NOTE: Golly, this is embarrassing, in my sloppiness, I seem to have forgotten about the Northern Indian ocean. Unfortunately the North Indian does not seem to have completely reliable data over the period in question (apparently before 1990 they only reported twelve hourlies instead of six) NEVERTHELESS you shall see that this update doesn’t change anything about the conclusions. This suggests that the lack of increase in 4’s and 5’s is “robust” in the Team sense.
One of the common claims made about global warming is that there will be an increase in the frequency of the strongest tropical cyclones in a warmer world, and this is already evident[1, 2]. These claims have been criticized on the grounds that early observational data is not as accurate as present data and ignoring decadal variability[4, 5]. Homogeneous satellite data fail to corroborate the large claimed trend, although they may indicate some intensity increase over the short period (note that they are thus still problematic due to decadal variability)[6, 7]. A previous look at data since 1986 did identify a small increase in the number of category 4 and 5 storms in the North Hemisphere, and a larger but more suspect trend in the Southern Hemisphere. Theoretically speaking, one might expect, all other things being equal, that hurricanes would become slightly more intense over warmer sea surface temperatures, in order for this to play out in the real world, the factors other than sea surface temperature must not counteract this tendency, which has not been shown-indeed, the “Maximum Potential Intensity” is not well correlated with TC activity except in the North Atlantic and it appears that while local warming may enhance activity, it does so only if it occurs more so than the tropical mean[11, 12] since widespread warming tends to increase “outflow” temperatures that act against increasing MPI. It is also worth pointing out that trends which have significance for society must pan out in terms of landfalling statistics, which have been under examined in terms of intensity. So even the small increases now claimed, if really due to sea surface warming, which has not been demonstrated, may have no significance for society, and if they do, because they are not large trends their importance may not manifest in terms of actually additional damages for some time, if at all.
Here, I will examine the issue of increases in the number-but not the proportion-of “intense” storms (category four and five on the Saffir Simpson Scale). Since 1987, the Western North Pacific basin has been entirely reliant on satellite monitoring due to discontinuing the Aircraft reconnaissance period, and satellite monitoring of the Atlantic and Eastern Pacific basin appears to have been adequate and fairly similar throughout this period. The Northern Indian appears problematic in the earliest years, but is nevertheless included as it does not appear to effect the results either way. In the North Hemisphere, there has been plenty of tropical warming over this period, so if there were an increase in storm intensity as a consequence in should be obvious. The presence of such a trend would not prove causation by warming (because of the decadal variability issue), but the absence of such a trend or a decrease would speak volumes about the validity of that hypothesis. Although this period is shorter than examined by the Elsner et al study again if that studies trend is actually related to steadily rising sea surface temperatures, it shouldn’t be confined entirely to 1981-86.
Figure one shows the number of category four and five tropical cyclones in the Eastern North Pacific, Western North Pacific, Northern Indian, and North Atlantic basins summed together over the 1987-2009 period according to “best track” data and Joint Typhoon Warning Center/HURDAT intensities, and an OLS fit line. As can be seen there is a slight decrease over this period despite significant warming, although the downward trend is obviously so small as to be insignificant.
Fig 1-Category 4 and 5 Tropical Cyclones in the North Atlantic, Western North Pacific, Northern Indian, and Eastern North Pacific from 1987 to 2009, with linear trend
The reason for the lack of increase in the Northern Hemisphere is primarily that the Eastern Pacific has seen a decline in these intense cyclone frequencies. The reasons for this tendency need to be better understood before attempting to attribute increases elsewhere to warming, since it appears that whatever has caused a decrease there must be some natural phenomenon which probably plays a role in the other basins. Indeed, it has been observed that overall activity in the ENP and NATL basins behave in complementary ways-when one is active, the other is inactive, and vice versa.
None of this is contradictory to the recent “consensus” statement by the research community that “it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes.”
With regard to the longer term, further study must be done to try and get homogeneous datasets as far back as possible, and the causes of decadal variability better understood.
Webster, P. J., G. J. Holland, J. A. Curry, and H.-R. Chang (2005), Changes in tropical cyclone number and intensity in a warming environment, Science, 309, 1844–1846.
Emanuel, K. A. (2005), Increasing destructiveness of tropical cyclones over the past 30 years, Nature, 326, 686– 688.
Landsea, C.W., B.A. Harper, K. Hoarau, J.A. Knaff. 2006. Can We Detect Trends in Extreme Tropical Cyclones? Science, 313, 452-454.
Chan, J. C. L., 2008: Decadal variations of intense typhoon occurrence in the western North Pacific. Proc. R. Soc. A, 464, 249−272.
Goldenberg, S. B., C. W. Landsea, A. M. Mestas-Nuñez, and W. M. Gray (2001), The recent increase in Atlantic hurricane activity: Causes and implications, Science, 293, 474– 479.
Kossin, J. P., Knapp, K. R., Vimont, D. J., Murnane, R. J. & Harper, B. A. A globally consistent reanalysis of hurricane variability and trends. Geophys. Res. Lett. 34, L04815 (2007).
Elsner, J. B., Kossin, J. P. & Jagger, T. H. The increasing intensity of the strongest tropical cyclones. Nature 455, 92–95 (2008).
Klotzbach, P.J., 2006. Trends in global tropical cyclone activity over the past twenty years (1986-2005). Geophysical Research Letters, 33, L010805, doi:10.1029/2006GL025881.
Emanuel, K. A. The dependence of hurricane intensity on climate. Nature 326, 483–485 (1987).
Chan, J.C.L. 2009. Thermodynamic control on the climate of intense tropical cyclones. Proceedings of the Royal Society A, 465, 3011-3021.
Swanson, K.L, 2007. Impact of scaling behavior on tropical cyclone intensities. Geophysical Research Letters, 34, doi:10.1029/2007GL030851.
Vecchi, G.A. and B.J. Soden. 2007. Effect of remote sea surface temperature change on tropical cyclone potential intensity. Nature, 450, 1066-1071.
Knapp, K. R., M. C. Kruk, D. H. Levinson, H. J. Diamond, and C. J. Neumann, 2010: The International Best Track Archive for Climate Stewardship (IBTrACS): Unifying tropical cyclone best track data. Bulletin of the American Meteor. Society, In press.
Maue, R. N. (2009), Northern Hemisphere tropical cyclone activity, Geophys. Res. Lett., 36, L05805, doi:10.1029/2008GL035946.
Knutson, T. R. et al (2010), Tropical cyclones and climate change, Nature Geoscience 3, 157 – 163, doi:10.1038/ngeo779