The Intersection

Here's the key passage from the SPM (the full IPCC report, to be released later, will have much more detail backing all this up):

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.

In addition to this passage, there's another section, also very important, where the SPM says: 1) it is "likely" that there have been "intense tropical cyclone activity increases" in some regions since 1970; 2) it is "more likely than not" that there has been a human contribution to this observed trend; and 3) it is "likely" that the trend will continue as warming continues. "More likely than not" is accompanied by a footnote explaining that this means, "Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgement rather than formal attribution studies."

What does all this signify? Unlike yesterday, we now know that "more likely than not" simply means that the IPCC's experts, noting a trend towards increased intense hurricane activity that is most uncontroversial in the Atlantic (i.e., "some regions"), and noting that this trend correlates with increasing sea surface temperatures (widely believed to reflect human induced global warming), essentially suspect there's a human influence on hurricanes. But they can't quantify the size of that influence or make the point any more strongly, because no formal detection and attribution research has been done. In short, this is not something that the community of experts as a whole can assert with strong confidence yet (although a number of prominent scientists, individually, are very convinced of it).

As Jeff Masters points out, even though the IPCC doesn't come right out and directly say it, all of this is very much about the Atlantic, where the basic hurricane trend since 1970 is least controversial (although attributing its cause is another matter entirely). The IPCC isn't making a strong statement of causal attribution. But the fact that it suspects/theorizes about such an attribution will hardly go unnoticed by those of us living alongside this ocean basin. (Incidentally, Masters thinks the IPCC should not have thrown in the whole "more likely than not" business. I suspect others may also question this choice, and thus there may be "controversy" after all.)

On that note, Dave Roberts over at Gristmill is asking why any of this "matters." If hurricanes are indeed intensifying, what's the policy implication? Well, the scientists involved in the debate have themselves released a prominent statement about this:

While the debate on this issue is of considerable scientific and societal interest and concern, it should in no event detract from the main hurricane problem facing the United States: the ever-growing concentration of population and wealth in vulnerable coastal regions. These demographic trends are setting us up for rapidly increasing human and economic losses from hurricane disasters, especially in this era of heightened activity. Scores of scientists and engineers had warned of the threat to New Orleans long before climate change was seriously considered, and a Katrina-like storm or worse was (and is) inevitable even in a stable climate.

In other words, in the U.S. we're not even prepared for today's hurricanes, much less super duper hurricanes that may be coming at us in the future. One policy implication, then, is to start getting much better adapted/protected against today's storms, and do something about the huge stream of people and property to the coasts.

At the same time, meanwhile, the new IPCC SPM says that future changes to hurricanes are indeed likely:

Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs. There is less confidence in projections of a global decrease in numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period.

Based upon this likelihood of at least somewhat stronger hurricanes in the future, even as we act immediately to deal with our coastlines, we can also consider the possibility that there might be an increased risk due to changes to storms themselves. That's something I suspect some insurance companies are already doing.