June 5, 2007

Tropical Cyclones Decreasing in China?

Filed under: Climate Extremes, Hurricanes

The greenhouse crowd had a field day following the active North Atlantic hurricane season of 2005, and they continue to do their best blaming any unusual tropical storm activity on global warming. Katrina remains the poster child for the link between warmer conditions and hurricanes. In the Gore film, Al explains how simple it is – warmer water will generate more storms and storms that are more powerful (and then run the Katrina footage – it seems to work every time). However, the North Atlantic hurricane season of 2006 was somewhat of a dud, so the blame machine is more than ready to go in 2007. You may have heard that tropical storm Andrea formed weeks before the 2007 official hurricane season (June 1 – November 30) got underway, and now with tropical storm Barry, we have two storms early on, and of course, global warming is to blame.

Perhaps not.

The 2007 Intergovernmental Panel on Climate Change (IPCC) report (Climate Change 2007: The Physical Science Basis) is now out, and in their Summary for Policymakers we find “There is observational evidence for an increase in 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.” So despite the alarmist hype, the IPCC continues to insist that there is no trend upward in the number of tropical cyclones.

A recent issue of the International Journal of Climatology contains an article providing more evidence in support of this stance.

The article is by three Chinese atmospheric scientists who explored tropical cyclone records and precipitation data from an area around Hainan Island. In case you are rusty in your geography of China, Hainan Island (Figure 1) in the far southeastern part of the country across the Gulf of Tonkin from Vietnam. Wu et al. note that Hainan Island is “one of the regions most frequently and seriously affected by TCs in China” and “TC precipitation is Hainan Island’s main water source, as well as the frequent cause of severe flood disasters.”

Figure 1. Tropical cyclone potential impact area for Hainan Island (from Wu et al., 2007).

Wu et al. collected two major datasets for their study. The first dataset came from the Shanghai Typhoon Institute and contained the 6-hourly positions of tropical cyclones from 1949-2005. This is one of the most populated areas of the world, and likelihood of an event being missed is slim to none. The second dataset consisted of daily precipitation records from 1962 to 2005 from 18 stations located around China’s Hainan Island.

Figure 2 shows some of there results—a plot of the number of tropical cyclones shows a highly statistically significant decrease over the period of historical records. In commenting on the figure below, Wu et al. note “A statistically significant decreasing trend at a rate −0.9 per decade (equivalent to 13% reduction per decade) is obvious in the time series.” Furthermore, they write “Before 1978, the number of impacting TCs was greater than nine. After 1978, however, there were only 2 years (1989, 1996) when the impacting TCs were more than nine. There were only 4 years in the period 1962–2005, in which TCs made no impact on the land on the Hainan Island–and these all occurred after 1978.” They obviously missed the memo from the greenhouse crusade about how hurricanes, typhoons, and tropical cyclones are increasing – time for another showing of the Gore film!

Figure 2. Variation in number of tropical cyclones impacting Hainan Island; thin smooth line is 11-point binomial filtered (from Wu et al., 2007).

Regarding rainfall from tropical cyclones (Figure 3), Wu et al. note “A slight decreasing trend is in the annual TC rainfall in the period studied. This was computed at a rate of −1.9% per decade. It should be noted that this trend is more pronounced since the 1970s.” In addition, they write “Owing to the impact of significant reduction in TC frequency, a decreasing trend at a rate of about 3% per decade was found in the proportion of TC impacting precipitation in the annual total.”

Figure 3. Variation in proportion of tropical cyclone precipitation in the total expressed as annual tropical cyclone precipitation divided by annual precipitation total in Hainan Island. Smooth lines are 11-point binomial filtered (from Wu et al., 2007).

We have reported similar findings from throughout the world, but as the 2007 hurricane gets started, we will surely be bombarded with the message that global warming is causing more hurricanes worldwide—yet historical evidence proves otherwise.


Wu, Y., S. Wu, and P. Zhai. 2007. The impact of tropical cyclones on Hainan Island’s extreme and total precipitation. International Journal of Climatology, DOI: 10.1002/joc.1464.

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