05pielke.txt

HURRICANES AND GLOBAL
WARMING
BY

R. A. PIELKE JR., C. L ANDSEA, M. MAYFIELD, J. L AVER, AND R. PASCH

An interdisciplinary team of researchers survey the peer-reviewed literature to assess the
relationships between global warming, hurricanes, and hurricane impacts.

D

ebate over climate change frequently conflates
issues of science and politics. Because of their
significant and visceral impacts, discussion of
extreme events is a frequent locus of such conflation.
Linda Mearns, of the National Center for Atmospheric Research (NCAR), aptly characterizes this context:
“There’s a push on climatologists to say something
about extremes, because they are so important. But
that can be very dangerous if we really don’t know
the answer” (Henson 2005). In this article we focus

AFFILIATIONS: PIELKE —Center for Science and Technology

Policy Research, University of Colorado, Boulder, Colorado;
L ANDSEA*—Hurricane Research Division, National Oceanic and
Atmospheric Administration/Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida; MAYFIELD —National Oceanic
and Atmospheric Administration/National Hurricane Center,
Miami, Florida; L AVER—National Oceanic and Atmospheric Administration/Climate Prediction Center, Camp Springs, Maryland;
PASCH —National Oceanic and Atmospheric Administration/National Hurricane Center, Miami, Florida
*Current affiliation: National Oceanic and Atmospheric Administration/National Hurricane Center
CORRESPONDING AUTHOR: Roger Pielke Jr., Center for
Science and Technology Policy Research, 1333 Grandview Ave.
UCB 488, University of Colorado, Boulder, CO 80309-0488
E-mail: pielke@colorado.edu
DOI: 10.1175/BAMS-86-11-1571
In final form 24 August 2005
©2005 American Meteorological Society

AMERICAN METEOROLOGICAL SOCIETY

on a particular type of extreme event—the tropical
cyclone—in the context of global warming (tropical
cyclones are better known in the United States as
hurricanes, i.e., tropical cyclones that form in the
waters of the Atlantic and eastern Pacific oceans with
maximum 1-min-averaged surface winds that exceed
32 m s–1).
In our discussion we follow distinctions between
event risk and outcome risk presented by Sarewitz
et al. (2003). “Event risk” refers to the occurrence of
a particular phenomenon, and in the context of hurricanes we focus on trends and projections of storm
frequencies and intensities. “Vulnerability” refers to
“the inherent characteristics of a system that create
the potential for harm,” but are independent from
event risk. In the context of the economic impacts of
tropical cyclones vulnerability has been characterized in terms of trends in population and wealth that
set the stage for storms to cause damage. “Outcome
risk” integrates considerations of vulnerability with
event risk to characterize an event that causes losses.
An example of outcome risk is the occurrence of a
$100 billion hurricane in the United States. To calculate such a probability requires consideration of both
vulnerability and event risk. This article discusses
hurricanes and global warming from both of these
perspectives.
EVENT RISK. At the end of the 2004 Atlantic
hurricane season, many scientists, reporters, and
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policymakers looked for simple answers to explain
the extent of the devastation, which totaled more
than $40 billion according to the National Hurricane
Center. Some prominent scientists proposed that the
intense 2004 hurricane season and its considerable
impacts, particularly in Florida, could be linked
to global warming resulting from the emissions of
greenhouse gases into the atmosphere (e.g., Harvard
Medical School 2004; NCAR 2004). But the current
state of climate science does not support so close a
linkage (Trenberth 2005).
Tropical cyclones can be thought of to a first
approximation as a natural heat engine or Carnot
cycle (Emanuel 1987). From this perspective global
warming can theoretically influence the maximum
potential intensity of tropical cyclones through alterations of the surface energy flux and/or the upper-level
cold exhaust (Emanuel 1987; Lighthill et al. 1994;
Henderson-Sellers et al. 1998). But no theoretical
basis yet exists for projecting changes in tropical cyclone frequency, though empirical studies do provide
some guidance as to the necessary thermodynamical
and dynamical ingredients for tropical cyclogenesis
(Gray 1968, 1979).
Since 1995 there has been an increase in the number of storms, and in particular the number of major
hurricanes (categorys 3, 4, and 5) in the Atlantic.
But the changes of the past decade in these metrics
are not so large as to clearly indicate that anything
is going on other than the multidecadal variability
that has been well documented since at least 1900
(Gray et al. 1997; Landsea et al. 1999; Goldenberg
et al. 2001). Consequently, in the absence of large or
unprecedented trends, any effect of greenhouse gases
on the frequency of storms or major hurricanes is necessarily very difficult to detect in the context of this
documented variability. Perspectives on hurricanes
are no doubt shaped by recent history, with relatively
few major hurricanes observed in the 1970s, 1980s,
and early 1990s, compared with considerable activity
during the 1940s, 1950s, and early 1960s. The period
from 1944 to 1950 was particularly active for Florida.
During that period 11 hurricanes hit the state, at least
one per year, resulting in the equivalent of billions of
dollars in damage in each of those years (Pielke and
Landsea 1998).
Globally there has been no increase in tropical cyclone frequency over at least the past several decades
(Webster et al. 2005; Lander and Guard 1998; Elsner
and Kocher 2000). In addition to a lack of theory for
future changes in storm frequencies, the few global
modeling results are contradictory (HendersonSellers et al. 1998; Houghton et al. 2001). Because
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historical and observational data on hurricanes
and tropical cyclones are relatively robust, it is clear
that storm frequency has not tracked recent tropical
climate trends. Research on possible future changes
in hurricane frequency due to global warming is
ambiguous, with most studies suggesting that future
changes will be regionally dependent, and showing a lack of consistency in projecting an increase
or decrease in the total global number of storms
(Henderson-Sellers et al. 1998; Royer et al. 1998; Sugi
et al. 2002). These studies give such contradictory
results as to suggest that the state of understanding
of tropical cyclogenesis provides too poor a foundation to base any projections about the future. While
there is always some degree of uncertainty about the
future and model-based results are often fickle, the
state of current understanding is such that we should
expect hurricane frequencies in the future to have
a great deal of year-to-year and decade-to-decade
variation as has been observed over the past decades
and longer.
The issue of trends in tropical cyclone intensity
is more complicated, simply because there are many
possible metrics of intensity (e.g., maximum potential
intensity, average intensity, average storm lifetime,
maximum storm lifetime, average wind speed, maximum sustained wind speed, maximum wind gust,
accumulated cyclone energy, power dissipation, and so
on), and not all such metrics have been closely studied
from the standpoint of historical trends, due to data
limitations among other reasons. Statistical analysis
of historical tropical cyclone intensity shows a robust
relationship to the thermodynamic potential intensity
(Emanuel 2000), suggesting that increasing potential
intensity should lead to an increase in the actual intensity of storms. The increasing potential intensity
associated with global warming as predicted by global
climate models (Emanuel 1987) is consistent with the
increase in modeled storm intensities in a warmer climate, as might be expected (Knutson and Tuleya 2004).
But while observations of tropical and subtropical sea
surface temperature have shown an overall increase of
about 0.2°C over the past ~50 years, there is only weak
evidence of a systematic increase in potential intensity
(Bister and Emanuel 2002; Free et al. 2004).
Emanual (2005) reports a very substantial upward
trend in power dissipation (i.e., the sum over the lifetime of the storm of the maximum wind speed cubed)
in the North Atlantic and western North Pacific, with
a near doubling over the past 50 years (Webster et al.
2005). The precise causation for this trend is not yet
clear. Moreover, in the North Atlantic, much of the
recent upward trend in Atlantic storm frequency

