A short article written for the GVSU Interchange in 2009.
A short article written for the GVSU Interchange in 2009.
West Antarctica and the threat of a sea level rise disaster
Patrick M. Colgan, Department of Geology
One of the most profound geoscience questions today is how quickly will glaciers and
ice sheets melt, and how rapidly will sea level rise in the next century? In particular, rapid
collapse of the West Antarctic ice sheet poses a possible threat to our civilization. Our
descendents could either thank us, or curse us for how we deal with this and other issues of
human-induced climate change.
Even by the late 1950s it was already apparent that humans were unintentionally
carrying out “a large-scale geophysical experiment” by burning coal, oil, and natural gas to
power our industrial society and thereby releasing large amounts of CO2 into the atmosphere
(p. 19, Revelle and Seuss, 1957). By the late 1970s, the amount of carbon dioxide (CO2) in the
atmosphere had already increased by about 20% from pre-industrial values and scientists
cautioned that this was causing global warming by enhancing the natural greenhouse effect.
In 1978, John Mercer published an article in Nature entitled, “West Antarctic Ice Sheet
and CO2 greenhouse effect: a threat of disaster”. Mercer, a glaciologist/glacial geologist at the
Ohio State University, hypothesized that as Earth’s climate warmed, the West Antarctic ice
sheet could thin, become unstable, and eventually break up. If this were to occur, he estimated
that 5 meters (16.4 feet) of sea level rise could occur rapidly.
What makes the West Antarctic ice sheet different from most other glaciers is that it lies
on land well below sea level. Much of this ice sheet is grounded on sea bed up to 2500 meters
deep (~8000 feet). If the ice sheet thinned significantly, due to global warming, it could allow
sea water to flood in under the ice, float the ice, and then break it up. Drop a few ice cubes in
your drink and observe what happens to the water level. Since Mercer’s paper in 1978, the
question of possible West Antarctic ice sheet collapse and rapid sea level rise has been a
dominant concern of glaciologists and climatologists working in Antarctica.
Today’s glaciers and ice sheets contain about 65 meters (~210 feet) of potential sea
level rise. If all the glaciers in the world melted, entire coastal nations would disappear under
the rising ocean. This is a fact that can be demonstrated using simple arithmetic, estimates of
ice sheet volume, and accurate topographic maps. About 99% of the potential sea level rise is
contained in the ice sheets of Greenland (~5 to 6 m) and Antarctica (~60 to 65 m). The other
1% is contained in all of the other glaciers in the world (~0.72 m).
During the last glacial maximum, about 21,000 years ago, sea level reached a level of
125 meters lower than today (~410 feet). As Earth’s climate warmed between 21,000 and
10,000 years ago, sea level rose rapidly, as it had done dozens of times over the last 2.5 million
years at the end of each glacial cycle. These glacial and interglacial cycles are driven by changes
in Earth’s orbit that vary the intensity of incoming solar radiation during the seasons. First
recognized by James Croll and then Milutin Milankovid, these astronomical cycles are the
pacemaker of Earth’s climate at glacial and interglacial time scales (21, 41, and 100 thousand
year cycles). The mean rate of sea level rise was probably about 12 mm per year during the
transition from the last glaciation to the present interglacial. Evidence of drowned coral reefs,
even suggests that sea level may have risen faster than 20 mm/y, during the most rapid periods
of ice sheet collapse.
At about the same time as Mercer’s paper, glaciologists began documenting that many
glaciers in the world were shrinking rapidly. The 1970s, 1980s and 1990s were especially bad
for glaciers. In 1997, glaciologists Mark Dyurgerov and Mark Meir of the University of Colorado
at Boulder published a comprehensive study, which estimated that mountain glaciers had
retreated and returned about 2650 cubic kilometers of water back into the ocean over the
period 1961-1991. This corresponded to about 7.4 mm of sea level rise in just 30 years (or
mean of 0.25 ± 0.10 mm/y). At this time, no one had any idea whether the Greenland and
Antarctic Ice sheets were melting or growing.
Estimates and direct measurements of interglacial, historical, and recent sea level rise
were also being made at the end of 20th century. During the last 8000 years, sea level has
slowly risen about 5 meters or at a rate of approximately 0.6 mm/y. Sea level rise during the
20th century has been estimated from tide gauge data to be about 1.7 ± 0.05 mm/y. This is 3 to
5 times the long term average for the last 80 centuries. Estimates for the next hundred years
vary from a low of 3 mm/y (no change in rate), or 5 to 7 mm/y (increased glacier melting due to
warming), or greater than 10 mm/y (beginning of collapse of West Antarctica), to a complete
collapse of West Antarctica with enhanced glacier melting producing > 5 meters of sea level rise
(rates higher than 20 mm/y).
