Thursday, November 08, 2012

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.


david lewis said...

When the US Navy asked the National Academy, what about sea level rise, the answer was this report: National Security Implications of Climate Change for US Naval Forces

The threat of sea level rise was described this way:

"neither regional nor global sea level is of primary interest in determining naval coastal installation vulnerability. Rather, it is the increased vulnerability associated with extreme events (storm surges) and their dependence on changes in regional sea level, tidal amplitudes, and the nature of extraordinary meteorological forces that are of greatest importance"

Simon Donner said...

Thanks David, that's a great quote.

EliRabett said...

Simon, as someone who played on the barrier beaches in the Rockaways and lived in low lying areas of South Brooklyn, let me tell you that Manhattan and the Bowery is the least of it. People we know who work in Hoboken NJ say that their buildings will not be open until January or later (a lot of Wall Street backoffices were located there.

Simon Donner said...

Absolutely, thanks for writing that. The recovery time will also be long in lots of communities along the Long Island coast and especially the Jersey Shore.

This is a powerful reminder of the worst potential inequalities of climate change. There's a huge range in our ability to adapt, and the people and communities with lower adaptive capacity might also be seen as "lower priorities" and receive less or less urgent public assistance.

Brian Schmidt said...

I've been on a kick for a few years that SLR is the strongest legal vulnerability for climate polluters because you don't have to wrestle with whether it was caused by climate change or something else.

Untangling damamges for SLR versus general damages from a storm won't be easy, but I think a judge who bought the argument that polluters are responsible for SLR isn't going to let them off because their damage is mixed in with other damages.