and intensity can be attributed to large multidecadal
fluctuations. Emanuel (2005) has just been published
as of this writing and is certain to motivate a healthy
and robust debate in the community. Other studies
that have addressed tropical cyclone intensity variations (Landsea et al. 1999; Chan and Liu 2004) show
no significant secular trends during the decades of
reliable records.
Because the global earth system is highly complicated, until a relationship between actual storm
intensity and tropical climate change is clearly demonstrated and accepted by the broader community,
it would be premature to conclude with certainty
that such a link exists or is significant (from the
standpoints of either event or outcome risk) in the
context of variability. Additionally, any such relationship between trends in sea surface temperature
and various measures of tropical cyclone intensity
would not necessarily mean that the storms of 2004
or 2005 or their associated damages could be attributed directly or indirectly to increasing greenhouse
gas emissions.
Looking to the future, global modeling studies
suggest the potential for relatively small changes in
tropical cyclone intensities related to global warming.
Early theoretical work suggested an increase of about
10% in wind speed for a 2°C increase in tropical sea
surface temperature (Emanuel 1987). A 2004 study
from the Geophysical Fluid Dynamics Laboratory
in Princeton, New Jersey, that utilized a mesoscale
model downscaled from coupled global climate
model runs indicated the possibility of a 5% increase
in the wind speeds of hurricanes by 2080 (Knutson
and Tuleya 2004; cf. Houghton et al. 2001). Michaels
et al. (2005) suggest that even this 5% increase may
be overstated, and that a more realistic projection is
on the order of only half of that amount. Even if one
accepts that the Knutson and Tuleya results are in
the right ballpark, these would imply that changes to
hurricane wind speeds on the order of 0.5–1.0 m s–1
may be occurring today. This value is exceedingly
small in the context of, for example, the more than
doubling in numbers of major hurricanes between
quiet and active decadal periods in the Atlantic
(Goldenberg et al. 2001). Moreover, such a change
in intensities would not be observable with today’s
combination of aircraft reconnaissance and satellite-based intensity estimates, which only resolves
wind speeds of individual tropical cyclones to—at
best—2.5 m s–1 increments.
VULNERABILITY AND OUTCOME RISK.
Understanding of trends and projections in tropical
AMERICAN METEOROLOGICAL SOCIETY

