Wednesday, April 7, 2010
Intensity or Frequency?
I have previously argued that – in my estimation – there's a strong causal association between sea-surface temperatures and the number of named storms (or tropical cyclones) in the Atlantic Basin. Statistically, the association is quite significant, and graphically, it is evident once you apply very simple smoothing filters.
This is not the prevailing scientific view, which essentially says that the intensity of storms should increase with global warming, and the frequency of storms should actually decline. This prevailing view, largely based on computer modeling and not observations, is best summarized by the IPCC in 4AR WGI 10.3.6.3.
I was wondering if both could be true: global warming increases the frequency and intensity of tropical cyclones. How can we test this idea using available observations? You can't just look at, say, Accumulated Cyclone Energy (ACE.) If the frequency of storms increases, ACE should also increase, even if the average intensity of each storm doesn't change.
It occurred to me that a much better test would be to look at the ratio of hurricanes to all named storms, and the ratio of major hurricanes to storms. I've done this, but I'll leave it as an exercise for the reader. It's a very easy analysis. You can use the named storm count data from the Hurricane Research Division of NOAA. If you have concerns that tropical storms were under-counted in the past relative to hurricanes (a reasonable assumption), you can use data starting in 1944, which is when systematic aircraft recognizance started. But remember, causality matters more than the trend in this case.
To make a long story short, observations do not appear to support the view that global warming will cause storm intensity to increase. The historical data is telling me the opposite of what the IPCC claims. What, if anything, am I missing? Could it be that things will work differently in the future?