Putting Climate Change in a Geologic Context - Are Models Under-predicting Future Changes in Sea Level?
By Frances Moore, Research Associate, The Climate Institute
One of the most direct, and potentially most serious, consequences of climate change is sea level rise. Even a rise of 1-2 feet over the 21st century would begin to inundate low-lying regions, accelerate coastal erosion and increase the exposure of coastal cities to hurricanes, storms and other extreme events. The EPA estimated the cost to the United States of a one meter (3ft) rise in sea level over the coming century at approximately four hundred billion dollars. Although the Greenland Ice Sheet is only about as large as Mexico or Algeria, complete melting would raise sea level by 6-7m (roughly 20 feet). Studies of the past make it clear that we should pay attention to what happens in Greenland because about half of Greenland apparently melted during the last interglacial. Additional cause for concern is that recent studies have suggested that the rate of melting has been accelerating, perhaps putting us on the slippery slope to much higher sea level in this century and beyond.
In its recently released Fourth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) estimated that sea level would rise by between 0.18 and 0.59 m between 1990 and 2100. This estimate was made using Global Climate Models, which predict temperature and precipitation around the world, combined with smaller-scale glaciological models which use the physics of ice sheets to predict the rate of melting. The treatment of glaciers and ice sheets in these models are limited both by our understanding of the processes that control ice sheet behavior and by a paucity of observations characterizing how glaciers and ice sheets respond to changes in the climate.
However, recent data coming from Greenland such as the increasing frequency of glacial earthquakes caused by glaciers slipping rapidly over bedrock and the formation of meltwater pools on the surface of the ice sheet suggest that the ice sheet is responding to warming faster than expected. Current glaciological models are not yet able to fully represent the mechanisms that govern the dynamics of ice-streams or the influence of meltwater, both of which could be crucial in controlling the loss-rate from the ice sheets. A recent comparison of the observed sea-level rise since 1990 with the IPCC projections shows that sea-level has been proceeding at the highest of the range of projections (1). These high-end projections are those that include an adjustment for 'land-ice uncertainty' suggesting that ice-sheet deterioration may already be exerting a significant influence on the rate of sea level rise.
Fig. 1 Satellite images showing the western edge of the Greenland Ice Sheet taken in August of 2001, 2002 and 2003. They show the dramatic increase in summer melt area that has occurred in recent years, and the unprecedented appearance of large meltwater pools on the ice surface. Meltwater pools, by absorbing more of the incoming solar energy than the surrounding ice act to accelerate melting. In this way, they tend to reduce the time it takes for the ice sheet to react to warmer arctic temperatures.
Photos from NASA GSFC
Fig. 2 When enough meltwater accumulates, a meltwater stream can form. This picture shows a meltwater stream on the surface of the Greenland Ice Sheet flowing into the ice through a tunnel called a moulin. These streams transport surface heat deep into the ice at a much faster rate than would otherwise occur by simple conduction of warming through the ice. This process thus has the potential to accelerate the deterioration of the ice sheet well beyond rates being projected in current model simulations.
Since model simulations may lead to underestimates of the future rate of loss of ice sheets, looking at ice sheet behavior during climatic changes in the distant past can offer some clues as to what can be expected in the coming decades.
The last time in the Earth's geological history that sea level and global temperatures were comparable to what they are today was during the last interglacial, 129,000 to 118,000 years ago. Elevated beaches and coral reefs from that time indicate that sea level was approximately 4-6m (roughly 13-19 feet) above where it is today while ice core data combined with modeling of the paleo-climate shows that much of southern Greenland was deglaciated, explaining between 2.2 and 3.4 m of the higher sea level (2). Simulations of the last interglacial climate also show that this deglaciation occurred when arctic temperatures were only 3-5° warmer than today and global annual average temperatures were not substantially different than present.
Overpeck et al. (3) compared arctic climate during the last interglacial with conditions projected to occur during the 21st century under a business-as-usual emissions scenario. They found that the Arctic will be substantially warmer before the end of 21st century than it was during the last interglacial (as shown in figure 3). This suggests that similar areas of Greenland could melt, raising sea level by at least two meters. Just how rapidly this melting might occur is a key question, with traditional models suggesting that it could take a thousand years, but the accelerating pace of melting recently observed suggesting that it might take only centuries.
Fig. 3 The two left panels show modern arctic summer temperatures and simulated differences from modern for the last inter-glaciation (LIG) when one third to one half of Greenland was deglaciated. The two right panels show projections of temperature change relative to modern for 2100 (at three times preindustrial CO2) and 2130 (at four times preindustrial CO2). These projections suggest that Arctic temperatures will exceed those of the LIG before the end of the century unless action is taken to curb emissions. From Overpeck et al., 2006.
Knowing just how fast Greenland could melt is crucial for coastal planning because it directly affects the rate of sea level rise. Looking back at what has happened in the past shows that ice sheet changes have happened at rates far faster than those predicted by current models for the 21st century. Scientists are able to reconstruct the rates of past increases in sea level through the dating of ancient coral reefs. Certain species of coral are known to grow only at certain depths in the ocean. For example Acropora palmata, or elkhorn coral, is known to live 5-7 m below the surface. As sea level rises, the original deeper coral dies and new coral grows on top. Scientists can determine the age of ancient coral using 14C or U-Th dating and so can put together a record of past sea level.
