Background
The authors write that "the homogeneity of historical observations is important in the study of tropical cyclones (TCs) and climate change," with "a large hurdle for climate change detection" being "the quality of TC historical databases" that they say "were populated over time without a focus on maintaining data homogeneity," which is obviously "a key requirement for databases that are used to assess possible climate-related trends."

What was done
In an effort to overcome this hurdle, which they describe as "a 'bottleneck' in tropical cyclone and climate change studies," Ren et al. took it upon themselves to carefully analyze three historical datasets for Western North Pacific TCs - those of the Joint Typhoon Warning Center (JTWC), the Japan Meteorological Agency (JMA) and the China Meteorological Administration (CMA) - focusing primarily on TC intensity and covering the 55-year period 1951-2005.

What was learned
After all their work, the five researchers say "it is still difficult to judge which one [of the three datasets] is best." However, they indicate that frequencies of the common TCs in all three datasets "show no obvious increasing or decreasing trend over the past 50 years." Instead, they find a weak inter-decadal variation with "more TCs from the mid-1960s to the mid-1970s and in the early 1990s." On the other hand, they state that the intensities of the common TCs "differed largely from one dataset to another, leading to quite opposite conclusions for TCs of category 4 and 5." In regard to this latter subject, for example, they say that "for the period after 1970, the JTWC dataset shows an increasing trend that complies with those of Webster et al. (2005) and Emanuel (2005)," but they say that "for a longer time scale, the result may be well consistent with that of Chan (2006)," which suggests that "the so-called 'trend' is a fragment of the longer inter-decadal variation."

What it means
All things considered, it would appear that there is no compelling evidence for an increase (or decrease) in the long-term trend of category 4 and 5 TCs in the Western North Pacific, although Ren et al. acknowledge that further studies and analyses may yet be needed to fully put the question to rest.

References
Chan, J.C.L. 2006. Comment on "Changes in tropical cyclone number, duration, and intensity in a warming environment." Science 311: 1713.

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

Webster, P.J., Holland, G.J., Curry, J.A. and Chang, H.-R. 2005. Changes in tropical cyclone number, duration, and intensity in a warming environment. Science 309: 1844-1846.