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HURRICANES AND GLOBAL WARMING - IS THERE A CONNECTION?
By Stefan Rahmstorf, Michael Mann, Rasmus Benestad, Gavin Schmidt, and
William Connolley
Real Climate
September 2, 2005

http://www.realclimate.org/index.php?p=181

On Monday August 29, Hurricane Katrina ravaged New Orleans, Louisiana and
Missisippi, leaving a trail of destruction in her wake. It will be some time
until the full toll of this hurricane can be assessed, but the devastating
human and environmental impacts are already obvious.

Katrina was the most feared of all meteorological events, a major hurricane
making landfall in a highly-populated low-lying region. In the wake of this
devastation, many have questioned whether global warming may have
contributed to this disaster. Could New Orleans be the first major U.S. city
ravaged by human-caused climate change?

The correct answer -- the one we have indeed provided in previous posts
(Storms & Global Warming II <http://www.realclimate.org/index.php?p3>, Some
recent updates <http://www.realclimate.org/index.php?p2> and Storms and
Climate Change <http://www.realclimate.org/index.php?p0>) -- is that there
is no way to prove that Katrina either was, or was not, affected by global
warming. For a single event, regardless of how extreme, such attribution is
fundamentally impossible. We only have one Earth, and it will follow only
one of an infinite number of possible weather sequences. It is impossible to
know whether or not this event would have taken place if we had not
increased the concentration of greenhouse gases in the atmosphere as much as
we have. Weather events will always result from a combination of
deterministic factors (including greenhouse gas forcing or slow natural
climate cycles) and stochastic factors (pure chance).

Due to this semi-random nature of weather, it is wrong to blame any one
event such as Katrina specifically on global warming - and of course it is
just as indefensible to blame Katrina on a long-term natural cycle in the
climate.

Yet this is not the right way to frame the question. As we have also pointed
out in previous posts, we can indeed draw some important conclusions about
the links between hurricane activity and global warming in a statistical
sense. The situation is analogous to rolling loaded dice: one could, if one
was so inclined, construct a set of dice where sixes occur twice as often as
normal. But if you were to roll a six using these dice, you could not blame
it specifically on the fact that the dice had been loaded. Half of the sixes
would have occurred anyway, even with normal dice. Loading the dice simply
doubled the odds. In the same manner, while we cannot draw firm conclusions
about one single hurricane, we can draw some conclusions about hurricanes
more generally. In particular, the available scientific evidence indicates
that it is likely that global warming will make - and possibly already is
making - those hurricanes that form more destructive than they otherwise
would have been.

The key connection is that between sea surface temperatures (we abbreviate
this as SST) and the power of hurricanes. Without going into technical
details about the dynamics and thermodynamics involved in tropical storms
and hurricanes (an excellent discussion of this can be found here
<http://www.aoml.noaa.gov/hrd/hrd_sub/dynamics.html>), the basic connection
between the two is actually fairly simple: warm water, and the instability
in the lower atmosphere that is created by it, is the energy source of
hurricanes. This is why they only arise in the tropics and during the season
when SSTs are highest (June to November in the tropical North Atlantic).

SST is not the only influence on hurricane formation. Strong shear in
atmospheric winds (that is, changes in wind strength and direction with
height in the atmosphere above the surface), for example, inhibits
development of the highly organized structure that is required for a
hurricane to form. In the case of Atlantic hurricanes, the El Nino/Southern
Oscillation tends to influence the vertical wind shear, and thus, in turn,
the number of hurricanes that tend to form in a given year. Many other
features of the process of hurricane development and strengthening, however,
are closely linked to SST.

Hurricane forecast models (the same ones that were used to predict Katrina's
path) indicate a tendency for more intense (but not overall more frequent)
hurricanes when they are run for climate change scenarios
<http://www.gfdl.noaa.gov/~tk/glob_warm_hurr.html>.

Figure 1. Model Simulation of Trend in Hurricanes (from Knutson et al,
2004):

http://www.realclimate.org/index.php?p=181

In the particular simulation shown above, the frequency of the strongest
(category 5) hurricanes roughly triples in the anthropogenic climate change
scenario relative to the control. This suggests that hurricanes may indeed
become more destructive (1) as tropical SSTs warm due to anthropogenic
impacts.

But what about the past? What do the observations of the last century
actually show? Some past studies (e.g. Goldenberg et al, 2001) assert that
there is no evidence of any long-term increase in statistical measures of
tropical Atlantic hurricane activity, despite the ongoing global warming.
These studies, however, have focused on the frequency of all tropical storms
and hurricanes (lumping the weak ones in with the strong ones) rather than a
measure of changes in the intensity of the storms. As we have discussed
elsewhere on this site <http://www.realclimate.org/index.php?p0>,
statistical measures that focus on trends in the strongest category storms,
maximum hurricane winds, and changes in minimum central pressures, suggest a
systematic increase in the intensities of those storms that form. This
finding is consistent with the model simulations.

