A vitalized symbol
Of earth and of storm,
Of Chaos contracted
To intricate form.
Oliver St. John
The Crab Tree
are the most serious of earthly storms. In the United States, damages from the rainfall of
intense hurricanes continually top the charts for property losses, both insured and
uninsured. But technology advances have improved survivability against these fierce
stormseven as luxury coastal development has made property losses seem to skyrocket.
In fact, hurricanes of the past were much deadlier. In 1938, for example, more than 600
people died in a hurricane that hit New England; and in 1900 more than 6,000 people were
killed in a hurricane that pelted Galveston, Texas.
So far in
1999 there have been five intense hurricanes (wind speeds above 130 miles per hour, or
categories 4 and 5 on the Saffir-Simpson scale), higher than the average of 2.2 per year
over the last 50 years. In searching for
what caused this uptick in intense hurricanes, can we look to our usual
suspectgreenhouse gas global warming?
must review what is known and unknown about hurricanes. Over the last 50 years, keen
observers noticed that the number of intense Atlantic hurricanes declined from 3 per year
(1944-1964) to about 1.5 (19651994) (Figure 1). Since 1995, hurricane activity seems
to have returned to its earlier, active phase, with the exception of 1997, when the strong
El Niņo suppressed hurricane formation in the Atlantic.
Figure 1. Number of intense (category 4 and 5) Atlantic
hurricanes, 19441999 (blue line); updated from C. Landsea NOAA/AOML, Miami. The change in annual Sahel region rainfall is also
shown (dashed red line, courtesy of M. Hulme, University of East Anglia).
One of the
keenest observers, William Gray of Colorado State University, has found an interesting
link between the decadal pattern of intense Atlantic hurricanes and rainfall in the
Western Sahel region of sub-Saharan Africasandwiched between the northern desert and
the southern wet zones. Sahel rainfall matches the pattern of changes in intense Atlantic
hurricanes. The connection might not be chance: Proto-hurricanes in their early,
tropical-storm stage are spawned in the eastern Atlantic near Western Africa. Sahel
rainfall does not create hurricanes per se; rather, it indicates conditions that favor
tropical storm formation.
cyclones in the North Pacific also show a decadal change, according to P.S. Chu and
J. D. Clark (Figure 2). Between 1966 and 1981 fewer tropical storms and hurricanes formed
than recently (19821997). As Chu and Clark note, the sudden, steplike jump in the
North Pacific storm activity is unlikely to be connected to a more gradual, greenhouse gas
Figure 2. Number of Northern Pacific tropical storms and
hurricanes, 19661997 (from Chu, University of Hawaii) (blue line), and change in
eastern equatorial Pacific sea surface temperature (dashed red line; from Mitchell,
University of Washington).
Is there a
simple explanation for the change in Pacific storm activity? Chu and Clark suggest storm
activity changes because the local sea surface temperature changeshigher local
sea-surface temperature tends to promote tropical storm genesis there.
surface temperature of the Eastern equatorial Pacific (6°N to 6°S, 180°W to 90°W),
also plotted in Figure 2, compared with the number of Pacific storms, suggests such a
correspondence. The jump in temperature starting circa 1982, beginning with the strong El
Niņo of that year, initiated a period of two more storms per year than the average over
the previous period.
relationship between sea-surface temperature and storm frequency, while statistically
significant, is not all that strong, as Figure 2 showsfurther evidence for the
complexity of the global warming/hurricane picture.
Just as the
rise in the number of Pacific storms is too sudden to be related to a gradual greenhouse
gas warming, so is the jump in Atlantic storms. The increases in storminess and
temperature are unrelated between the two oceans because they describe only local
conditions. If the independent storm patterns of both oceans follow the decadal averages
they are currently showing, then both regions are currently in phase with conditions that
tend to promote high storm activity. Coincidence? Absolutely.
empirical results on storm activity highlight the enormous difficulties in engaging in
storm prediction for a greenhouse-enhanced climate. Modeling storminess requires paying
attention to the small spatial scales on which storms are formed, while understanding
influences of variability that occur on much broader scales such as the El
NiņoSouthern Oscillation index.
Princetons GFDL model, F. Vitart and colleagues point out some deficiencies of storm
modeling. For example, the modeled storms are less intense and shorter-lived; and the
distribution of storms disagrees with the observed locations. Notably, the model
incorrectly predicts two storms per year spawning over land, and one storm per year in the
south Atlantic, where none is seen. The most obvious disagreement between the model and
reality is in the number of storms predicted for the Eastern North Pacific, three per
year, compared with the recorded average of 17 per year.
real-world storminess using a GCM is still a challenge, and models cannot yet match the
observations. Model predictions about storms in a greenhouse gasenhanced world
remain premature and unreliable.
This bimonthly contribution is made
possible by the George C Marshall Institute, Washington, D.C., where Sallie Baliunas is
senior scientist and Willie Soon is a visiting fellow.
Chu. P.-S., and J.D.
Clark, 1999, Decadal variations of tropical cyclone activity over the central north
Pacific, Bulletin of the American Meteorological
Society, 80, 18751881.
F., et al., 1997, Simulation of interannual variability of tropical storm frequency in an
ensemble of GCM integrations, Journal of Climate, 10, 745760.