February 27, 2007

Global Hurricane Intensity NOT Increasing

Filed under: Hurricanes

Global hurricane intensity not increasing…so concludes a just-published paper by University of Wisconsin atmospheric scientist Jim Kossin and colleagues. In order that we can’t be accused of misrepresenting the authors’ meaning, here is the complete conclusion section of their Geophysical Research Letters paper [we added emphasis just for fun –eds]:

The time-dependent differences between the UW/NCDC and JTWC best track records underscores the potential for data inconsistencies to introduce spurious (or spuriously large) upward trends in longer-term measures of hurricane activity. Using a homogeneous record, we were not able to corroborate the presence of upward trends in hurricane intensity over the past two decades in any basin other than the Atlantic. Since the Atlantic basin accounts for less than 15% of global hurricane activity, this result poses a challenge to hypotheses that directly relate globally increasing tropical SST to increases in long-term mean global hurricane intensity.

Efforts are presently underway to maximize the length of our new homogeneous data record but at most we can add another 6–7 years, and whether meaningful trends can be measured or inferred in a 30-year data record remains very much an open question. Given these limitations of the data, the question of whether hurricane intensity is globally trending upwards in a warming climate will likely remain a point of debate in the foreseeable future. Still, the very real and dangerous increases in recent Atlantic hurricane activity will no doubt continue to provide a heightened sense of purpose to research addressing how hurricane behavior might change in our changing climate, and further efforts toward improvement of archival data quality are expected to continue in parallel with efforts to better reconcile the physical processes involved. If our 23-year record is in fact representative of the longer record, then we need to better understand why hurricane activity in the Atlantic basin is varying in a fundamentally different way than the rest of the world despite similar upward trends of SST in each basin.

All we can say is “oh my.” Can it be possible that the small band of global warming alarmists who is going around pushing the concept that global warming has led to measurably more dangerous hurricanes is wrong? Horrors.

Let’s see what the IPCC has to say about this. Here is the section on hurricanes from their recent Summary for Policymakers (SPM):

There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones.

Well, it looks like the SPM authors decided to play it a bit cautious. Only reporting “suggestions” of increases in oceans basins other than the Atlantic. Wise move. These suggestions now seem to be wrong.

Certainly it is true that tropical cyclones have increased in frequency and intensity in the Atlantic Ocean over the past 30 years. And certainly sea surface temperatures have risen there as well. But as we have been harping on over and over and over again, this does not mean that anthropogenic alterations to the earth’s atmospheric composition are the primary reason why (see any of the articles listed here). Many other factors besides sea surface temperatures impact the tendency for weak tropical disturbances to grow into fierce hurricanes. And many of these other factors have been trending in a way that is towards favoring stronger storms, but that is away from the direction that global warming should be taking them. Yes, you read it correctly, aside from increasing sea surface temperatures, anthropogenic climate changes from increasing greenhouse gas concentrations are generally projected to make the tropical environment less favorable for tropical cyclone intensification. Nobody told you that one before, did they (besides us, of course)?

What about the high profile results from papers in Nature (Emanuel, 2005) and Science (Webster et al, 2005) that made a lot of noise that intense hurricanes were increasing the world over and that anthropogenic global warming was to blame? Well, according to the new results by Kossin et al., and as has been suggested by Landsea (2006) and others, it looks like they were the result of a reliance on data of poor quality data. The figure below, from Kossin et al., shows a comparison of their results from a consistent application of a new procedure for determining hurricane intensity during the period 1984-2005, against the results of using the currently existing data that apparently suffers from data quality issues (the data that underlies the analyses of Emanuel and Webster et al., and virtually everyone else).


Figure 1. A global comparison of Kossin et al.’s new hurricane analysis (blue lines) against the existing data (red lines) for Power Dissipation Index (top), the number of category 4 and 5 hurricanes (middle), and the percentage of category 4 and 5 hurricanes (bottom). (Source: Kossin et al., 2007).

Notice how the blue curves (from the Kossin data) show a decidedly lack of upward trend since 1984, instead, they are dominated by decadal variability.

Sure, individual scientists can have individual opinions about the results and implications of their work and that of others. And, to be sure, the issue of how anthropogenic global warming may be, or may someday be, impacting hurricanes is far from being settled. But the one thing that appears to be quite settled, is that currently, anyone who insists that present-day hurricanes are definitely and measurably being made worse the world over because of human emissions is outside of the consensus of our best scientific understanding and as such, represents an extremist’s viewpoint on this issue. Don’t let them try to convince you otherwise.

References:

Emanuel, K., 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 686-688.

Kossin, J.P., et al., 2007. A globally consistent reanalysis of hurricane variability and trends. Geophysical Research Letters, 34, L4815, doi: 10.1029/2006GL028836.

Landsea, C.W., B.A. Harper, K. Hoarau, J.A. Knaff. 2006. Can We Detect Trends in Extreme Tropical Cyclones? Science, 313, 452-454.

Webster, P. J., et al., 2005. Changes in tropical cyclone number, duration, and intensity in a warming environment. Science, 309, 1844-1846.




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