Global sea levels are currently rising as a
result of the melting of the Greenland and Antarctica ice
sheets and thermal expansion of the world’s ocean waters due
to global warming. We are presently in an interglacial stage of
an ice age. Sea levels have risen nearly 400 feet (122 m) since
the last glacial maximum 20,000 years ago and about 6 inches
(15 cm) in the past 100 years. The rate of sea-level rise seems
to be accelerating and may presently be as much as an inch
every eight to 10 years. If all the ice on both the Antarctic and
Greenland ice sheets were to melt, global sea levels would rise
by 230 feet (70 m), inundating most of the world’s major
cities and submerging large parts of the continents under shallow
seas. The coastal regions of the world are densely populated
and are experiencing rapid population growth.
Approximately 100 million people presently live within one
meter of the present-day sea level. If sea levels were to rise
rapidly and significantly, the world would experience an economic
and social disaster of a magnitude not yet experienced
by the civilized world. Many areas would become permanently
flooded or subject to inundation by storms, beach erosion
would be accelerated, and water tables would rise.
The Greenland and Antarctic ice sheets have some significant
differences that cause them to respond differently to
changes in air and water temperatures. The Antarctic ice sheet
is about 10 times as large as the Greenland ice sheet, and since
it sits on the South Pole, Antarctica dominates its own climate.
The surrounding ocean is cold even during summer, and
much of Antarctica is a cold desert with low precipitation
rates and high evaporation potential. Most meltwater in
Antarctica seeps into underlying snow and simply refreezes,
with little running off into the sea. Antarctica hosts several
large ice shelves fed by glaciers moving at rates of up to a
thousand feet per year. Most ice loss in Antarctica is accomplished
through calving and basal melting of the ice shelves at
rates of a 10–15 inches per year (25–38 cm/yr).
In contrast, Greenland’s climate is influenced by warm
North Atlantic currents and its proximity to other landmasses.
Climate data measured from ice cores taken from the top
of the Greenland ice cap show that temperatures have varied
significantly in cycles of years to decades. Greenland also
experiences significant summer melting, abundant snowfall,
has few ice shelves, and its glaciers move quickly at rates of
up to miles per year. These fast-moving glaciers are able to
drain a large amount of ice from Greenland in relatively short
amounts of time.
The Greenland ice sheet is thinning rapidly along its
edges, losing an average of 15–20 feet in different areas
(4.5–6 m) over the past decade. In addition, tidewater
glaciers and the small ice shelves in Greenland are melting an
order of magnitude faster than the Antarctic ice sheets, with
rates of melting between 25–65 feet per year (7–20 m/yr).
About half of the ice lost from Greenland is through surface
melting, forming water that runs off into the sea. The other
half of ice loss is through calving of outlet glaciers and melting
along the tidewater glaciers and ice shelf bases.
These differences between the Greenland and Antarctic
ice sheets lead them to play different roles in global sea-level
rise. Greenland contributes more to the rapid short-term fluctuations
in sea level, responding to short-term changes in climate.
In contrast, most of the world’s water available for
raising sea level is locked up in the slowly changing Antarctic
ice sheet. Antarctica contributes more to the gradual, longterm
sea-level rise.
What is causing the rapid melting of the polar ice caps?
Most data suggests that the current melting is largely the
result of the gradual warming of the planet in the past 100
years through the effects of greenhouse warming. Greenhouse
gases have been increasing at a rate of more than 0.2 percent
per year, and global temperatures are rising accordingly. The
most significant contributor to the greenhouse gas buildup is
carbon dioxide, produced mainly by the burning of fossil
fuels. Other gases that contribute to greenhouse warming
include carbon monoxide, nitrogen oxides, methane (CH4),
ozone (O3), and chlorofluorocarbons. Methane is produced
by gas from grazing animals and termites, whereas nitrogen
oxides are increasing because of the increased use of fertilizers
and automobiles, and the chlorofluorocarbons are
increasing from release of aerosols and refrigerants. Together
the greenhouse gases have the effect of allowing short-wavelength
incoming solar radiation to penetrate the gas in the
upper atmosphere, but trapping the solar radiation after it is
reemitted from the Earth in a longer wavelength form. The
trapped radiation causes the atmosphere to heat up, leading
to greenhouse warming. Other factors also influence greenhouse
warming and cooling, including the abundance of volcanic
ash in the atmosphere and solar luminosity variations,
as evidenced by sunspot variations.
Measuring global (also called eustatic) sea-level rise and
fall is difficult because many factors influence the relative
height of the sea along any coastline. These vertical motions
of continents are called epeirogenic movements and may be
related to plate tectonics, rebound from being buried by
glaciers, or to changes in the amount of heat added to the
base of the continent by mantle convection. Continents may
rise or sink vertically, causing apparent sea-level change, but
these sea-level changes are relatively slow compared to
changes induced by global warming and glacial melting.
Slow, long-term sea-level changes can also be induced by
changes in the amount of seafloor volcanism associated with
seafloor spreading. At some periods in Earth’s history, seafloor
spreading was particularly vigorous, and the increased volume
of volcanoes and the mid-ocean ridge system caused
global sea levels to rise.
Steady winds and currents can mass water against a particular
coastline, causing a local and temporary sea-level rise.
Such a phenomena is associated with the ENSO, causing sealevels
to rise by 4–8 inches (10–20 cm) in the Australia-Asia
region. When the warm water moves east in an ENSO event,
sea levels may rise 4–20 inches (10–50 cm) across much of
the North and South American coastlines. Other atmospheric
phenomena can also change sea level by centimeters to meters
locally, on short time scales. Changes in atmospheric pressure,
salinity of seawaters, coastal upwelling, onshore winds,
and storm surges all cause short-term fluctuations along segments
of coastline. Global or local warming of waters can
cause them to expand slightly, causing a local sea-level rise. It
is even thought that the extraction and use of groundwater
and its subsequent release into the sea might be causing sealevel
rise of about 0.78 inches per year (1.3 mm/yr). Seasonal
changes in river discharge can temporarily change sea levels
along some coastlines, especially where winter cooling locks
up large amounts of snow that melt in the spring.
It is clear that attempts to estimate eustatic sea-level
changes must be able to average out the numerous local and
tectonic effects to arrive at a globally meaningful estimate of
sea-level change. Most coastlines seem to be dominated by
local fluctuations that are larger in magnitude than any global
sea-level rise. Recently, satellite radar technology has been
employed to precisely measure sea surface height and to document
annual changes in sea level. Radar altimetry is able to
map sea surface elevations to the sub-inch scale, and to do
this globally, providing an unprecedented level of understanding
of sea surface topography. Satellite techniques support the
concept that global sea levels are rising at about 0.01 inches
per year (0.025 cm/yr).
See also EL NIÑO; GLACIER; PLATE TECTONICS.
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