ScienceWeek

CLIMATOLOGY: INCREASED ESTIMATES OF FUTURE SEA-LEVEL RISE

The following points are made by Julian A. Dowdeswell (Science 2006 311:963):

1) The changing mass of the great ice sheets of Greenland and Antarctica represents the largest unknown in predictions of global sea-level rise over the coming decades. At 1.7 million km^(2), up to 3 km thick, and a little smaller than Mexico, the Greenland Ice Sheet would raise global sea level by about 7 m if it melted completely. This could take from a millennium to a few thousand years (if melting were the only mechanism by which it lost mass) depending on the magnitude of future warming [1]. Of more immediate concern are several sets of new observations, derived largely from remote-sensing satellites. As reported by Rignot and Kanagaratnam [2], the velocities of several large glaciers draining the ice sheet to the sea, already among the fastest-flowing on Earth, have recently doubled to reach over 12 km/year. In addition, the ice sheet has experienced a greater area of surface melting this year than at any time since systematic satellite monitoring began in 1979 [3]. Both these changes increase mass loss from the ice sheet, with the implication that current estimates of global sea-level rise over the next century, of about 0.5 +- 0.4 m [4], may be underestimated.

2) The Greenland Ice Sheet gains mass through snowfall and loses it by surface melting and runoff to the sea, together with the production of icebergs and melting at the base of its floating ice tongues. The difference between these gains and losses is the mass balance; a negative balance contributes to global sea-level rise and vice versa. About half of the discharge from the ice sheet is through 12 fast-flowing outlet glaciers, most no more than 10 to 20 km across at their seaward margin, and each fed from a large interior basin of about 50,000 to 100,000 km^(2). As a result, the mass balance of the ice sheet depends quite sensitively on the behavior of these outlet glaciers.

3) Two changes to these glaciers have been observed recently. First, the floating tongues or ice shelves of several outlet glaciers, each several hundred meters thick and extending up to tens of kilometers beyond the grounded glaciers, have broken up in the past few years [5]. Second, measurements of ice velocity made with satellite radar interferometric methods have demonstrated that flow rates of these glaciers have approximately doubled over the past 5 years or so [2,5]. The effect has been to discharge more ice and, thus, to increase the mass deficit of the ice sheet from a little more than 50 km^(3)/year to in excess of 150 km^(3)/year [2]. Increased velocity, combined with rapid dynamic thinning of up to 15 m/year that cannot be accounted for by increased melting, may be linked to the loss of the mechanical buttressing effect of the ice tongues [2,5].

4) The outlet glaciers in question, including Jakobshavn Isbrae in the west and Kangerdlugssuaq Glacier on the east coast of Greenland, are all south of 70 deg N, suggesting that there may be some linkage with changing climate. Satellite data from passive microwave instruments show that there has been a very marked increase in the area affected by summer melting and the length of the melt season on Greenland. Indeed, 2002 and 2005 are records for melt extent over the 27 years of observations [3]. With these observations and a meteorological model to retrieve annual accumulation, runoff, and surface mass balance for the ice sheet, a declining mass balance over the past 6 years (to 2003) was calculated. Not only did this change in surface balance yield a contribution of 0.15 mm/year to global sea-level rise, but it may also be implicated in the changing velocity structure of the ice sheet.

References (abridged):

1. J. M. Gregory et al., Nature 428, 616 (2004)

2. E. Rignot, P. Kanagaratnam, Science 311, 986 (2006)

3. K. Steffen, R. Huff, Greenland Melt Extent, 2005 (Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, 2005); available at cires.colorado.edu/science/groups/steffen/greenland/melt2005

4. J. A. Church et al., in Climate Change 2001: The Scientific Basis, Intergovernmental Panel on Climate Change, J. T. Houghton et al., Eds. (Cambridge Univ. Press, Cambridge, 2001), pp. 639-693

5. I. Joughin et al., Nature 432, 608 (2004)

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ENVIRONMENTAL SCIENCE: ON ICE-SHEET AND SEA-LEVEL CHANGES

The following points are made by R.B. Alley et al (Science 2005 310:456):

1) Because a heavy concentration of the population lives along coastlines, even small amounts of sea-level rise would have substantial societal and economic impacts through coastal erosion, increased susceptibility to storm surges, groundwater contamination by salt intrusion, and other effects. Over the last century, sea level rose ~1.0 to 2.0 mm/year, with water expansion from warming contributing 0.5 +- 0.2 mm (steric change) [1,2] and the rest from the addition of water to the oceans (eustatic change) due mostly to melting of land ice [2]. By the end of the 21st century, sea level is projected to rise by 0.5 +- 0.4 m in response to additional global warming [2], with potential contributions from the Greenland and Antarctic ice sheets dominating the uncertainty of that estimate.

2) These projections emphasize surface melting and accumulation in controlling ice-sheet mass balance, with different relative contributions for warmer Greenland and colder Antarctica[3]. The Greenland Ice Sheet may melt entirely from future global warming[4], whereas the East Antarctic Ice Sheet (EAIS) is likely to grow through increased accumulation for warmings not exceeding ~5øC [5]. The future of the West Antarctic Ice Sheet (WAIS) remains uncertain, with its marine-based configuration raising the possibility of important losses in the coming centuries [2]. Despite these uncertainties, the geologic record clearly indicates that past changes in atmospheric CO2 were correlated with substantial changes in ice volume and global sea level.

