Since Typhoon Haiyan is in the news, it seems timely to look at what the IPCC AR5 WG1 report says on this subject. The word “typhoon” is used to describe strong cyclones in the West Pacific – apparently it is just the Chinese for “big wind”.
The key graph plotting the numbers of cyclones and typhoons is Figure 2.34:
It is quite clear from this graph that there has been no increase. The IPCC says so:
The Executive Summary at the start of Chapter 2 says
“Confidence remains low for long-term (centennial) changes in tropical cyclone activity”
and page 2-60 says
“Current datasets indicate no significant observed trends in global tropical cyclone frequency over the past century”.
In the Summary for Policymakers, the IPCC says that there has been an increase in intense tropical cyclones in the North Atlantic since 1970. Looking at the above graphs, this seems to be a severe case of “start-date cherry-picking”, combined with “regional cherry-picking”.
Update: It has been pointed out that the concern is more that the intensity of cyclones may increase, rather than their frequency. Here’s what the IPCC says in Ch 14 (which is about regional projections):
“it is likely that the global frequency of occurrence of tropical cyclones will either decrease or remain essentially unchanged, concurrent with a likely increase in both global mean tropical cyclone maximum wind speed and precipitation rates.”
It goes on to say that there is low confidence in regional projections of frequency and intensity.
Elsner et al (2008) gives some evidence of increasing intensity in the Atlantic over the last 30 years, but their graphs show no trend in the Pacific region.