Storm surges, sea level and climate change

In the inevitable discussion about the relationship between climate change and Hurricane Sandy, there's been much focus on the storm surge. Hallelujah.

There are a lot of ways climate change could influence tropical cyclones. In the past, most of the public discussion had focused on warmer water temperatures driving more intense storms, based largely on research by Kerry Emanuel and others. The water temperatures are, however, just one factor.

Other important issues include how climate change may affect upper levels winds, which can 'shear' off storms; El Nino events, which itself affect upper level winds; mid-latitude pressure systems, which divert storms and affect their dissolution; patterns of ocean temperature change and hence storm generation and path; atmospheric moisture, and in turn, rainfall. All of these are complicated and uncertain. When the experts try to add it all together, the verdict is that it's hard to say just how climate change will affect tropical cyclone frequency and intensity.

The one area in which we can speak with some certainty is cyclone impacts - things like storm surges - rather than cyclone formation and frequency. There's no reasonable doubt that the global mean sea level has risen due to climate change. The rise to date is largely because of thermal expansion of sea water; water gets less dense and expands as it warms (above the maximum density, 4 deg C for freshwater). If we are wrong about that, we may as well throw every physics textbook in a bonfire, but don't let me give the first year students here any ideas. Here's the sea level data from Battery Park in New York City since 1856

The rise of almost 40 cm is not entirely due to sea level rise. As is explained clearly in this article by Chris Mooney, land subsidence, a New York legacy of the end of the last ice age, account for roughly half of the observed change. The rest is largely driven by global sea level rise. Thanks in part to climate change, the storm surge from Sandy was certainly higher than that of an identical hurricane at the identical time in the tidal cycle striking the coast 50, 100 or 150 years ago.

Now the difference noted here - 20 cm or so - is not large. Keep in mind, though, that there's a non-linear relationship between the surge height and the "run-up": how far the water runs up on to land. An increase in surge height can have a disproportionate affect on the the distance water travels inland and the erosive power. The exact relationship depends on the profile of the shoreline and the type of ground or sediment; this is evident in the stunning before and after Sandy images of the New Jersey coastline put together by NOAA (or images from the Japanese tsunami).

You might argue this is not so different from the ocean temperature argument:  if everything else is equal, the same cyclone passing over warmer water will be more intense. The problem is that, in reality, everything else is never equal. That, in effect, is the theme of my article on sea level variability in Kiribati and the video we put together, using footage I'd gathered during field trips to Kiribati, about the causes of the loss of the island of Bikeman.