The buzz in North Carolina is how fast sea level (especially in the estuaries) seems to have increased in the last few years.  I haven’t seen any measurements confirming this, but anecdotal observations suggest somethings is happening. [I know, this may be a short term thing, unrelated to AGW]  I am on the outer banks near Cape Hatteras and the beachside erosion from fall storms has been amazing.  Sand dune constructed by the army core of engineers in the 40s are recently beginning to erode. And the sea level at soundside beach behind our house has been noticeably higher since 2008.  And this isn’t due to sand loss.  The water level is higher in the marsh at most high tides than it should be given the vegetation.

It could be rapid sea level rise.  One of my colleagues (a physical oceanographer who has also observed this) says his colleagues think something is going on with the NAO.  And a new paper (Engelhart et al. 2009) just published in Geology and covered at Futurity indicates that some of the observed increase in sea level here is due to glacial isostatic adjustment (GIA).  Simply put, the continent is still rebounding (sinking) from the prior release of the weight of the glaciers during the last ice age.

Correction: Fish, rightly commented that I was wrong about GIA and thus the implications of the paper.  I am sorry about the error.  The north American land masses are in general still rising or “rebounding” from the removal of the weight of glaciers during the last ice age.  Thus, as Fish correctly pointed out, such “glacial rebound” in effect works against the various factors casing sea level rise.  Although the main conclusion, from my perspective, still holds: the regional variability in GIA emphasizes the inaccuracies of simplistic “bathtub models” that use surface elevation to project future sea level rise (related to AGW).

Now that I am correctly interpreting the paper, I see the point the authors were making near the end of the Discussion.  They point out that the melting of glaciers on Greenland will increase glacial rebound, predominantly in the northern sections of NE north American, with diminishing rebound down the coast.  All things being equally, which is probably not the case, then sea level rise due to AGW would increase from Maine south towards the Carolinas and Georgia.  In the authors words:

The effects of Greenland mass loss on the U.S. Atlantic coast would result in a north to south increase in sea-level rise… Rignot et al. (2008) suggested that Greenland is currently losing mass at the equivalent sea-level rise rate of ~0.6 mm a–1. Steric effects may also play an important role in 20th century sea-level change (Miller and Douglas, 2004; Wake et al., 2006; Church et al., 2008). Church et al. (2008) proposed significant spatial variation in ocean thermal expansion for the upper 700 m along the U.S. Atlantic coast with areas possessing negative and positive thermal contributions to sea-level rise over the period 1993–2003. Wake et al. (2006) analyzed hydrographic data sets of the Atlantic coast and identified a large steric effect for the southernportion of the coastline that would influence 20th century RSLR, but Miller and Douglas (2006, 2007) concluded that there were only minor steric contributions to sealevel rise during the 20th century, north of Cape Hatteras, North Carolina.

The paper reports land “subsidence rates of <0.8 mm a–1 in Maine, increasing to rates of 1.7 mm a–1 in Delaware, and a return to rates <0.9 mm a–1 in the Carolinas.” This subsidence is part of observed sea level rise.  The other two main contributions (particularly in the future) to sea level rise are increases in the volume of the seas from glacial and ice cap melting and from the thermal expansion of sea water.  The authors also estimate  ”a mean 20th century sea-level rise rate for the U.S. Atlantic coast of 1.8 ± 0.2 mm a–1, similar to the global average”.

The regional variability in GIA (subsidence) also emphasizes the inaccuracies of simplistic “bathtub models” that use surface elevation to project future sea level rise (related to AGW).

U. PENN—Sea level along the Atlantic Coast is rising faster now than at any time in the past 4,000 years.

Coastal subsidence enhances sea-level rise, which leads to shoreline erosion and loss of wetlands and threatens coastal populations.

Further, the researchers find that the mid-Atlantic coastlines of New Jersey, Delaware, and Maryland are subsiding twice as much as areas to the north and south. The study results were published in a recent issue of the journal Geology.

This is the first demonstrated evidence of this phenomenon from observational data alone. Researchers believe this may be related to the melting of the Greenland Ice Sheet and ocean thermal expansion.

