A Host of Katrinas and the Damage Function

If you are interested in the interface between weather and climate, then I would highly recommend Dr. Jeff Master’s Wunderblog. Yesterday, Jeff highlighted a paper by Aslak Grinstead of the Danish Niels Bohr Institute looking at storm surge frequencies.

Grinstead has decided to take as his unit of measurement a Katrina-sized storm surge. While we all know that this is pretty big, the high water marks in the photos below from Master’s post (click for larger image) bring home just how amazing the storm surge was:

High Water Marks Katrina jpeg

Turning to the institute’s press release, Katrina-scale storms surges are recorded as having taken place around once every 20 years since 1923. However, Grinstead’s main finding is that this frequency no longer holds:

We find that 0.4 degrees Celcius warming of the climate corresponds to a doubling of the frequency of extreme storm surges like the one following Hurricane Katrina. With the global warming we have had during the 20th century, we have already crossed the threshold where more than half of all ‘Katrinas’ are due to global warming.

And he goes on:

If the temperature rises an additional degree, the frequency will increase by 3-4 times and if the global climate becomes two degrees warmer, there will be about 10 times as many extreme storm surges. This means that there will be a ‘Katrina’ magnitude storm surge every other year.

You can see the relationship in the chart below (click for larger image). Note the log scale for Katrinas per decade on the left, which means such events do not rise linearly with temperature—rather they go through the roof.

Storm Surge Frequency jpeg

Stuart Staniford, over at Early Warning, put up an intriguing chart covering the Katrina impact on New Orleans GDP titled “Modern Cities Are Hard to Kill” some months back (click for larger image). This suggests that these storm surges can be taken in our stride—I am not so sure.

GDP of New Orleans jpeg

First, GDP only covers half of the economic security equation; that is, it is a measure of flow not stock. In short, if someone was hired post Katrina to take on a clean-up position at a higher wage than their pre-Katrina job, they would not think of themselves as better off if their house had also been destroyed. Second, the Federal funds that poured into New Orleans post Katrina had to have been paid for by someone (putting aside Keynesian considerations), even if spread across the other 51 U.S. States. Katrina led to higher taxes, or less alternative public spending or more debt somewhere in the system. Third, the fallacy of composition will rear its ugly head in due course: just because the economy can absorb one Katrina, doesn’t mean it could absorb a series of Katrinas in quick succession.

Further, at two degrees Celsius of warming, Grinstead suggests that we could be experiencing Katrina-type surges every other year. Unfortunately, current emissions trajectories indicate that we are likely to move well above 2 degrees, suggesting storm surges will explode in number.

Nonetheless, the most famous economic model looking at climate change impacts, the Dynamic Integrated Climate-Economy (DICE) developed by William Nordhaus, is also relaxed about our progression into a new climate regime. Their damage function (see chart below) with respect to temperature is mildly non-linear, but nothing like the storm surge chart.

Of course, storm surges are just one hit to GDP. But as findings like those of Grinstead continue to be published, my belief is that the damage estimates relating to climate change and GDP, which have a major impact on policy perceptions, will prove woefully low (see my previous posts on this issue here and here).

damage function

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