Geoscience Reference
In-Depth Information
Global ChanGe in the oCeans
6
The Sea Level Rise
the ice sheets re-form and advance toward
the south. However, the return of the gla-
ciers in full bloom may well have been de-
layed by warming of the atmosphere due to
greenhouse gases.
After the last glacial maxima of eigh-
teen to twenty thousand years ago, the sea
level rose almost to its present level about
four to five thousand years ago. From then
on the sea generally rose very slowly to its
present location. In different parts of the
globe the rate of change differed, because
the Earth is not a perfect sphere and also
because the land surface is moving up and
down in some locations. In the Southern
Hemisphere sea level after the last glacia-
tion actually rose two or three feet above
the current level, and for the last two to
three thousand years it has slowly dropped
in places like Brazil, South Africa, and
Australia. But with the sudden increase
in greenhouse gases and warming of the
oceans, sea level is rising almost every-
where today, and the rate of rise is acceler-
ating. Currently the level of the sea is ris-
ing at an overall rate of 3.2 to 3.5 mm (1.26
to 1.38 inches) per year, as determined by
both the last thirty years of tide gauges (a
The level of the sea changes for many rea-
sons and on many time scales. But melt-
ing land ice will be the major influence on
global sea level changes on the time scale
that should concern humans (chapter 5).
The behavior of the world's ice sheets in
the next four or five decades will determine
whether sea level rise becomes the first
great global catastrophe of climate change,
flooding the coastal cities and displacing
millions of rural dwellers in countries like
Vietnam and Bangladesh.
Eighteen thousand years ago, a mere blip
on the geologic time scale, the sea was 120
meters (400 feet) lower than it is today,
thanks to the huge amount of water tied
up in the massive glaciers and ice sheets
grinding their way across the landscape
from polar regions mostly in the North-
ern Hemisphere. Over the last two million
years the sea level has risen and fallen in a
major way at least seven times in rhythm
with the advance and retreat of the gla-
ciers. Currently geologists believe that we
are in an interglacial stage, and we can ex-
pect the sea level to go down once more as