cyclone frequencies and intensities takes on a different perspective when considered in the context of
rapidly growing societal vulnerability to storm impacts (Pielke and Pielke 1997; Pulwarty and Riebsame
1997). There is overwhelming evidence that the most
significant factor underlying trends and projections
associated with hurricane impacts on society is
societal vulnerability to those impacts, and not the
trends or variation in the storms themselves (Pielke
and Landsea 1998). Growing population and wealth
in exposed coastal locations guarantee increased
economic damage in coming years, regardless of the
details of future patterns of intensity or frequency
(Pielke et al. 2000). Tropical cyclones will also result
in death and suffering, in less developed countries in
particular, as seen in Haiti during Hurricane Jeanne
(cf. Pielke et al. 2003).
Over the long term the effects of changes in society
dwarf the effects of any projected changes in tropical
cyclones according to research based on assumptions
of the Intergovernmental Panel on Climate Change
(IPCC), the scientific organization convened to report
on the science of climate change. By 2050, for every
additional dollar in damage that the IPCC expects to
result from the effects of global warming on tropical
cyclones, we should expect between $22 and $60 of
increase in damage due to population growth and
wealth (Pielke et al. 2000). The primary factors that
govern the magnitude and patterns of future damages
and causalities are how society develops and prepares
for storms rather than any presently conceivable future
changes in the frequency and intensity of the storms.
Consider that if per capita wealth and population grow
at a combined 5% per year, this implies a doubling in
the real costs of hurricanes about every 15 years. In
such a context, any climate trend would have to be
quite large to be discernible in the impacts record.
With no trend identified in various metrics of
hurricane damage over the twentieth century (Pielke
and Landsea 1998), it is exceedingly unlikely that scientists will identify large changes in historical storm
behavior that have significant societal implications.
In addition, looking to the future, until scientists
conclude a) that there will be changes to storms that
are significantly larger than observed in the past,
b) that such changes are correlated to measures of
societal impact, and c) that the effects of such changes
are significant in the context of inexorable growth in
population and property at risk, then it is reasonable
to conclude that the significance of any connection of
human-caused climate change to hurricane impacts
necessarily has been and will continue to be exceedingly small.
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CONCLUSIONS. To summarize, claims of linkages between global warming and hurricane impacts
are premature for three reasons. First, no connection has been established between greenhouse gas
emissions and the observed behavior of hurricanes
(Houghton et al. 2001; Walsh 2004). Emanuel (2005)
is suggestive of such a connection, but is by no means
definitive. In the future, such a connection may be
established [e.g., in the case of the observations
of Emanuel (2005) or the projections of Knutson
and Tuleya (2004)] or made in the context of other
metrics of tropical cyclone intensity and duration
that remain to be closely examined. Second, the
peer-reviewed literature ref lects that a scientific
consensus exists that any future changes in hurricane intensities will likely be small in the context
of observed variability (Knutson and Tuleya 2004;
Henderson-Sellers et al. 1998), while the scientific
problem of tropical cyclogenesis is so far from being
solved that little can be said about possible changes
in frequency. And third, under the assumptions of
the IPCC, expected future damages to society of its
projected changes in the behavior of hurricanes are
dwarfed by the influence of its own projections of
growing wealth and population (Pielke et al. 2000).
While future research or experience may yet overturn these conclusions, the state of the peer-reviewed
knowledge today is such that there are good reasons
to expect that any conclusive connection between
global warming and hurricanes or their impacts will
not be made in the near term.
Yet, claims of such connections persist (cf. Epstein
and McCarthy 2004; Eilperin 2005), particularly in
support of a political agenda focused on greenhouse
gas emissions reduction (e.g., Harvard Medical
School 2004). But a great irony here is that invoking the modulation of future hurricanes to justify
energy policies to mitigate climate change may prove
counterproductive. Not only does this provide a great
opening for criticism of the underlying scientific
reasoning, it leads to advocacy of policies that simply
will not be effective with respect to addressing future
hurricane impacts. There are much, much better ways
to deal with the threat of hurricanes than with energy
policies (e.g., Pielke and Pielke 1997). There are also
much, much better ways to justify climate mitigation
policies than with hurricanes (e.g., Rayner 2004).
ACKNOWLEDGMENTS. The views expressed are
those of the authors, and for the four coauthors employed by the U.S. government, do not necessarily represent those of the National Oceanic and Atmospheric
Administration.

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