Fig. 4 Cartoon showing the effect of sea level rise on coral reefs. The coral in the first diagram is growing 5-7 m below sea level but as sea level rises, the coral dies and a new, younger coral grows 5-7 m below the new sea level.
Graphics by Molly Clare Wilson
Data combined from several reefs dating from the last interglacial suggest that sea level may have risen at rates of 30-50 mm/year, indicating that the disintegration of the ice sheets in the penultimate deglaciation was sudden and catastrophic (4). This rate of sea level rise is approximately 10 times the projected rate for the coming century and yet occurred under climatic conditions similar to those we can expect to see in less than a hundred years unless serious steps are taken to curb greenhouse gas emissions.
Further evidence that ice sheet collapse can be relatively rapid comes from evidence of sea level rise during the most recent deglaciation. At the last glacial maximum 21,000 years ago, sea level was 120 m lower than today. The rise in sea level to present day levels is characterized by several episodes of particularly rapid sea level rise known as meltwater pulses, each representing the sudden collapse of a large area of continental ice. There is evidence for meltwater pulses at 19,000 years ago, 14,200 years ago, 11,500 years ago, and 7,600 years ago (5 and 6), each one adding the equivalent of 1.5 to 3 Greenland Ice Sheets to the ocean over a period of one to five centuries (7). The most well-documented of these pulses occurred about 14,200 years ago and is known as meltwater pulse 1a (MWP 1a). Evidence for MWP 1a has been found in corals from Barbados, Tahiti, and Southeast Asia (8,9 and 10). These coral records show that sea level rose by 16 m over 300 years, an average rate of about 50 mm/year - twenty times faster than the current rate of sea level rise.
Fig. 5 Curve showing sea level over the last deglaciation as derived from coral records. Meltwater pulse 1a represents a sudden rise in sea level of 16 m over 300 years. Other meltwater pulses have been proposed at 19, 11.5 and 7.6 thousand years ago. Each pulse represents the equivalent of adding between 1.5 and 3 Greenland Ice Sheets to the ocean over the period of a few centuries.
Image from the Global Warming Art Project
Perhaps the most remarkable aspect of the post-glacial sea level curve is the stability of sea level over the period of human civilization. For the past six thousand years, shorelines have remained unusually stable and humans have flocked to the coasts where many of our major cities have been built. But the geological record tells us that this long-term stability is the exception rather than the rule. Sea level has risen rapidly in the past and ongoing emissions of greenhouse gasses greatly increase the likelihood that it will do so again in the future. The risk of a sudden and significant increase in the rate of sea level rise to rates seen during past meltwater pulses, potentially critical for the future of many coastal cities, cannot yet be evaluated by current models which do not capture the dynamic ice processes that would be involved. Without action to curb emissions, temperatures in the Arctic will likely exceed those of the last interglacial before the end of the century suggesting that, based on what has happened in the past, we should take seriously the risk of having to deal with a 4-6m sea level rise over the next few centuries.
Frances Moore graduated in June 2006 from Harvard University where she studied Earth and Planetary Science. She has spent time on Svalbard investigating the links between climate and glacial change in the high Arctic and has performed research on carbon cycling in the Cretaceous ocean.
REFERENCES
1. Rahmstorf, S., Cazenave, A., Church, J.A., Hansen, J.E., Keeling, R.F., Parker, D.E., and Somerville, R. C. J., 1 Feb 2007, Recent climate observations compared to projections, Science Express
2. Otto-Bliesner, B. L., Marshall, S. J., Overpeck, J. T., Miller, G. H., Hu, A. and CAPE Last Interglacial Project Members, 2006, Simulating arctic climate warmth and icefield retreat in the last interglaciation, Science, 311 p1751-1753
3. Overpeck, J. T., Otto-Bliesner, B. L., Miller, G. H., Muhs, D. R., Alley, R. B., and Kiehl, J. T., 2006, Paleoclimatic evidence for future ice-sheet instability and rapid sea level rise, Science, 311 p1747-1750
4. McCulloch, M. T. and Esat, T., 2000, The coral record of last interglacial sea levels and sea surface temperatures, Chemical Geology, 169 p107-129
5. Clark, P. U., McCabe, A. M., Mix, A. C., and Weaver, A. J., 2004, Rapid rise of sea level 19,000 years ago and its global implications, Science, 304 p1141-1144
6. Blanchon, P., and Shaw, J., 1995, Reef drowning during the last deglaciation: Evidence for catastrophic sea level rise and ice-sheet collapse, Geology, 23 p4-8
7. Alley, R. B., Clark, P. U., Huybrechts, P., and Joughin, I., 2005, Ice-sheet and sea level changes, Science, 310 p456-460
8. Bard, E., Hamelin, B., Fairbanks, R. G., and Zindler, A., 1990, Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals, Nature, 345 p405-410
9. Bard, E., Hamelin, B., Arnold, M., Montaggioni, L., Cabioch, G., Faure, G., and Rougerie, F., 1996, Deglacial sea level record from Tahiti corals and the timing of global meltwater discharge, Nature, 382 p241-244
10. Hannebuth, T., Stattegger, K., and Grootes, P. M., 2000, Rapid flooding of the Sunda Shelf: A late-glacial sea level record, Science, 288 p1033-1035
February 8, 2007