A recent study in Nature by Emanuel (2005) examined, for the first time, a
statistical measure of the power dissipation associated with past hurricane
activity (i.e., the "Power Dissipation Index" or "PDI"--Fig. 2). Emanuel
found a close correlation between increases in this measure of hurricane
activity (which is likely a better measure of the destructive potential of
the storms than previously used measures) and rising tropical North Atlantic
SST, consistent with basic theoretical expectations. As tropical SSTs have
increased in past decades, so has the intrinsic destructive potential of
hurricanes.

Figure 2. Measure of total power dissipated annually by tropical cyclones in
the North Atlantic (the power dissipation index "PDI") compared to September
tropical North Atlantic SST (from Emanuel, 2005).

http://www.realclimate.org/index.php?p=181

The key question then becomes this: Why has SST increased in the tropics? Is
this increase due to global warming (which is almost certainly in large part
due to human impacts on climate)? Or is this increase part of a natural
cycle?

It has been asserted (for example, by the NOAA National Hurricane Center)
that the recent upturn in hurricane activity is due to a natural cycle, e.g.
the so-called Atlantic Multidecadal Oscillation ("AMO"). The new results by
Emanuel (Fig. 2) argue against this hypothesis being the sole explanation:
the recent increase in SST (at least for September as shown in the Figure)
is well outside the range of any past oscillations. Emanuel therefore
concludes in his paper that "the large upswing in the last decade is
unprecedented, and probably reflects the effect of global warming." However,
caution is always warranted with very new scientific results until they have
been thoroughly discussed by the community and either supported or
challenged by further analyses. Previous analysis of the AMO and natural
oscillation modes in the Atlantic (Delworth and Mann, 2000; Kerr, 2000)
suggest that the amplitude of natural SST variations averaged over the
tropics is about 0.1-0.2 șC, so a swing from the coldest to warmest phase
could explain up to ~0.4 șC warming.

What about the alternative hypothesis: the contribution of anthropogenic
greenhouse gases to tropical SST warming? How strong do we expect this to
be? One way to estimate this is to use climate models. Driven by
anthropogenic forcings, these show a warming of tropical SST in the Atlantic
of about 0.2 - 0.5 șC. Globally, SST has increased by ~0.6 șC in the past
hundred years. This mostly reflects the response to global radiative
forcings, which are dominated by anthropogenic forcing over the 20th
Century. Regional modes of variability, such as the AMO, largely cancel out
and make a very small contribution in the global mean SST changes.

Thus, we can conclude that both a natural cycle (the AMO) and anthropogenic
forcing could have made roughly equally large contributions to the warming
of the tropical Atlantic over the past decades, with an exact attribution
impossible so far. The observed warming is likely the result of a combined
effect: data strongly suggest that the AMO has been in a warming phase for
the past two or three decades, and we also know that at the same time
anthropogenic global warming is ongoing.

Finally, then, we come back to Katrina. This storm was a weak (category 1)
hurricane when crossing Florida, and only gained force later over the warm
waters of the Gulf of Mexico. So the question to ask here is: why is the
Gulf of Mexico so hot at present - how much of this could be attributed to
global warming, and how much to natural variability? More detailed analysis
of the SST changes in the relevant regions, and comparisons with model
predictions, will probably shed more light on this question in the future.
At present, however, the available scientific evidence suggests that it
would be premature to assert that the recent anomalous behavior can be
attributed entirely to a natural cycle.

But ultimately the answer to what caused Katrina is of little practical
value. Katrina is in the past. Far more important is learning something for
the future, as this could help reduce the risk of further tragedies. Better
protection against hurricanes will be an obvious discussion point over the
coming months, to which as climatologists we are not particularly qualified
to contribute. But climate science can help us understand how human actions
influence climate. The current evidence strongly suggests that:

(a) hurricanes tend to become more destructive as ocean temperatures rise,
and

(b) an unchecked rise in greenhouse gas concentrations will very likely
increase ocean temperatures further, ultimately overwhelming any natural
oscillations.

Scenarios for future global warming show tropical SST rising by a few
degrees, not just tenths of a degree (see e.g. results from the Hadley
Centre model <http://www.realclimate.org/RC_HadCM3_tropical-SST.gif> and the
implications for hurricanes shown in Fig. 1 above). That is the important
message from science. What we need to discuss is not what caused Katrina,
but the likelyhood that global warming will make hurricanes even worse in
future.

Complete article, including diagrams and references:
http://www.realclimate.org/index.php?p=181

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Published by David Sunfellow
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