Tag Archives: Eric Berger

Hurricane Sandy and the Climate Hens

Image courtesy of NASA, used under Creative Commons














Tropical cyclones, climate, and consensus

A favorite denialist canard goes something like this: “You alarmists swore that your global warming caused Katrina and would cause more hurricanes every year, but the last few years have been a bust. Yet another algore lie exposed!” If you think I’m unfairly exaggerating, read the comments section of any major newspaper article or online forum discussing climate change.

Lest you believe that this (genuine or otherwise) confusion about the mainstream view of climate change and hurricanes is restricted to random commenters, you see something similar (although less frothy) from even mainstream science writers, like the Houston Chronicle’s science blogger, Eric Berger [emphasis mine]:

When An Inconvenient Truth came out I believed the movie to be scientifically accurate. Carbon dioxide levels were rising and so were temperatures. And hurricane activity, especially after the disastrous 2005 season, was out of control.




We’re in some familiar denialist territory here- even if there were an alleged consensus about an increase in “hurricane activity”, drawing conclusions based on a period of 2005-2009 is, well, inexcusable. Especially for a science writer. Similarly, he seems to believe that global warming should likewise be monotonic– which is to say that Mr. Berger can’t seem to understand that there can be long term upward trends despite substantial year-to-year variation.

In any event, the idea that there was even a robust, precise consensus about the current and future impacts warming is supposed to have on hurricanes (tropical cyclones, typhoons, etc.) is easily refuted by simply looking at the relevant sections of the IPCC Fourth Assessment Report, which I’ll add to the end of this post.

Whenever someone asks me “What happened to all the hurricanes?” or “What’s the consensus on climate and hurricanes?”, my stock answer is something like this-

First, in terms of a broad, robust consensus- such as those around the issues of whether or not the planet is warming, and the anthropogenic nature of that warming- one doesn’t exist for tropical cyclones. To grossly oversimplify, we could say that in a warming world- all other things being equal– we would expect warmer sea surface temperatures (SSTs) to contribute to an increase in tropical cyclone intensity. However, not all other things will be equal. While SSTs play an enormous role in cyclogenesis, they are not the only factor in play. For example, wind shear is another huge player in tropical cyclone behavior– too much wind shear can both prevent the formation of  and destroy tropical cyclones- and we expect wind shear to increase in a warming world. And of course there are “unknown unknowns”, as well as some known unknowns like the behavior of relevant ocean-atmosphere patterns (like ENSO* and the NAO) in a warmer world.

So roughly speaking, I think it would be fair to say that (setting aside the unknowns) while warmer SSTs imply an increase in intensity, increased wind shear implies a decrease in total storms, so that we’re left with the possibility of a warming world with fewer but more intense hurricanes- something that I’ve categorized in the past as a “proto-consensus”.

There have been a number of recent high profile attempts to model tropical cyclone behavior from some of the big names in the field, like Knutson 2008 and Emanuel 2008. Although Knutson 2008’s and Emanuel 2008’s findings were misrepresented by some in the media and governmental offices as evidence of a shift in opinion that GHGs had little effect on the behavior of tropical cyclones, both papers supported this proto-consensus, albeit with significant caveats and from different angles. One of the biggest areas of concern in these kinds of studies is that the models used often didn’t have the resolution necessary to look at the behavior of the strongest (Cat 3 and higher) storms, which seem to be getting stronger as we warm.

Knutson is part of a new study in Science (or here), lead by Morris Bender, which seeks to remedy this flaw entitled “Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes”. Bender et al. use a downscaling process to examine the effect of anthropogenic warming in GCMs on the behavior of Atlantic storms in an operational hurricane model. They find that while the total number of storms decreases, there is “nearly a doubling of the frequency of category 4 and 5 storms by the end of the 21st century”.

Tracks for all storms reaching category 4 or 5 intensity, for the control and the warmed 18-model ensemble conditions, as obtained using the GFDL/NWS hurricane model.

This has serious implications for policy-makers, especially in the US, where the strongest storms (Cat 3+) are responsible for the overwhelming majority (80%) of economic losses from Atlantic hurricanes.

So- is this the last word? Have we reached A Consensus? Of course not- although this paper certainly adds weight to the proto-consensus, there is still a tremendous amount of uncertainty on this issue. But the next time someone tries to tell you about how “climate science got it all wrong re: hurricanes”, you’ll know more or less where things stand.

*Intriguingly, while normal El Niños tend to suppress Atlantic hurricanes, so-called Modoki El Niños are actually positively correlated with them, and as the planet warms it looks as though Modoki El Niños are becoming more common.

Relevant sections from the AR4 are below the fold:

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