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West Antarctica and threat of a sea level rise disaster

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A short article written for the GVSU Interchange in 2009.

A short article written for the GVSU Interchange in 2009.

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  • 1. 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
  • 2. 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.
  • 3. 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? Further Reading 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.
  • 4. 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.