The Rising Tide -
Cause, Effects and Planning
for
Rising Sea Level
 Science,
measurements and predictions
Relative sea level rise is made up of a number
of factors. As global warming occurs, the oceans are heating
up, and as water heats it expands. At the same time, many land-based
glaciers are melting at an accelerated rate, adding water to
the oceans. These two factors constitute the absolute sea
level rise, estimated to be about 1.8 to 2.8 mm/year (308 mm
= 1 foot). (This base rate of rise is significant since vegetated
tidal marshes retain enough sediment and plant material to
keep up with a 2 mm/year sea level rise – so our vegetated
tidal wetlands would just keep their heads above water at that
rate.)
In addition, there is subsidence
of the land mostly caused by groundwater withdrawals,
in the Chesapeake Bay region. In populated areas using
groundwater or around large industrial facilities withdrawing
large volumes of groundwater (paper mills, refineries,
etc.) there is localized subsidence of the land that increases
the relative sea level rise over the base rate.
Another contributor to subsidence within the Chesapeake
Bay region is linked to the recession of continental glaciers
at the end of the last ice age. During the height of
the ice age, the mass of the ice that covered the northeastern
US depressed the land in that region and like a teeter-totter,
uplifted the mid-Atlantic region south of the glaciation. After
the ice receded, the previously glaciated region has slowly
rebounded while our region has slowly been subsiding. This
shift, called “isostatic
rebound,” means
that the relative sea level rise is greater here than to the
north or south because we are slowly sinking back to our original
elevation.
Finally – remember that comet ? Geologists
think that we are still seeing subsidence at the lower end
of the Chesapeake Bay as we continue to settle back into the
crater created by the comet’s impact, further increasing
relative sea level rise.
Together, all
of these factors are causing the relative sea level
rise in Hampton Roads to run about 4.25 mm/year or about
1½ feet
per century. Other estimates of the relative sea level
rise are higher – University of Maryland researchers
have put the relative sea level rise in the mid-Atlantic
closer to 3 feet per century. And if any of the more catastrophic
predictions hold true, such as massive melting of the Greenland
or Antarctic glaciers, the scale of sea level rise will
be significantly greater.
This base rate of sea level rise is a conservative estimate,
relying upon historic data and historic data on temperature
and sea level rise may not apply. Some of the recent data shows
a troubling increase
in global temperature starting about twenty years ago.
This illustrates the need for two sets of responses to sea
level rise: reducing greenhouse gas emissions to keep the warming
and sea level rise from getting even worse, and mitigating
the damage from the ~2-foot rise that is certain to come. Other
groups are taking on the greenhouse gas reductions to reduce
higher sea level rise. Wetlands Watch is concentrating on preparing
for the sea level rise that we know is coming.
The International Panel on Climate Change, the United Nations
organization collecting global warming data worldwide, estimated
that North America will lose 50% of its tidal wetlands in the
next 100 years. In regions surrounding the Chesapeake Bay,
aerial surveys have already found that more than half of the
tidal vegetated wetlands are already degraded, in part due
to sea level rise. With more rapid sea level rise, this figure
will undoubtedly increase, with localized rates of wetlands
loss much higher, approaching 100% if development
or shoreline hardening has been allowed behind the
tidal marshes.
In the southern Chesapeake Bay, the tidal range is about 2.5
feet and it is within this 2.5-foot elevation – the intertidal
zone - that the vegetated tidal marshes grow. When you raise
the sea level ~2 feet, most of the marshes will drown if they
cannot gradually move “uphill” as the shoreline
gets flooded.
We have laws to protect the upland shoreline behind the tidal
marshes. In Tidewater Virginia the Chesapeake Bay Preservation
Act (CBPA) is supposed to insure that no development comes
within 100 feet of the shoreline. Maryland has similar provisions
in its Critical Areas Act.
However, waivers are routinely granted in Virginia and the
law is not well implemented (http://jlarc.state.va.us/Meetings/October02/CBLAD.pdf).
A recent study on the Maryland shoreline protection system
showed similar problems (http://www.law.umaryland.edu/specialty/environment/documents/Final_Critical_Area_Report.pdf)
In a perverse twist, sometimes out of a desire to avoid wetlands
impacts, shoreline alterations and erosion protection work
is moved into the CBPA buffer, dooming the remaining fringe
wetland as sea level rises.
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