October 27, 2006

Reverse Spin on Tropical Cyclones

Filed under: Climate Extremes, Hurricanes

Look at any popular presentation on the threat of global warming, and it is highly likely you will be led to believe that the buildup of greenhouse gases and the accompanying rise in planetary temperature will surely increase hurricane activity in the years to come. Who could ever forget the drumbeat during and after Katrina from the global warming crowd; Al Gore was all over this concept in his movie. Somewhat surprisingly to most, the Intergovernmental Panel on Climate Change (IPCC) concluded in the Summary for Policymakers “Changes globally in tropical and extra-tropical storm intensity and frequency are dominated by inter-decadal and multi-decadal variations, with no significant trends evident over the 20th century.”

What you call hurricanes, the professional climatologists around the world call tropical cyclones to avoid confusion regarding different names for the same meteorological phenomenon (e.g., “hurricanes” in the western Pacific are called typhoons). Anyway, a team of scientists from various Chinese agencies examined trends in the number of tropical cyclones and the rainfall from these events over the period 1957 to 2004. The results are published in a recent issue of Geophysical Research Letters, and given what they found, they are absolutely certain to get zero press coverage.

Ren et al. begin their article by covering the debate about increased tropical cyclone (TC) activity given the buildup of greenhouse gases, with special interest in the Western North Pacific (WNP). They note “Some studies indicate that there was no statistically significant long-term trend in both the number and the intensity of TCs in WNP over the past 4–5 decades.” They also write “However, two recent studies argued that TC intensity has increased markedly in recent decades.”

As good scientists, they decided to collect hard data on tropical cyclone activity to determine if trends exist in their datasets. Ren et al. write “The information about the WNP TC positions used in this study is from Shanghai Typhoon Institute of China Meteorological Administration (CMA). The data set ranges from 1949 to 2004 and is believed to be more accurate for those TCs that affected China because the positions have been carefully validated. The daily precipitation data set used in this study includes 677 stations in China that represented the spatial distribution of precipitation in China, in which 659 stations are over the mainland of China and the Hainan Island from National Meteorological Information Center/CMA and the other 18 stations are in the Taiwan Island.” After reviewing the data, they determined that missing data prior to 1957 might impact the accuracy of the study, so they limited their analyses to 1957-2004.

Figure 1 below will not be welcomed in many global warming circles, for it surely shows downward trends in the data. Regarding the total annual volume of rain from tropical cyclones in China, Ren et al. write “First, a downward trend can be found during 1957~2004, with a rate of -3.0 km3/yr. A Kendall test indicates that the trend is statistically significant (0.01 significance level).” Regarding variations in the total annual frequency of the torrential TCP events (≥50 mm/day), the team reports “A significant (0.05 level) decreasing trend can be clearly seen with similar interdecadal variations and year-to-year fluctuations” “for the annual TCP volume.” Not only is there no evidence of increasing tropical cyclone activity, the trends in the real-world data are downward and the trends are statistically significant!

Figure 1. Variations of TC precipitation: (a) total annual volume of TC precipitation for China (km3) and (b) accumulated number of times with torrential TCP events (≥50 mm/day) for individual stations (from Ren et al., 2006).

Next up, Ren et al. examined “the frequency of TCs and typhoons that affected China from 1957 to 2004. The frequencies of the influencing TCs and typhoons also display significant decreasing trends during the period, with 0.05 and 0.01 significant levels, respectively” (Figure 2). We at World Climate Report wonder if this figure will ever be shared with the public at large?

Figure 2. Variations of the frequency of influencing TCs and typhoons in China (from Ren et al., 2006)

To sum up their work, Ren et al. conclude “Together with interdecadal and interannual variations, significant downward trends are found in the TCP volume, the annual frequency of torrential TCP events, and the contribution of TCP to the annual precipitation over the past 48 years. The downward trends were accompanied with the decreases in the numbers of TCs and typhoons that affected China during the period 1957–2004. These changes strongly suggest that China has experienced decreasing TC influence over the past 48 years, especially in terms of the TCP.”

So last year, the tropical cyclone season was active in the Atlantic, two papers were published suggesting tropical cyclone activity was related to global warming, and the two papers were turned into front page news all over the world. This year’s tropical cyclone season has been quiet in the Atlantic, and the Ren et al. paper is published showing many tropical cyclone indicators are decreasing significantly over a substantial area of the western Pacific.

Sorry Ren et al, your “reverse spin” on tropical cyclone trends is simply not newsworthy!


Ren, F., G. Wu, W. Dong, X. Wang, Y. Wang, W. Ai, and W. Li, 2006. Changes in tropical cyclone precipitation over China, Geophysical Research Letters, 33, L20702, doi:10.1029/2006GL027951.

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