Geoscience Reference
In-Depth Information
total record of 150 years) and satellites (go-
ing back 18 years). This is more than 30 cen-
timeters (1 foot) per century.
Sea level rise is actually a combination
of the change in the volume of sea water
(eustatic change) and the local up or down
movement of the land (tectonic change).
Eustatic change includes the increase in
volume of the uppermost two thousand
feet of the ocean water column because of
warming water, and also the rise due to the
introduction of “new” water from melting
glaciers and ice sheets. Where the land is
sinking, the sea level rise is enhanced. For
instance, on parts of the Mississippi Delta
where the sediment is compacting and the
land is sinking because of the extraction of
oil and gas, the rise is as much as 1.2 meters
(4 feet) per century. Along the Pacific Coast
of Colombia, the land at the coast is rap-
idly sinking, because of the forces actively
building up the Andes mountain range. The
coast sinks as the mountains rise, pushing
the relative sea level rise rate up to as much
as three meters (ten feet) per century. On
the other hand, in the high latitudes the
land may be rebounding from the recently
removed weight of glacial ice. This is why
the sea level is dropping in parts of Scandi-
navia, Nova Scotia, and Alaska.
During the twentieth century the pri-
mary cause of sea level rise was thermal
expansion of the ocean water: an increase
in the volume of seawater as it warms up.
Roughly 80% of the warming by green-
house gases is stored in the upper 760 me-
ters (2,500 feet) of the ocean. The rest of
the heat is in the atmosphere. Because of
the downward momentum of the warm wa-
ter through the ocean's water column, ther-
mal expansion would continue for centuries
even if global warming and surface ocean
heating stopped tomorrow. The chemical
oceanographer Philip Froelich likens the
downward transfer of heat through the wa-
ter column to a flywheel: like the flywheel
in an engine, it provides momentum. As
the cold deep water is heated, it expands.
Simultaneously, the loss of this warm water
that sank into the deep has a cooling effect
on the shallow waters and causes the water
there to contract. The expansion is greater
than the contraction, so overall the ocean
expands and the sea level continues to rise.
The next-biggest contributor to sea level
rise over the twentieth century was the
melting of mountain glaciers, followed by
melting of the Greenland Ice Sheet. In the
twenty-first century global change scien-
tists believe that the Antarctic ice sheets
will be the single biggest source of water
causing the sea to rise, followed by the
melting of the Greenland Ice Sheet, ther-
mal expansion, and the melting of moun-
tain glaciers.
The short of it is that we should expect a
minimum
of a one-meter (three-foot) rise in
sea level by the year 2100, even in the un-
likely event that we quickly reduce the
amount of greenhouse gas emissions. The
momentum furnished by the flywheel of
sinking water will cause the system to roll
on and on. Hal Wanless, a geologist at the
University of Miami, believes that the most
