Pielke argues that any real trend like that found by Emanuel should be accompanied by a corresponding increase in coastal damage normalized for inflation and level of development:
If hurricanes are indeed becoming
more destructive over time, then this trend
should manifest itself in more destruction.
However, my analysis of a long-term data set
of hurricane losses in the United States shows
no upward trend once the data are normalized
to remove the effects of societal changes.
He offers an analysis indicating that such a correlation does not exist, and concludes that either Emanuel is in error, or else total power dissipation is not an interesting measure with regard to societal impact:
These loss data indicate two possibilities
with respect to Emanuel’s analysis1: if the
power-dissipation index metric is an accurate
indicator of hurricane destructiveness, then
the trend identified by Emanuel could be an
artefact of the data and/or methods; alternatively,
the trend he identifies is an accurate
reflection of trends in the real-world characteristics
of storms, but the power-dissipation
index is a weak indicator of hurricane destructiveness
— which would call for the identification
of climate metrics more directly
associated with societal outcomes. In any case,
it is misleading to characterize Emanuel’s
results as indicating an increase in “destructiveness”
or as an indication of future increases
in destruction resulting from changes in the
power-dissipation index.
Emanuel replies:
In my original Article1, I showed that there has
been a significant upward trend in a measure
of tropical-cyclone power dissipation over the
past 30 years1. It is important to note that this
measure is integrated over the life of the storm,
and that the upward increase is evident in all
major ocean basins prone to tropical cyclones.
However, Pielke2 finds no discernible trend in
hurricane damage in the United States after
correction for inflation and demographic
trends, and Landsea3 finds no trend in US
landfall-based hurricane power dissipation
back to the turn of the last century.
Pielke suggests that this apparent disparity
could be explained if the power-dissipation
trend I find is an artefact of the data and/or
analysis methods, or if the trend is accurate but
not a good predictor of damage. As this trend
is large and universal — having about the same
value in all the major ocean basins, despite different
measurement techniques — and as it is
well correlated with sea surface temperature
(SST), which is relatively well measured, I
stand by my conclusions about the trends in
tropical-cyclone power dissipation.
I cannot discount the second of Pielke’s conjectures,
but the reason for the disparity may
be more prosaic. Although Atlantic hurricanes
do most of their destruction within 6–12
hours after landfall, they last for an average of
180 hours; moreover, only a fraction of hurricanes
ever affect the US coastline. This means
that the power-dissipation index (PDI) I used,
which is accumulated over all storms and over
their entire lives, contains about 100 times
more data than an index related to wind
speeds of hurricanes at landfall. There is large
variability in wind speed over the life of each
storm and large storm-to-storm random variability:
detecting a temporal trend in the presence
of this variability requires separation of
the signal from the noise. With 100 times more
data, my index has a signal-to-noise ratio that
is ten times that of an index based on landfalling
wind speeds. It is therefore possible that
the real trend is detectable in the power dissipation
but not in landfalling statistics.
Christopher Landsea leads off with this interesting statement:
Anthropogenic climate change has the potential
for slightly increasing the intensity of tropical
cyclones through warming of sea surface
temperatures.
Which strikes me as a major concession relative to some of the quotes and/or representations of his and some of his colleagues' positions on this issue.
Emanuel replies:
Landsea starts by saying that increasing
SST has the potential for “slightly” increasing
the intensity of tropical cyclones. But, as I discussed1,
the existing theory and modelling5 on
which this assertion is based suggest that the
predicted ~2 C increase in tropical SST would
increase wind speeds by 10% and, accounting
for increased storm lifetime, increase power
dissipation by 40–50%. This is hardly slight.
The existing theory and modelling work5 are
limited, however, in that they do not account
for changes in environmental conditions, such
as wind shear, and so only provide a loose
guide as to what to expect.
Landsea goes on to raise specific issues with Emanuel's methodology:
My first concern is that Emanuel’s figures2
do not match their description: his Figs 1–3
aim to present smoothed power-dissipation
index (PDI) time series with two passes of a
1-2-1 filter, but the end-points — which are
crucial to his conclusions — instead retain
data unaltered by the smoothing; this is
important because the last data point plotted
in Emanuel’s Fig. 1 is far larger than any other
portion of the time series. Even after adding
last year’s busy hurricane season into the
analysis and then properly using the filter,
as described, the crucial end-point of the
smoothed time series no longer jumps up dramatically
in the last couple of years (Fig. 1a).
