Hurricane Heat

The big problem with much of the discussions about trends in hurricane activity is that the databases that everyone is working from are known to have significant inhomogeneities due to changes in observing practice and technology over the years. So it’s not surprising that a new re-analysis (Kossin et al, published yesterday) has been generating significant interest and controversy among the hurricane research community (see e.g. Prometheus or Chris Mooney). However, rather than this study being taken for what it is – a preliminary and useful attempt to make homogeneous a part of the data (1983 to 2005) – it is unfortunately being treated as if it was the definitive last word. We’ve often made the point that single papers are not generally the breakthroughs that are sometimes implied in press releases or commentary sites and this case is a good example of that.

Kossin et al develop an algorithm based on North Atlantic data that can be theoretically used with the coarsest data available from the earlier parts of the record and in more remote regions. While the technique works well in the North Atlantic (picking up almost all of the storms seen in the standard data), it doesn’t work as well in other basins – possibly because the characteristics of tropical cyclones are not universal, or because the coarse early remote sensing data are still not sufficient. The poorer performance in the other basins is surely a reason to anticipate that further work will be necessary to refine these estimates, and should serve as a caution to those wanting definitive conclusions.

How does this fit in with some of the previous work? Well, it confirms the large trend in the North Atlantic (seen in Emanuel, 2005), but doesn’t show significant trends in the other basins (from 1983). This isn’t directly comparable with Webster et al (2005) though, since their trends start in the 1970s, and the shortness of the new reanalysis (only 23 years) emphasizes interannual and decadal variability associated with e.g. El Nino. The Kossin et al study is therefore unlikely to shed much light on the potential global warming/hurricane intensity link.

In summary, read the papers and the comments but don’t believe the hype.

We’ll start off the discussion with a few comments we have already received on the provocative study:

1. Chip Knappenberger:

Based upon the new results of Kossin et al. (GRL, 2007), it looks like the IPCC SPM just barely covered itself in its proclamations on observed hurricanes:

There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones.

From the results presented in Kossin et al. the “suggestions” of increases in intense tropical cyclone activity in regions other than the Atlantic basin are not really so well supported, at least for the last 23 years.

We’ve tried to make this point a bit more clearly here, despite the implications of the headline of Kossin’s press release.


(disclosure: I have, to some degree, been funded by the fossil fuel industry since 1992)

2. Dr. Kevin Trenberth:

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