Cyclone activity and climate change

There is an interesting discussion over on Frogblog about cyclone activity increasing due to climate change. Frog assumed it was increasing, but was wise to point out that:

“I’m always wary of linking specific weather events to climate change because while you can argue that climate change will and is leading to more storms, droughts and floods, you can’t realistically pin it to specific storms, droughts or floods as they occur.”

Unfortunately not all commenters have been so realistic about the situation. The fact as I can see at the moment is that cyclone activity does not appear to be increasing at all. Although there are heaps of media articles blaming cyclones on climate change, this does not appear to be supported by the research. It is important to actually read the peer-reviewed papers behind issues before jumping to conclusions based on media reports.

Here is some actual information from peer-reviewed papers:

Henderson-Sellers et al. (1998) Tropical Cyclones and Global Climate Change – a post-IPCC assessment. Bulletin of the American Meteorological Society 79(1): 19-38.

“There are no discernable trends in tropical cyclone number, intensity or location from historical data analysis”

Landsea et al. (1996) Downward trend in the frequency of intense Atlantic hurricanes during the past five decades. Geophysical Research Letters 23: 527-530.

“…a long-term (five decade) downward trend continues to be evident primarily in the frequency of intense hurricanes. In addition, the mean maximum intensity (i.e., averaged over all cyclones in a season) has decreased, while the maximum intensity attained by the strongest hurricane each year has not shown a significant change.”

Elsner, J., Kossin, J. P. & Jagger, T. H. (2008) Nature 445, 92–95

“The number of cyclones per year over the globe … there is no trend in these counts. Also, there is no trend in the median lifetime-maximum wind speed”

This paper does note a slight upward trend in the lifetime-maximum wind speed of the worst cyclones.

From these three papers we have two claiming no change in number of cyclones per year or cyclone strength (one finding a slight increase in the strength of the worst cyclones), and one paper claiming a decrease in number of cyclones and cyclone strength. Cyclone numbers and strength are NOT increasing.

Interestingly however, it is possible to hype up these figures if you want to sell a certain line. This Nature News article is based on the 2008 paper referred to above. It is hyped and on initial reading you get the impression that cyclone activity is increasing dramatically due to climate change. It is only when you read the actual peer-reviewed paper and look at the graphs that you can see the facts more clearly.

Number and maximum windspeed of tropical cyclones (Elsner et al., 2008)

Number and maximum windspeed of tropical cyclones (Elsner et al., 2008). (Red line = median, Green = 0.75 quantile, Blue = 1.5x the inter-quartile range)

Clearly, from the actual data, cyclone numbers and median intensity are not increasing. The worst hurricane appears to have been in around 1987 according to that graph. There is a slight increase in intensity of the worst hurricanes, but this is not supported by the other papers I refer to.

We cannot conclude that climate change is causing an increase in storm activity, whatever the media hype.


StephenR has drawn my attention to a paper that does appear to show a strong correlation between sea surface temperature and total energy dissipated by hurricanes:

Emanuel, K (2005) Nature 436:686-688

Power distributed annually (solid line) and sea surface temperature (dashed line) for Western North Pacific and North Atlantic tropical cyclones (Emanuel, 2005)

There appears to be a clear increase in total energy (and therefore total destructiveness) of hurricanes with rising sea temperature, and according to this paper sea surface temperature has risen by about 0.4 degrees C from 1950 to 2000. Due to the date of this paper it doesn’t show what has happened since 2000, when global temperatures have apparently been falling, so whether this trend has continued will have to be confirmed in a future paper. Some of the older data is probably questionable too, as it was recorded before the satellites used by Elsner et al. were available. However it does indicate that, should temperatures rise, we could most likely expect increased destruction from hurricanes. It does not show that humans are causing an increase in hurricane activity, as it doesn’t address the question of whether humans are causing the temperature increase, so don’t jump to conclusions. But it is one paper that does contradict the three I quoted earlier and actually supports the “global warming could cause terrible destruction” line that is popular.