About one-third of the increase in Atlantic
PDI in Emanuel’s graph for the past ten years
is incorrect owing to inappropriate plotting
of the data, even if the active 2004 season is
incorporated.
Emanuel agrees with this point, but adds that treating the endpoints correctly but adding in 2005 still shows a significant increase:
Landsea correctly points out that in applying
a smoothing to the time series, I neglected
to drop the end-points of the series, so that
these end-points remain unsmoothed. This
has the effect of exaggerating the recent
upswing in Atlantic activity. However, by
chance it had little effect on the western Pacific
time series, which entails about three times as
many events. As it happens, including the 2004
and 2005 Atlantic storms and correctly dropping
the end-points restores much of the
recent upswing evident in my original Fig. 1
and leaves the western Pacific series, correctly
truncated to 2003, virtually unchanged. Moreover,
this error has comparatively little effect
on the high correlation between PDI and SST
that I reported.
Landsea continues:
A second concern is the bias-removal
scheme used to alter the data for the Atlantic
for 1949–69. Emanuel can demonstrate
“unprecedented” activity in the past ten years
only by markedly reducing the tropicalcyclone
winds for the first two decades of the
time series. He attempts to use a bias-removal
scheme3 that recommends reduction of the
tropical-cyclone winds by 2.5–5.0 m s1 for
the 1940s–60s because of an inconsistency in
the pressure–wind relationship during those
years compared with subsequent (and presumably
more accurate) data. However, the
function used by Emanuel to reduce the winds
in the earlier period goes well beyond this
recommendation, as the bias removal used
continued to increase with increasing wind
intensity and reached a reduction of as much
as 12.2ms1 for the strongest hurricane in the
1949–69 original data set.
(there's a substantial amount more to this argument, but I won't quote the whole thing since this firs paragraph expresses the gist of it)
Emanuel accepts this modification to his methodology as well, but goes on to argue that nonetheless his analysis indicates a strong indication of an emeging global warming signal in global hurricane intensity:
In correcting for biases in the original
Atlantic tropical-cyclone data, I relied on a
bias correction applied by Landsea6, presented
as a table. I had fitted a polynomial to that
correction, as I felt that a continuous rather
than discrete correction was more defensible.
Landsea believes that this had the effect of
overcorrecting the most intense storms in the
pre-1970 record, and I accept his revision to
my analysis (Fig. 1b of ref. 3).
The Atlantic hurricane-intensity record by
itself is not long enough to infer any connection
between hurricanes and either global
warming or multi-decadal cycles, but the high
correlation between hurricane activity and
tropical SST is remarkable (and largely unaffected
by the corrections discussed), and the
SST record is long enough to show the influence
of global warming. To detect correlations
with hurricane activity, tropical cyclones in the
North Atlantic can be counted, assuming that
detection of the presence of a storm by ships
and islands is reliable (although intensity estimation
is dubious before the mid-1940s). This
count is highly correlated with both tropical
Atlantic SST and Northern Hemispheric mean
surface temperature through the entire record,
casting doubt on whether the recent multidecadal
variability in tropical SST and hurricane
activity is due purely to natural causes, as
Landsea implies.
I maintain that current levels of tropical
storminess are unprecedented in the historical
record and that a global-warming signal is
now emerging in records of hurricane activity.
This is especially evident when one looks at
global activity and not just the 12% of storms
that occur in the Atlantic. But I agree that there
is a pressing need for a storm-by-storm
reanalysis of tropical cyclones, not only in the
North Atlantic, but also in the western North
Pacific, where aircraft reconnaissance records
also extend back to the 1940s.
The full exchange has been posted by Pielke at http://sciencepolicy.colorado.edu/prometheus/archives/nature04477.pdf
If that gets taken down (Nature may object to his posting this) I have a copy which I'd be willing to email to those interested.
I commend this to everyone as an example of how real scientific disputes are discussed (as opposed to the wild claims of "fraud" etc. which have become all too common in popular discussion of these issues).
Jan
