May 26, 2006

Hurricane/Global Warming Linkage Takes Another Hit

Filed under: Hurricanes

We have covered many papers in the recent scientific literature that do not support the hypothesis that global warming has led, or will lead, to large changes in the intensity of tropical cyclones (see here and here and here). Michaels et al. (2006), Pielke Jr. et al. (2006), and Hoyos et al. (2006), all present evidence that the tropical cyclone regime, at least in the Atlantic basin, during the past 20-30 years, is a complex combination of the interactions of several different environmental factors that include sea surface temperatures (SST), vertical wind shear, and atmospheric stability, among others. The variations and trends of these parameters are often not what has been projected by models for anthropogenerated global warming. The climate models also project far more modest changes in hurricane intensity than are being observed. This is further evidence that factors other than those directly related to anthropogenic climate change are influencing observed trends and variations in tropical cyclones. These other factors include cyclical, or quasi-cyclical, oscillations as well as possible observational biases in the record resulting from changing technology and observing practices that have evolved over the past century or so.

These results do not support the hypotheses of Emanuel (2005) and Webster et al. (2005) that link large changes in the intensity of tropical cyclones primarily to increased SST caused by global warming

The latest iteration was just published by Philip Klotzbach, a paper titled “Trends in global tropical cyclone activity over the past twenty years (1986-2005)” in the journal Geophysical Research Letters (GRL). Klotzbach examined “worldwide tropical cyclone frequency and intensity to determine trends in activity over the past twenty years during which there has been an approximate 0.2-0.4ºC warming of SSTs.” He found “a large increasing trend in tropical cyclone intensity and longevity for the North Atlantic basin and a considerable decreasing trend for the North Pacific.” Other tropical cyclone-producing ocean basins showed only small variations (Figure 1).

Figure 1. Accumulated Cyclone Energy (ACE) index values for the ocean basins examined by Klotzbach. The ACE index is a measure of the energy contained in the tropical cyclone over its lifetime. There has been an increase in the North Atlantic, a decrease in the Northeast Pacific, and not much long-term change anywhere else. (Source: Klotzbach, 2006)

Overall, Klotzbach noted “no significant change in global net tropical cyclone activity” but a “small increase in global Category 4-5 hurricanes from the period 1986-1995 to the period 1996-2005.” From this analysis, he concluded that factors other than SSTs are important in governing tropical cyclone frequency and intensity (Figure 2) and noted the likelihood that “improved observational technology” has had an influence on the small increases that he did observe.

Figure 2. (top) Global and hemispheric ACE index values from the combination of the individual basins, (bottom) tropical SST anomalies. While there has been a rise in tropical SSTs, there has not been a concomitant rise in global ACE values. (Source: Klotzbach, 2006)

Klotzbach ultimately summed up his findings as:

These findings are contradictory to the conclusions drawn by Emanuel (2005) and Webster et al. (2005). They do not support the argument that global TC [tropical cyclone] frequency, intensity and longevity have undergone increases in recent years. Utilizing global ‘‘best track’’ data, there has been no significant increasing trend in ACE [accumulated cyclone energy] and only a small increase (~10%) in Category 4–5 hurricanes over the past twenty years, despite an increase in the trend of warming sea surface temperatures during this time period.

The results of this paper are more in line with a prior study by Shapiro and Goldenberg (1998) and a project report by Gray and Klotzbach (2005). Shapiro and Goldenberg (1998) showed only marginally significant correlations between SSTs in the tropical Atlantic and major hurricane development in the basin. Vertical wind shear was shown to be a much more fundamental component for major hurricane development and maintenance. Gray and Klotzbach (2005), while developing seasonal hurricane forecasts for TC activity, found only a modest correlation (~0.4) between seasonal and monthly Atlantic basin SSTs and major (Category 3–4–5) hurricane frequency. This study indicates that, based on data over the last twenty years, no significant increasing trend is evident in global ACE or in Category 4–5 hurricanes.

The more you look, the less obvious it becomes that anthropogenic global warming has significantly (i.e., measurably) contributed to the current increase in hurricane activity in the North Atlantic basin, or anywhere else in the world, for that matter.

References:

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

Hoyos, C.D., et al., 2006. Deconvolution of the factors contributing to the increase in global hurricane intensity. Science, 312, 94-97.

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.

Michaels, P.J. et a.l, 2006. Sea-surface temperatures and tropical cyclones in the Atlantic basin. Geophysical Research Letters, 33, doi:10.1029/2006GL025757.

Pielke Jr., R. A., et al., 2006. Reply to “hurricanes and Global Warming—Potential Linkages and Consequences”. Bulletin of the American Meteorological Society, 87,628-631.

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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