Outer Banks Sea Level Rise: Worth Getting Exercised Over?

Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

The Washington Post, yesterday, fanned the flames of a dispute over how much sea level rise the residents of the North Carolina Outer Banks should plan upon for this century.

The dispute arose when, a few years ago, politicians in Raleigh decided to get involved in the business of climate forecasting,  and decreed that the Outer Banks region should expect a 39-inch sea level rise by the year 2100 and that people need to plan for a  future based upon this number. Some of the rumored plans include abandonment of the region’s major roadways, stopping new construction, and re-zoning the land to declare all property at an elevation less than 39 inches to be uninhabitable. The state government under then-governor Beverly Perdue (D) was “helping” by preparing a website that showed all property that would be under water by the year 2100, deep-sixing the equity held in many beach houses.

It’s no surprise that there’s a pushback against the state’s 39-inch forecast, which was based on a selection of outdated science that foretold a much more alarming story than newer scientific studies.

For example, the latest (fifth) assessment report from the U.N.’s Intergovernmental Panel on Climate Change (IPCC) projects that the global average sea level rise over the course of the 21st century would be in the range of 10 to 32 inches, with a mean value of about 19 inches.  This is only about 50% of the 39-inch projection.

And, the IPCC projection is probably too high because it was driven by a collection of climate models which new science indicates produce too much warming given a rise in atmospheric carbon dioxide levels.  If the models were forced to run with a lower sensitivity to carbon dioxide emissions, their sea level rise projections would decline proportionally,  down to about 13 inches.  This arguably better value is only 1/3rd of the 39-inch value forwarded by the NC state government.  No wonder the realtors and mortgage bankers were up in arms about Bev Purdue’s map.

Not so fast, said the supporters of the 39-inch rise, pointing to a 2012 by Asbury Sallenger and colleagues from the U.S Geological Survey (USGS), which identified a sea level rise “hot spot” that stretched from Cape Hatteras northward to Maine.  It showed that the rate of sea level rise in the “hot spot” was substantially greater than the global average, and postulated that it was the result of an anthropogenic climate change-induced slowdown of the Gulf Stream.  This slowdown was only expected to get worse in the future (according to the same climate models that can’t get the climate response to carbon dioxide level increases correct) and therefore, the Outer Banks should expect substantially more sea level rise than the average experienced by the rest of the world.

Unfortunately, Sallenger et al. was typical of so many climate projections that seem to thrive in a data-poor environment. Six months ago, we highlighted a study by Tom Rossby and colleagues which looked directly and more completely at the recent behavior of the Gulf Stream. Rossby identified no slowdown at all, and directly questioned the Sallenger explanation for the “hot spot”:

Recently, two papers have suggested that the [Gulf Stream] may be weakening based on the well-documented accelerated Sea Level Rise (SLR) along the U.S. east coast (Sallenger et al., 2012; Ezer et al., 2013). …In contrast to these recent assertions of a weakening Gulf Stream our direct measurements of Gulf Stream currents for the past 20 years indicate no such trend…”

The Rossby study did not dispute the existence of the “hot spot,” but, rather, the very shaky hypothesis that it was due to a slowdown of the Gulf Stream resulting from global warming, and even more warming would make things (warning:  familiar meme ahead) worse than we thought!

In fact, one only needs logic to refute the hypothesis.  The Gulf Stream is the principal transporter of tropical warmth to high latitudes in the western hemisphere.  If it were significantly weaker, that would obviously increase the temperature gradient (difference) between the pole and the tropics, something that is obviously happening in the opposite direction (unless all those stories about disproportionate north polar warming are hooey).

Another recent study by Rutgers University’s Robert Kopp took the “hot spot” analysis further and found that, while it has existed since the mid-1970s, it was largely explained by natural  variability present within various patterns of atmospheric circulation. No need to invoke global warming.  Kopp went so far as to calculate that it would take another 20 years or so of a continuation of the “hot spot” sea level rise before the rate would even rise about the level of natural variability observed during the 20th century. In other words, it is way too soon to tell whether  the Outer Banks will experience a sea level rise greater than the global average.

Recognizing this, and that the level of uncertainty is large and grows larger the further out into the future, the current North Carolina governor Pat McCrory (R) and Republican-led state legislature rescinded the 39-inch projection and are working on new guidance that reportedly extends out only 30 years with projections of no more than 8 inches of rise during that time (which is still probably on the high side of what will occur).

It doesn’t take a rocket scientist to notice that the Outer Banks is a pretty unstable environment to begin with, one that is continually reshaped by ocean currents, hurricanes, nor’easters, and more recently, human engineering.

All these shaping forces will continue into the future—with or without climate change.

So, too, will fancy houses on stilts out in the waves.  That is part and parcel of the dynamic environment there and our desire to live (or vacation) as close to the edge of the sea as possible.

The degree to which that unstable environment will be preserved, reshaped or shored up is best left to the locals, although the situation is infinitely more complicated in that many of those large vacation homes would not exist if it weren’t for federal subsidies in the form of reduced flood insurance rates.

This is a complex system, partly of our own making. Managing it going forward isn’t exactly helped by scare-mongering. Proclaiming a 39-inch sea level rise in the face of a large body of contradictory science will only further rachet up the public’s distrust of gloomsaying scientists.

In closing, it’s appropriate to remember President Dwight Eisenhower’s telling paragraph in his iconic 1961 Farewell Address:

Yet, in holding scientific research and discovery in respect, as we should, we must also be alert to the equal and opposite danger that public policy could itself become the captive of a scientific-technological elite. 

…as in the publicly-funded bureaucrats and cherry-pickers in Raleigh.

References:

Kopp, R. E., 2013. Does the mid-Atlantic United States sea level acceleration hot spot reflect ocean dynamic variability? Geophysical Research Letters, 40, 3981–3985, doi:10.1002/grl.50781

Rossby, T., et al., 2013. On the long-term stability of Gulf Stream transport based on 20 years of direct measurements. Geophysical Research letters, doi: 10.1002/2013GL058636

Sallenger Jr., A. H., K. S. Doran, and P. A. Howd, 2012. Hotspot of accelerated sea-level rise on the Atlantic coast of North America. Nature Climate Change, 2, 884-888.