Posted by: coastlinesproject | April 27, 2016

Why is SLR so fast on the East Coast? Thanks Bill Ryan!

You may find the attached article interesting, if a little dense. I have extracted some of the main points
below. It is also summarized at:
http://www.climatecentral.org/news/sinking-atlantic-coastline-meets-rapidly-rising-seas-20247

A big question today is: Why is sea level rise occurring at a faster rate along the Atlantic seaboard
compared to other locations?

The four major culprits (though there are smaller factors as well) are:

1. Climate induced ocean level rise. Primarily expansion of the ocean due to heating (steric rise), but
also includes increases due to melting of land ice (including glacial melt).

2. Slowing of the Atlantic Meridional Overturning Circulation [Sallenger et al., 2012; Yang et al., 2016]
and variations in the Gulf Stream location and intensity [Ezer et al., 2013] have recently been identified
as possible drivers of accelerated sea-level rise in this region.

3. Ground level subsidence. As the ice sheet moved south during the last glacial period, it pushed
much of North America down under its weight. However, just ahead (south) of the ice sheet, the
land actually “bulged” upward – think of trying to get lumps out of a rug, as you push, the rug
bulges ahead of you. So, when the ice retreats, areas in inland Maine and Quebec “bounce”
back up (rise) while areas along the coast will “relax” downward.

4. Subsidence as ground water is depleted. This is a huge problem in the Central Valley of
California these days but also occurs world wide. It is a factor along the Atlantic coastal plain
[Depaul et al., 2008; Boon et al., 2010].
See also:

As Groundwater Withdrawals Increase, Jakarta Sinks

This paper uses GPS and geologic data to try to determine the effects of (3) and (4) above.

Karegar, M. A., T. H. Dixon, and S. E. Engelhart (2016), Subsidence along the Atlantic Coast of North
America: Insights from GPS and late Holocene relative sea level data, Geophys. Res. Lett., 43, doi:10.1002/
2016GL068015.

Selections from the article, lightly edited:

Eastern North America is a passive continental margin. Most of this margin is experiencing spatially variable,
long-term vertical motion due to glacial isostatic adjustment (GIA), a viscoelastic response of the Earth’s crust
and mantle to retreat of the Laurentide Ice Sheet since the last glacial maximum ~ 20,000 years ago [e.g.,
Peltier, 2004].

Definition: Viscoelastic – There are two ways in which materials can deform, elastically or viscously.
In practice a material can exhibit either/both type of behaviour, under different conditions or different rates
of deformation. This hybrid behaviour is known as viscoelastic.

GIA drives land uplift in areas under the former Laurentide Ice Sheet and land subsidence in
peripheral areas as the forebulge beyond the former ice sheet margin collapses. Subsidence along parts of
the Atlantic Coast constitutes the largest-amplitude proglacial forebulge collapse on Earth

New GPS data based on longer time series (average record length of 8.5 years) and additional stations
(~190 new sites) are now available, allowing substantial refinement of the present-day vertical velocity field.
We compare these data to high-quality geological records of Holocene RSL describing vertical land motion
in the region from 4 ka B.P. to 1900 A.D.

Conclusions:

The GPS and geologic data indicate that the highest subsidence rates concentrate in a coastal region from
northern Delaware and Maryland (~40°N) to the northern part of North Carolina (~35°N) (mean ~1.5 mm/yr,
up to ~3 mm/yr).

Note: Global average sea level rise rate – 20th Century average ~ 1.7 mm/yr; since 1993: ~3.4 mm/yr
http://climate.nasa.gov/vital-signs/sea-level/

In the central and southern Atlantic coastal plains (38°N–32°N), we observe a correlation between rapid
subsidence and groundwater depletion, consistent with the idea that excessive groundwater extraction
is driving rapid land subsidence in these regions. The short-term GPS subsidence rates are double the
long-term geologic subsidence rates. Our result also suggests that recent changes in groundwater
management have been effective at reducing aquifer compaction [in portions of the southern Chesapeake].

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