## Half a Degree May Double Hurricane Intensity

Environment Monday, August 1, 2005. Post by apsmith

From the New Scientist, this report on a study by MIT meteorologist Kerry Emanuel of hurricane wind intensities over the past 50 years in the North Atlantic and Northwest Pacific seems to indicate a strong correlation between small increases in sea-surface temperatures and total “destructive potential” of the storms. Destructive potential here refers to the integrated square of the wind speeds (total wind energy) over the duration of the storm – the increases, roughly double the destructive potential for a half-degree sea-surface temperature change, come from both greater wind speeds and longer duration storms, sustained by the higher temperatures.

A previous article noted that intensity and rainfall from hurricanes have been consistently increasing in recent years. There do remain some doubts about the way Emanual has come up with some of his numbers. However, if this holds up, future temperature increases with global warming suggest things could get much worse. Meanwhile in other reports, sea life on the Pacific coast seems to be suffering from the high temperatures this year, with plummeting catches of fish, lots of dead birds, and very little plankton.

This is not as bad as it sounds, and I’m going to dissect the phrase, “integrated square,” to show what I mean.

Let’s start with “square.” Whenever you hear the words “energy” or “power,” immediate start thinking about exponents, and especially powers of two. The common measurement of power, the decibel, is a way of linearizing exponents, where every 3 decibels approximately corresponds to a doubling. So 30 decibels is (approximately) equivalent to multiplying by 1000. (Decibels are unitless, however there are some versions that are anchored to a specific value at 0dB. You may be most familiar with the audio version, dBA, which is typically shortened just to “decibels” and should not be confused with this conversation.)

Anyway, back to the discussion. So in the topic, we are dealing with the square of the wind speed. A small change in wind speed will create a larger change in the square of the wind speed, especially when the wind speed is high, like it is in hurricanes. After doing some sample math with actual numbers, however, I realized that a few percentage points of wind speed change, say 2%, will only change the square by about 4%. So this is not the primary source of the doubling.

Most of the doubling, therefore, must come from the other part of the original phrase: “integrated.” Think of the “integrated square” as a (curvy) rectangle, with the square of the wind speed as the height and time as the width. The area of the square is the “destructive potential” of the hurricane, as mentioned in the topic. This width is the “integrated” part, and it is how long the winds last. The increase in temperature prolongs the duration of the winds. The increase in temperature, therefore, makes the rectangle wider, and this is where we get most of the increase in the “destructive potential” of the hurricane.

The New Scientist equates destructive potential with intensity. I think this is a bit of a misnomer. I believe most people consider intensity to be synonymous with wind speed. So I return to my original statement, which is, this isn’t nearly as bad as it sounds. I do, however, agree that sustaining high wind speeds for a longer period, especially as the hurricane continues to move inland, would definitely increase its destructiveness. It would be nice to have some actual figures on how much longer the winds are sustained.

SEWilco

Notice the “50 year” limit.
In 1975, 30 years ago, the 1945-1975 global cooling period ended and warming began. Behavior of hurricanes may have been different during that time. There was a period of warming from about 1850-1945; how did hurricanes behave then?

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