Storm Surges, Global Warming, and Delaware Beaches

ADDENDUM (Nov.-7, 2012): Time lapse video from Delaware Sea Grant.

Rising seas and flood waters cause most of the damage during storms such as Sandy did last week. Tides, waves, and storms all contribute. We can debate how global warming impacts any of the above, but the arguments are involved. So lets assume, that neither tides, waves, nor storms are impacted by global warming, but that the globally averaged rise in sea level over the last 50 or 100 years is. This global warming induced sea level rise is about half a foot in 50 years (3 mm/year), but why would we care about global averages, when we live in Delaware? Furthermore, why worry about the whimpy surges we get ever 2-3 weeks. We don’t, we worry most about the most extreme events like Sandy and want to know how often they occur. Below I show a Sandy-like event to occur about once every 10 years. Furthermore, over time Delaware’s most extreme storm surges are rising twice as fast as global averages do. So, how much does the global warming impact our local flooding in Delaware?

Market Street on the beach in Lewes is in one of the lowest lying areas of town and takes its good old-time draining. This photograph looks northeastward toward the beach, just west of the intersection with Massachusetts Avenue. [Credit: Cape Gazette]

More than I initially thought: the largest storm surge that has hit Delaware was the Ash Wednesday storm on March 6, 1962 which added 5.8 feet to the regular tides and waves. I wrote about this yesterday using public NOAA data. This same storm today would add 6.8 feet to the regular tides and waves. For comparison, Sandy’s storm surge added 5.3 feet. So Sandy was a weak storm by comparison. If it had hit in 1962, it would have added only 4.3 feet. The difference of 1 foot in 50 years is due to steadily rising sea levels:

Largest storm surge at Lewes, Delaware each year from 1957 to present. The red line is a linear fit to the data. The slope indicates that the largest storm surge increase by almost 3 inches every 10 years.

On average each year has a larger largest surge than the year before. While this steady increase by 2.8 +/- 1.7 inches each 10 years is statistically significant (95% confidence), picking the extreme each year is perhaps not the best statistic as extremes do not happen often. Please note that a 95% confidence means that there is a 5% chance that the true increase is either smaller than 0.9 inches/decade or larger than 4.5 inches/decade.

What about the mean or average surge each year? From hourly data, I pick the middle surge, that is, half the surges each year are larger and half are smaller:

This increase of 1.4 +/- 0.2 inches per decade (95% confidence) is more in line of the global average. The uncertainty in this trend is smaller than that of the trend for the extreme, because the median sea level varies little from year to year, while the extreme value varies more from year to year. So, from these results we can conclude, that while the mean or median sea level at Lewes increases by perhaps 1.5 inches in 10 years, the extremes increase twice as fast. So, storm surges like Sandy will become more common than they are today mostly because of global warming.

Over the last 50 years we had at least 5 such events in 1962, 1968, 1996, 1998, and 2012. So, on average we have a Sandy-type storm surge greater than 5 feet every 10 years. This contradicts a Wilmington News Journal article today which quotes John Ramsey to describe “… Sandy as a 1-in-200-years storm, unlikely to be repeated anytime soon. That could give coastal communities time enough to deal with the real threats and realities of sea level rise and climate change.” There is no such time, as it is mis-leading to describe Sandy as a 1-in-200-year event when it has happened about every 10 years during the last 55 years. Instead of a 0.5% percent chance of a Sandy-like event to hit Lewes each year, I would raise this chance to be larger than 10%.

5 responses to “Storm Surges, Global Warming, and Delaware Beaches

  1. In a sense, this comment is a little unfair as it relates to a book of mine that has not been available for a couple of years and is unlikely to be made available again until sometime next year. (A very long story I will not bore you with today).

    There is no doubt here in the UK, (where, being an island sitting on the edge of the Atlantic Ocean, downwind of the prevailing winds, we are always driven by changing weather), we have been observing an alarming increase in precipitation. Generally, over the last decade or so, we have seen a ~25% increase in rainfall. Yes, just as with tides, there is variation; swinging between droughts to months on end of heavy rainfall. But it is clear we are observing more rain.

    That must, in turn, mean that the atmosphere, in locations associated with that increase of rainfall; is carrying again, ~25% more water vapour and in turn again, that the gravitational structure of the weather systems carrying that increased water vapour are carrying ~25% more mass. More mass means more gravitational effects from the increase; higher mass and thus associated higher gravity ABOVE the surface from that increased cloud mass will increase the height of any associated storm surge. That exactly the same effects as the tides are also to be observed from the increase of the mass within the clouds.

    That the increased mass of the clouds, must also affect the surrounding environment; that higher mass means, for the same external inputs; for example, wind and rotation of the planet; slower moving storms; longer time periods between changes within weather systems; such as we are now observing from the reported increases in the wave length of the Jet Stream.

    That when we look at the dynamics of weather systems, we must also take account of the increase of the mass of water vapour within the clouds; and thus, in which case, the greatest increase has not been temperature, or rising sea levels; but water vapour mass above the surface of the planet..

  2. I think it may be a bit problematic to “fit” a trend line to annual storm surge maxima. Severe storms are highly episodic, and using annual max to derive a trend line may discount years in which multiple severe surge events occured.

  3. Hi Andreas,

    Hope you and yours were not too badly affected by the storm.

    A couple of points: I haven’t seen you mention that much of the US East Coast “sea level rise” is actually caused by “land level fall”.

    And this recent study, finding evidence of sustained change in the heat of the Carribean might be relevent…

    Or not. Your guess is probably better than mine.

  4. Wow – just reading that Wilmington News Journal article. I agree with you that the Delaware shore is completely vulnerable to more storms like Sandy. It’s definitely not going to take 200 years for us to see something like this again. We’re lucky Delaware didn’t have more damage.
    Folks who live down on the shore have to constantly be prepared for storms like this. When you live near the water, you are basically open to hurricanes and massive storms coming and destroying the beach and even your home. It’s a big decision to buy beachfront property. (It’s one of the reasons I hesitate to buy on the water!) And with the flood insurance companies denying coverage to many who live on the beach, you have an even bigger issue. Yes, Sandy was destructive but anyone who lives on the shore must constantly be prepared for the worst to hit at anytime. (Or atleast that’s the way we see it.)

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