“There is universal agreement that sea level will rise as a result of global warming but by how much, when and where it will have the most effect is unclear,” says Ben Horton, assistant professor of earth and environmental science.

“Such information is vital to governments, commerce and the general public. An essential prerequisite for accurate prediction is understanding how sea level has responded to past climate changes and how these were influenced by geological events such as land movements.”

The study provides the first accurate dataset for sea-level rise for the U.S. Atlantic coast, identifying regional differences that arise from variations in subsidence and demonstrate the possible effects of ice-sheet melting and thermal expansion for sea level rise.

The study was supported by the National Science Foundation, the Thouron Family, and the University of Pennsylvania. Researchers from the International Hurricane Research Center at Florida International University, the University of Toronto, and the Tulane/Xavier Center for Bioenvironmental Research at Tulane University contributed to the study.

Figure 1 Rate of late Holocene relative sea-level rise with 2 errors for 19 locations along U.S. Atlantic coast. Inset plots are examples of locations with sea-level index points plotted as calibrated age before A.D. 1900 versus change in relative sea level (RSL) relative to mean sea level (MSL) in A.D. 1900 (m). Red line is linear regression for each site. Rates and errors shown to 1 decimal place. MA—Massachusetts; ME—Maine; NY—New York; DE—Delaware; NC—North Carolina.


Abstract: Accurate estimates of global sea-level rise in the pre-satellite era provide a context for 21st century sea-level predictions, but the use of tide-gauge records is complicated by the contributions from changes in land level due to glacial isostatic adjustment (GIA). We have constructed a rigorous quality-controlled database of late Holocene sea-level indices from the U.S. Atlantic coast, exhibiting subsidence rates of <0.8 mm a−1 in Maine, increasing to rates of 1.7 mm a−1 in Delaware, and a return to rates <0.9 mm a−1in the Carolinas. This pattern can be attributed to ongoing GIA due to the demise of the Laurentide Ice Sheet. Our data allow us to define the geometry of the associated collapsing proglacial forebulge with a level of resolution unmatched by any other currently available method. The corresponding rates of relative sea-level rise serve as background rates on which future sea-level rise must be superimposed. We further employ the geological data to remove the GIA component from tide-gauge records to estimate a mean 20th century sea-level rise rate for the U.S. Atlantic coast of 1.8 ± 0.2 mm a−1, similar to the global average. However, we find a distinct spatial trend in the rate of 20thcentury sea-level rise, increasing from Maine to South Carolina. This is the first evidence of this phenomenon from observational data alone. We suggest this may be related to the melting of the Greenland ice sheet and/or ocean steric effects.

Reference:  Engelhart, Simon E., Horton, Benjamin P., Douglas, Bruce C., Peltier, W. Richard, Tornqvist, Torbjorn E. Spatial variability of late Holocene and 20th century sea-level rise along the Atlantic coast of the United States. Geology 2009 37: 1115-1118

 

6 Responses to Atlantic Coast sea level rises at faster pace

  1. Fish says:

    First of all the sentence: “I haven’t seen any measurements confirming this, but anecdotal observations suggest somethings is happening” is quite moronic and in no way scientific. Certainly a scientist of your education should know better. Also, I think someone is confused about isostatic rebound, when a weight is lifted, the substrate will rise not sink. Every action has an equal and opposite reaction. The weight of the ice is pushing down, therefore the ground is pushing up; remove the ice and the ground rises. Not that isostatic rebound has anything to do with coastal subsidence, that sediment subsides as it compacts because it is newly deposited sediment.

  2. J.Roff says:

    Fish:

    First of all the sentence: “I haven’t seen any measurements confirming this, but anecdotal observations suggest somethings is happening” is quite moronic and in no way scientific. Certainly a scientist of your education should know better.

    John knows better. This isn’t moronic – it’s how science works. Someone comes up with an observation or prediction (in ecology this is often anecdotal), and then formulates a hypothesis to test this observation or prediction.