Sea level is thought to rise or fall mainly because of changes in the mass of water in the
ocean, and changes in the volume of the water in the ocean. Changes in mass are controlled by
how much water is locked up in glaciers, streams, lakes, and groundwater. Since glaciers are
the biggest reservoir in the hydrological cycle on land (>90%), the size of glaciers and ice sheets
are the most important factor in how much water is left in the ocean. The volume of the water
in the ocean depends directly on temperature. As water warms it expands and as it cools it
shrinks like most liquids. As the ocean warms it expands. Scientists currently believe that the
observed sea level rise from 1993 to 2003 (~3.1 ± 0.07 mm/y) is 50% due to thermal expansion,
and 50% due to glacial melting (Bindoff et al., 2007).
Today in 2009, we have direct measurements of what is happening with our glaciers and
ice sheets, and the findings are not all that calming. Recent improvements in satellite
technology, geodesy, and mapping have provided ways to map, measure the thickness, and
determine the mass changes of entire ice sheets. Direct measurements suggest that the recent
melt rates of all mountain glaciers are causing sea level to rise much faster than the 0.25 ± 0.10
mm/y rate estimated by Dyuregov and Meir (1997). Recent studies show that much of this
melting is occurring in southeast Alaska (~0.47 mm/y), Patagonia (0.11 mm/y), Arctic (~0.19
mm/y) and in the high mountains of Asia (0.10 mm/y). The total melting (~1 mm/yr) in these
areas is probably at least three times the rate estimate by Dyuregov and Meir (1997) for 1961-
1991. Most of the recent measurements indicate that glaciers experienced accelerated melting
in the 1991-2009 period compared to 1961-1991.
Gravity measurements of ice sheet mass taken during the GRACE mission in the last few
years (Gravity Recovery and Climate Experiment) show that the Greenland ice sheet is losing
mass and providing about 0.34 mm/y to sea level rise (Ramillien et al. 2006). Measurements
for West Antarctica indicate a loss of about 0.30 mm/y of sea level rise, while the larger East
Antarctic ice sheet seems to be growing and actually lowers sea level by a rate of 0.19 mm/y
(Ramillien et al. 2006). Based on these newest measurements the three large ice sheets are
contributing a net 0.45 mm/y of sea level rise per year. In sum, these data confirm that glaciers
and ice sheets are probably contributing between 1 and 2 mm/y to current sea level rise
(0.45mm/y + 0.9 mm/y). Thermal expansion of sea water, due to warming of the oceans can
explain the rest of the observed 3.1 mm per year rise.
None of the newest estimates of sea level rise or predictions of future sea level rise take
into account a collapse of the West Antarctic ice sheet. Since 1978, the greenhouse gas CO 2
levels have increased by about another 20% for a total increase of roughly 40% since pre-
industrial levels. Temperatures have warmed in many regions including in West Antarctica. We
know that West Antarctica is melting according to the GRACE measurements. If the ice sheet
crosses a tipping point and begins to collapse, then sea level rise could reach rates higher than
10 or even 20 mm/y. This is 3 to 6 times the current observed rate of rise (3.1 mm/y). Over a
billion people live within one meter of the Earth’s current sea level and even the current rates
of sea level result in an enormous cost to society. Can we really afford to wait and see what
happens, or should we quickly make changes to try to mitigate the effects human-induced
climate change and the possible sea level changes that have already been set in motion?
Bindoff, N.L., J. Willebrand, V. Artale, A, Cazenave, J. Gregory, S. Gulev, K. Hanawa, C. Le Quéré,
S. Levitus, Y. Nojiri, C.K. Shum, L.D. Talley and A. Unnikrishnan, 2007: Observations:
Oceanic Climate Change and Sea Level. In: Climate Change 2007: The Physical Science
Basis. Contribution of Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen,
M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press,
Cambridge, United Kingdom and New York, NY, USA.
Mercer, John H. (1978). West Antarctic Ice Sheet and CO2 greenhouse effect: A threat of
disaster. Nature, v. 271, p. 321-325.
Dyurgerov, Mark B. & Meier, Mark F. (1997). Year-to-Year Fluctuations of Global Mass Balance
of Small Glaciers and Their Contribution to Sea-Level Changes. Arctic and Alpine
Research, v. 29, no. 4, p. 392-402
Ramillien, G., Lombard, A., Cazenave, A., Ivins, E. R., Llubes, M., Remy, F., & Biancale, R. (2006).
Interannual variations of the mass balance of the Antarctica and Greenland ice sheets
from GRACE. Global and Planetary Change, v. 53, no. 3, p. 198-208.
Revelle, Roger, and Hans E. Suess (1957). Carbon Dioxide Exchange between Atmosphere and
Ocean and the Question of an Increase of Atmospheric CO2 During the Past Decades.
Tellus, v. 9, no. 1, p. 18-27.