The following is from Judith Curry:
Whether changes in the characteristics of tropical storms observed in the last few decades [1,2] are the result of only natural variability, due to climate change, or a combination of both factors is the subject of intense debate . Central to the debate is the quality of the tropical cyclone data [3,4].
Here we examine what is inarguably the most reliable data in the global tropical cyclone database: the data in the North Atlantic since 1944-5 [4,5]. Since 1944, aircraft reconnaissance flights have been made in nearly all of the North Atlantic tropical cyclones, and since 1970 satellite observations have made observing and monitoring tropical cyclones even more accurate. The data are obtained from the NOAA National Hurricane Center best track data, and are plotted in Figure 1. To reveal the decadal and longer term variability and eliminate the year-to-year variability (e.g. El Nino), an 11-year running mean has been applied to the data. The figure addresses another issue of contention regarding the North Atlantic tropical cyclone data has recently emerged [1,4]: whether or not Landsea’s6 adjustment to the intensity of major (categories 3, 4, 5) hurricanes prior to 1970 should be made. In Figure 1, the data are presented both with (solid) and without (dash) the intensity adjustment prior to 1970 recommended by Landsea6.
Figure 1a shows the time series in the North Atlantic since 1994 of the numbers of named storms (tropical cyclones), hurricanes, and category 4+5 hurricanes. Figure 1b shows the time series of accumulated cyclone energy (ACE) and potential destructive index1 (PDI), which are integral measures of tropical cyclone activity that include the number, intensity and duration of tropical cyclones. All of the measures of tropical cyclone activity shown in Figure 1 indicate that the period 1944-1964 was associated with elevated tropical cyclone activity and the period 1965-1994 was associated with relatively low activity. The period since 1995 shows tropical cyclone activity that is elevated substantially relative to the active period of 1944-1964.
No attempt is made to determine a linear trend in the time series 1944-2005, owing to the combination of multidecadal natural variability in the North Atlantic and the nonlinearity of the global temperature variations during that period3. Instead, we compare the annual average statistics for the recent period of elevated activity since 1995 with averages from the previous period of elevated activity during 1944-1964. Relative to the previous active period 1944-1964, the period since 1995 has averaged (annually) 50% more named storms, 37% more hurricanes, 167% more category 4+5 storms, 55% greater ACE, and 63% greater PDI. If Landsea’s intensity adjustment for major hurricanes is not used, the differences between periods are not as large, but are still substantial: 44% more category 4+5 storms, 41% greater ACE, and 31% greater PDI.
This analysis indicates that North Atlantic tropical cyclone data are consistent with assertions1-3 that a warming sea surface temperature induced by greenhouse warming is contributing to an increase in the intensity and number of North Atlantic tropical cyclones.
Figure 1: Seasonal tropical cyclone data, filtered by an 11-year running mean. Data are obtained from http://www.aoml.noaa.gov/hrd/hurdat/. 1A: total number of named storms (blue), hurricanes (red), and category 4 + 5 hurricanes. 1B: accumulated cyclone energy (ACE; blue) and potential destructive index (PDI; red). Dashed lines reflect values that do not include the Landsea correction8.
(1) Emanuel, K., 2005, Nature, 436, 686-688 (2005).
(2) Webster, P.J., G.J. Holland, J.A. Curry, and H.-R. Chang, 2005, Science, 309, 1844-1846 (2005).
(3) Curry, J.A. P.J. Webster, G.J. Holland, 2006, Bull. Amer. Meteorol. Soc., 87, 1025-1037 (2006)
(4) Landsea C.W., B.A. Harper, K. Horau, J.A. Knaff, Science, 313, 452-454 (2006)
(5) Landsea, C.W., R.A. Pielke, A. Mestas-Nunez, J.A. Knaff, Clim. Change, 42, 89-129 (1999)
(6) Landsea, C.W., Mon. Weather Rev., 121, 1703-1713 (1993)