Friday, November 2, 2012
Hurricane/Tropical Storm Strengths, 1851 to 2010
Hurricane Sandy has caused much discussion on all the climate blogs I have visited lately, such as climate etc. The continuing question -- already considered answered in the affirmative by climate alarmists -- is whether Sandy was somehow due to global warming. Here is my short response:
What is the effect, upon the available atmospheric energy, of a change of about 1°C of global warming over the last century? The equipartition theorem tells us that the molecular energy is proportional to the absolute (Kelvin) temperature, so the fractional change in energy is equal to the fractional change in temperature, or 1°/T, with T the average temperature of the lower atmosphere. The mass mean temperature of the troposphere in the Standard Atmosphere model (which I confirmed in my Venus/Earth temperatures comparison) is 259.4K. So an increase of 1°C, or 1K, in the mean temperature means a fractional increase in the available energy of 1/259.4, or about 0.004.
If we define a hurricane by a representative wind velocity within it, then the energy in the storm should vary as the square of that velocity. Suppose we have a hurricane defined by a 75 mph windspeed; the square of that is 5625. Now raising the temperature by 1°C should increase the energy to 1.004 its former value -- 1.004 times 5625 = 5647.5, and the square-root of that, 75.15, would be the new wind velocity expected for the storm. But distinguishing a storm with 75 mph winds from one with 75.15 mph winds, in my estimation at least, is not possible (this estimate of the effect is rough, of course, but it is certainly within an order of magnitude, and if the effect were ten times larger, we would still be looking at trying to distinguish 76.5 mph winds from 75 mph ones). So we shouldn't expect to see any increase in hurricane intensities, or destructive power, on average, with only a 1°C rise in global temperature.
And if we look at the tropical storm and hurricane data, from 1851 to the present, at this site, we can find and plot the average intensity of all tropical storms (including those that became hurricane force) over that time period:
The horizontal black line in the above image is the best fit linear trend line. It has a slope of -0.0006/decade, or essentially zero. There has in fact been no observable increase in average atlantic storm strength, over the range of the data. (In the above graph, the decadal average strength of all atlantic tropical storms and hurricanes is calculated according to the hurricane categories, 1 to 5, and each tropical storm taken as a category zero.) This is not the whole story, of course, but in my view it is the bottom line conclusion to be drawn from the data, as well as from the simple theoretical estimate above.