3) Recent observations of startling changes at the margins of the Greenland and Antarctic ice sheets indicate that dynamical responses to warming may play a much greater role in the future mass balance of ice sheets than previously considered. Models are just beginning to include these responses, but if they prove to be important, sea-level projections may need to be revised upward. Also, because sites of global deepwater formation occur immediately adjacent to the Greenland and Antarctic ice sheets, any notable increase in freshwater fluxes from these ice sheets may induce changes in ocean heat transport and thus climate.

4) The record of past glacial changes provides important insight to the behavior of large ice sheets during warming. At the last glacial maximum about 21,000 years ago, ice volume and area were more than twice modern values. Deglaciation was forced by warming from changes in Earth's orbital parameters, increasing greenhouse gas concentrations, and other attendant feedbacks. Deglacial sea-level rise averaged 10 mm/year, but with variations including two extraordinary episodes at 19,000 years before present (19 kyr B.P.) and 14.5 kyr B.P., when peak rates potentially exceeded 50 mm/year. Each of these "meltwater pulses" added the equivalent of 1.5 to 3 Greenland Ice Sheets to the oceans over a period of one to five centuries.

5) In summary: Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic ice sheets, with the potential to raise sea level ~70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these ice sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to ice-sheet behavior, but recently detected, rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered.

References (abridged):

1. W. Munk, Science 300, 2041 (2003)

2. J. Church et al., in Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, J. T. Houghton et al., Eds. (Cambridge Univ. Press, Cambridge, 2001), pp. 639 693.

3. P. Huybrechts, J. Gregory, I. Janssens, M. Wild, Global Planet. Change 42, 83 (2004)

4. J. M. Gregory, P. Huybrechts, S. C. B. Raper, Nature 428, 616 (2004)

5. P. Huybrechts, J. de Wolde, J. Clim. 12, 2169 (1999)

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RAPID WASTAGE OF ALASKA GLACIERS AND THEIR CONTRIBUTION TO RISING SEA LEVEL

The following points are made by A.A. Arendt et al (Science 2002 297:382):

1) Mountain glaciers (1) constitute only about 3% of the glacierized area on Earth, but they are important because they may be melting rapidly under present climatic conditions and may therefore make large contributions to rising sea level. Previous studies (2-5), based on observations and model simulations of glacier mass balance, estimated the contribution of all mountain glaciers to rising sea level during the last century to be 0.2 to 0.4 mm/year. The range of uncertainty is large, and it stems from insufficient measurements of glacier mass balance: Conventional mass balance programs are too costly and difficult to sample adequately the >160,000 glaciers on Earth. At present, there are only about 40 glaciers worldwide with continuous balance measurements spanning more than 20 years. High-latitude glaciers, which are particularly important because predicted climate warming may be greatest there, receive even less attention because of their remote locations. Glaciers that are monitored routinely are often chosen more for their ease of access and manageable size than for how well they represent a given region or how large a contribution they might make to changing sea level. As a result, global mass balance data are biased toward small glaciers ( < 20 km2) rather than those that contain the most ice ( > 100 km^(2)). Also, large cumulative errors can result from using only a few point measurements to estimate glacier-wide mass balances on an individual glacier.

2) Glaciers in Alaska and neighboring Canada (labeled "Alaska" glaciers herein) cover 90,000 km^(2), or about 13% of the mountain glacier area on Earth, and include some of the largest ice masses outside of Greenland and Antarctica. Additionally, many of these glaciers have high rates of mass turnover. However, they are underrepresented by conventional mass balance studies, which include only three or four long-term programs on relatively small glaciers. Dyurgerov and Meier (5), by necessity, extrapolated the data from these few small glaciers to estimate the contribution of all Alaska glaciers to sea-level change, and they specifically pointed to the need for further data in this region, especially on the larger glaciers. The authors report they used airborne laser altimetry to address this problem. They have measured volume and area changes on 67 glaciers, representing about 20% of the glacierized area in Alaska and neighboring Canada, and they use these data to develop new estimates of the total contribution of Alaska glaciers to rising sea level.

3) In summary: The authors have used airborne laser altimetry to estimate volume changes of 67 glaciers in Alaska from the mid-1950s to the mid-1990s. The average rate of thickness change of these glaciers was -0.52 m/year. Extrapolation to all glaciers in Alaska yields an estimated total annual volume change of -52 15 km^(3)/year (water equivalent), equivalent to a rise in sea level (SLE) of 0.14 0.04 mm/year. Repeat measurements of 28 glaciers from the mid-1990s to 2000-2001 suggest an increased average rate of thinning, -1.8 m/year. This leads to an extrapolated annual volume loss from Alaska glaciers equal to -96 35 km^(3)/year, or 0.27 0.10 mm/year SLE, during the past decade. These recent losses are nearly double the estimated annual loss from the entire Greenland Ice Sheet during the same time period and are much higher than previously published loss estimates for Alaska glaciers. They form the largest glaciological contribution to rising sea level yet measured.

References (abridged):

1. Mountain glaciers are those not in Greenland and Antarctica

2. M. Meier, Science 226, 1418 (1984)

3. M. Meier, in Ice in the Climate System, W. R. Peltier, Ed. (Springer-Verlag, Berlin, 1993), pp. 141-160

4. Z. Zuo and J. Oerlemans, Clim. Dyn. 13, 835 (1997)

5. M. Dyurgerov and M. Meier, Arctic Alpine Res. 29, 392 (1997)

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