Spinning the Atlantic Hurricane Season

Recently blogger Steve McIntyre, a strong skeptic of any large climate-related trend in hurricane activity, had this to say: "Some of you have been noticing a tendency for almost any gust of wind in the Atlantic to now become a named storm ... At this point, despite a couple of intense hurricanes, 2007 is even quieter thus far than 2006."

And yet the famed Colorado State University hurricane forecasting team, comprised of storm science legend William Gray and Ph.D. student Phil Klotzbach, say this in their latest update (PDF): "Information obtained through 30 September 2007 shows that we have so far experienced a slightly above-average Atlantic basin hurricane season."

Who's right? What the heck is going on here?

What the difference between McIntyre and Klotzbach-Gray shows is that there are multiple metrics for assessing hurricane activity. Different metrics give very different answers, especially in a year like 2007, in which we have seen a fairly large number of named storms but not many long lived or very intense ones. Or as Klotzbach-Gray put it regarding September: "The month witnessed the formation of eight named storms, tying a record for most named storm formations during the month. However, most of these tropical cyclones were quite short-lived and not particularly intense."

So depending on what you emphasize, you can take these data and say that the season is either above or below average.

Klotzbach and Gray are using a combined metric called "Net Tropical Cyclone activity," which takes into account how many storms there are, how long they last, how intense they become, and other parameters. By this standard, we are slightly above average so far in the Atlantic in 2007.

McIntyre is using narrower metrics: "Hurricane days" and "storm days" -- measurements of how many days there have been with either a named storm or a hurricane out there. McIntyre is also using a metric called "Accumulated Cyclone Energy" or ACE, which again depends heavily on the duration of storms combined with their numbers and intensities.

But precisely because so many storms have been short-lived, this approach gives you a lower number.

The upshot, for me, is that the behavior of the Atlantic this year is mixed, or as I said in the last "Storm Pundit" post, schizophrenic. That's weather for you. Clearly, we ought to be very cautious about taking these ambiguous data and strongly arguing any position with them. Indeed, for the moment our outlook on hurricane-climate risks should remain the same as always: It is very worrisome that storms might be changing, and there are reasons to think that they are -- but the scientific jury remains very much out on precisely what is going on out there.

One season in the Atlantic is not going to resolve the hurricane-climate debate one way or another.