  3. John Bruno says:

    Dear Fish,

    1) I knew someone would make that point, which is why I added the “I may be confusing weather for climate here, but…” disclaimer, i.e., “I know, this may be a short term thing, unrelated to AGW”

    2) As Jez points out, this is precisely how science works. I know people are working on confirming whether this common observation [even by non-scientists; this is a very common conversation starter on the outer banks, "hey Joe, what the frak is up with the tides these days?!"] are in fact real, i.e., supported by quantitative pattern data, and if so trying to figure out why, i.e., testing alternative hypotheses including local erosion, changing wind patterns, actual sea level rise, and if so due to the NAO, something else, etc.

    3) I didn’t say there were no data confirming the pattern. I suspect there are. But that isn’t my field. I just said ” I haven’t seen any measurements confirming this”.

    4) Finally, most of our observations turn out to be unrepresentative, many of our ideas are wrong (moronic?), we disprove our own hypotheses every day, etc.

    BUT MOST IMPORTANTLY:
    “Also, I think someone is confused about isostatic rebound, when a weight is lifted, the substrate will rise not sink. Every action has an equal and opposite reaction. The weight of the ice is pushing down, therefore the ground is pushing up; remove the ice and the ground rises. Not that isostatic rebound has anything to do with coastal subsidence, that sediment subsides as it compacts because it is newly deposited sediment.”

    Mea culpa; You are correct. I screwed up. I am familiar with isostatic rebound and do know that when the ice melted the continents rose due to the release of the weight which deforms the plates. But I thought they eventually sink back down into equilibrium. I also, therefore, misinterpreted the implications of the paper (to a degree). Ill make a correction to the post. Thanks for catching that. Frankly, this is a big reason I blog, i.e., to learn shit.

  4. Fish says:

    I was not trying to be rude, I don’t think you are moronic or anything, I just think that your first sentence was and I stand by that. There is a reason why eyewitness testimony is unreliable. We are by definintion subjective, that is where science and testing comes in to be objective, take out that human opinion. Now, to show my ignorance, what does the unit a^-1 mean, I assume per year. This is the reason I like to read blogs, to learn things (not just to police the web for geologic inaccuracies)

  5. John B says:

    No worries Fish. I didnt take it as such. And I did feel like an idiot for screwing that up. My Geology friends are gonna smack me. So any scorn was deserved and your correction was appreciated.

    I was confused initially about that too. Frankly, the paper was so full of science jargon, I found it pretty difficult to interpret. 1.7 mm a−1 or unit per a-1 means change per year, “a” = annum, a form of the latin word annus which means year. Why geologists use unit per a-1 instead of unit per y -1? Just to be fancy and confuse us I guess.

    “I was not trying to be rude, I don’t think you are moronic or anything, I just think that your first sentence was and I stand by that. There is a reason why eyewitness testimony is unreliable.”

    You know, even though I disagreed from one perspective (and what I was trying to do was put the science paper into a non-science, every day context) I agree with you from another. And I like your point about removing the subjective humanness from science. I do a fair amount of meta-analyses where I combine quantitative survey data from lots (thousands) of reefs to look at broad scale patterns. You would be amazed at how frequently I get criticized by colleagues for never having been to many of the reefs that I use data from. But I think that is in a sense a benefit: I have never seen it, don’t have any subjective views, emotional attachments to it, etc. I just have the number, cold and objective. Like the blind watchmaker.

  6. Dave says:

    A better late than never comment: I believe that there is a misunderstanding of glacial rebound here. When one end of the North American plate is unweighted (deglaciation), not all of the plate is going to rise up. There will be areas that actually subside under those circumstances.

    One can use a large wooden raft out on a still lake as an analogy. Put a bunch of partying teenagers over on one side of the raft and you will get some sinking on one end of the raft and some lifting on the other end. When the kids all jump in the water, the opposite occurs. Structural geology makes this much more complex in the case of the North American plate, but the concept is the same.

    From WikiP:
    “Since the glacial isostatic adjustment process causes the land to move relative to the sea, ancient shorelines are found to lie above present day sea level in areas that were once glaciated. On the other hand, places in the peripheral bulge area which was uplifted during glaciation now begins to subside. Therefore ancient beaches are found below present day sea level in the bulge area.”
    http://en.wikipedia.org/wiki/Post-glacial_rebound
    There is a map currently there that shows North Carolina to be in this bulge area.

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