the Mississippi River delta.
Hurricanes that pass over the Loop typically get an energy boost, but
the extra kick is brief, since they usually cross it and move on. But
Rita and Katrina surfed it across the Gulf, picking up an even more
powerful head of steam before slamming into the coastal states. Even
if those unlucky beelines had been entirely random, the general trend
toward warmer Gulf water may well have made the Loop even deadlier
"We don't know the temperature within the Loop Current," says Nan
Walker, director of Louisiana State University's Earth Scan
Laboratory. "It's possible that below the surface, it's warmer than
normal. This needs to be investigated."
Other greenhouse-related variables may also be fueling the storms.
Temperature-boosting carbon dioxide, for example, does not linger in
the atmosphere forever. Some of it precipitates out in rain, settling
partly on the oceans and sinking at least temporarily out of sight.
But the violent frothing of the water caused by a hurricane can
release some of that entrained CO2, sending it back into the sky,
where it resumes its role in the warming cycle. During Hurricane
Felix in 1995, measurements taken in one area the storm struck showed
local CO2 levels spiking 100-fold.
So, are hurricanes actually speeding the effects of global warming
and thus spawning even more violent storms? That's a matter of some
dispute. While many scientists agree that this outgassing process
goes on, not everyone agrees that it makes much of a difference. "The
amount of CO2 given off is fairly insignificant in terms of the total
CO2 in the atmosphere," says atmospheric scientist Chris Bretherton
of the University of Washington in Seattle. "I am fairly confident in
saying that there is no direct feedback from hurricanes."
Thus scientific uncertainty enters the debate--a debate already
intensified by the political passions that surround any discussion of
global warming. The fact is, there is plenty of room for doubt on
both sides of the argument. Chris Landsea, a science and operations
officer at the National Hurricane Center in Miami, is one of many
experts who believe that global warming may be boosting the power of
hurricanes--but only a bit, perhaps 1% to 5%. "A 100-mile-per-hour
wind today would be a 105-mile-per-hour wind in a century," he
says. "That is pretty tiny in comparison with the swings between
Skeptics are also troubled by what they see as a not inconsiderable
bias in how hurricane researchers collect their data. Since most
hurricanes spend the majority of their lives at sea--some never
making land at all--it's impossible to measure rainfall precisely and
therefore difficult to measure the true intensity of a storm.
What's more, historical studies of hurricanes like Emanuel's rely on
measurements taken both before and during the era of satellites. Size
up your storms in radically divergent ways, and you're likely to get
radically divergent results. Even after satellites came into wide use-
-adding a significant measure of reliability to the data collected--
the quality of the machines and the meteorologists who relied on them
was often uneven. "The satellite technology available from 1970 to
1989 was not up to the job," says William Gray of Colorado State
University. "And many people in non-U.S. areas were not trained well
enough to determine the very fine differences between, say, the 130-
m.p.h. wind speed of a Category 4 and, below that, a Category 3."
There's also some question as to whether there's a subtler, less
scientific bias going on, one driven not by the raw power of the
storms but by where they do their damage. Hurricanes that claw up
empty coasts don't generate the same headlines as those that strike
the places we like to live--and increasingly we like to live near the
shore. The coastal population in the U.S. jumped 28% between 1980 and
2003. In Florida alone, the increase was a staggering 75%. Even the
most objective scientists can be swayed when whole cities are being
demolished by a hurricane.
"The storm activity this year is not necessarily higher than in
previous high-activity years. It's just where they are going," says
meteorologist Stan Goldenberg of the National Oceanic and Atmospheric
Administration in Key Biscayne, Fla. "If you've got a guy shooting a
machine gun but he's not shooting toward your neighborhood, it
doesn't bother you."
Even correcting for our tendency to pay more attention to what is
happening in our backyard, however, the global census of storms and
the general measurement of their increasing power don't lie. And what
those measurements tell scientists is that this already serious
problem could grow a great deal worse--and do so very fast.
Some scientists are studying not just climate change but the even
more alarming phenomenon of abrupt climate change. Complex systems
like the atmosphere are known to move from one steady state to
another with only very brief transitions in between. (Think of water,
which when put over a flame becomes hotter and hotter until suddenly
it turns into steam.) Ice cores taken from Greenland in the 1990s by
geoscientist Richard Alley of Pennsylvania State University show that
the last ice age came to an end not in the slow creep of geological
time but in the quick pop of real time, with the entire planet
abruptly warming in just three years.
"There are thresholds one crosses, and change runs a lot faster,"
Alley says. "Most of the time, climate responds as if it's being
controlled by a dial, but occasionally it acts as if it's controlled
by a switch." Adds Laurence Smith, an associate professor of
geography at UCLA who has been studying fast climate change in the
Arctic: "We face the possibility of abrupt changes that are
economically and socially frightening."
Do we have the time to avert even a relatively slow climate change,
or at least the nimbleness to survive it? That's what a lot of
scientists are trying to determine. Japanese climatologists, for
example, are using the Earth Simulator in Yokohama--one of the most
powerful supercomputers in the world--to develop climate models that
are more and more sophisticated. Scientists like geologist Claudia
Mora of the University of Tennessee at Knoxville are going in another
direction, studying isotopes locked in old tree rings to look for
clues to past eras of heavy and light rainfall. Pair that information
with global-temperature estimates for the same periods, and you can
get a pretty good idea of how heat and hurricanes drive each
other. "We've taken it back 100 years and didn't miss a storm," said
It's impossible to say whether any of that will convince the
lingering global-warming skeptics. What does seem certain is that the
ranks of those skeptics are growing thinner. In Washington successive
administrations have ignored greenhouse warnings, piling up
environmental debt the way we have been piling up fiscal debt. The
problem is, when it comes to the atmosphere, there's no such thing as
creative accounting. If we don't bring our climate ledgers back into
balance, the climate will surely do it for us.
Reported by Mike Billips/Atlanta, Rita Healy/Denver, Kristin
Kloberdanz/Chicago, Terry McCarthy/Los Angeles and Siobhan
Copyright 2005 Time Inc.