Does the hurricane get worse than before?

It seems like that we hear about hurricane news more often than before. (Well, Hurricane Gonzalo has just gone, nearly two weeks ago.) Maybe many people hold the thought that the environment gets worse, and then extend the scope of this perspective to all branches such as the climate system, atmosphere and water cycle. How about hurricanes? Does it get worse than before?

Frequency of hurricanes

Global frequency
According to the conclusion of the latest IPCC report, it is in low confidence that changes of tropical cyclone activities are obvious in long term. From global time series pictures below, although the data set is limited, it can be seen that there is no significant trend of hurricanes’ occurrences from 1970 to 2004. Based on the scenario from Knutson et al. 2010, the projection shows that the global frequency of tropical cyclones maybe will keep generally still or decrease when average intensity increases by 2~11% and rainfall rates of tropical cyclones increase by nearly 20% within 100km of the cyclone centre. (IPCC chapter 14)

Global time series for 1970-2004 of number of storms

Regional frequency

But since 1970, some places have significant increasing trends of intense tropical cyclone activities, North Atlantic should be mentioned here. Webster et al. 2005 indicates that North Atlantic Ocean has an obvious increasing frequency in comparison with regional time series of other oceans. And this trend has a very high confidence level, which reaches at 99%. Based on time series of various cyclone indices including the duration, the power dissipation, the intensity and a compound of tropical cyclone frequency, since the late 1970s, the rising trend is presented in North Atlantic and the slight rising trend is found in western North Pacific. (IPCC chapter 2) What should be noted is that the trend of annual numbers of tropical cyclones is not robust, which has been observed over past one hundred years, as well as in North Atlantic. The annual regional frequency of tropical cyclones projection has similar trends with the annual global frequency, but has lower confidence level.(IPCC chapter 14)

Factors

The tropical cyclone is a complicated system, so it is influenced by a mix of natural and anthropogenic factors. IPCC AR4 (chapter 10) points out that anthropogenic factors more likely than not result in an upward trend in the intensity of tropical cyclones. Greenhouse gases increased by humanity affect the potential intensity of tropical cyclones, via leading to the change of Sea Surface Temperatures (SST). the SST is also influenced by the change of the radiation. Aerosols (including lots of types) also is a factor which influences different areas in different ways such as kinematics and thermodynamics. For example, in North Atlantic, the emission of pollution aerosols decreases which cause the increase of tropical Sea Surface Temperatures (SST) by changing cloud albedo. However, in northern Indian Ocean, the rising of aerosols lead to reduced vertical wind shear. It is still uncertain of the link between tropical cyclones and these factors.(IPCC chapter 14)

From above, the phenomenon, at least on global scale, may be not that terrible like some people think. Even though, we also need pay attention to hurricanes, because of the variability characteristics of them.