Monday, June 9, 2014

Antarctica: The Problem Child of future sea level change in this century

icebergs in the Ross Sea, Antarctica. Photo © Bruce Luyendyk
Iceberg Ross Sea, Antarctica. Photo © Bruce Luyendyk

A few weeks ago I posted about discoveries that showed dynamic collapse of marine-based parts of the West Antarctic Ice Sheet is underway (May 20, 2014). What I want to explore now is the current thinking of scientists on the role of Antarctica in sea level change this century.

It will come as no surprise that sea level is projected to rise from the influence of global warming. This is discussed in both the Intergovernmental Panel on Climate Change (IPCC) AR5 report (Assessment Report Five 2013) and the 2014 National Climate Assessment. What do these reports say about sea level change and Antarctica’s role – and do the reports address the dynamic collapse scenarios revealed by studies published in the last few months?

First some basics - how much would sea level rise if ALL the polar ice caps melted (this would take thousands of years even at today’s warming levels and higher). Answers, all of Antarctica, 58.3 meters (188 feet)1, West Antarctic Ice Sheet (WAIS) alone 4.3 meters (13.9 feet)1, Antarctic Peninsula 0.2 meters (0.6 feet)1,  Greenland, about 7 meters (22.6 feet)2. Thus, 211 feet of sea level rise for all the ice sheets – my home on a Santa Barbara hill would be under 50 feet of water if it lasts a few thousand years!3 But let’s focus on this century –three human generations – and the rise expected by 2100.

For this century the IPCC has synthesized the research on the course of sea level and comes up with a range of 0.28 to 0.98 meters of rise (10 inches to 3.2 feet) by 21002 (Figure below). The 2014 National Climate Assessment using a different methodology comes up with 1 to 4 feet. The largest contributor to sea level rise is thermal expansion of a warmer ocean – over one-third of the total. Other contributions are melting of mountain glaciers, runoff from depletion of ground water, and ice sheet melting. In the later parts of the 20th century sea level rise has accelerated to about 3 millimeters/year (0.12 inches/yr)2. Ice sheet melting has contributed about 20% or so of this. The polar ice sheets are not loosing mass (ice) at the same rate. Greenland is melting faster2. Numbers for the Antarctic ice sheet show a rise of about 4 mm/19 years calculated from mass loss estimated by satellite radar and gravity data and 7 mm/19 years for Greenland4. This decrease in mass through the steady process of ice outflow and melting represents a change in Surface Mass Balance – a loss. This is different than the ice sheet collapse that I discussed in my May 20 entry.

From IPPC (2) below. Figure 13.27 | Compilation of paleo sea level data, 
tide gauge data, altimeter data (from Figure 13.3), and central estimates 
and likely ranges for projections of global mean sea level rise for 
RCP2.6 (blue) and RCP8.5 (red) scenarios (Section 13.5.1), 
all relative to pre-industrial values.

Projecting forward towards the year 2100 the IPCC summarizes that Antarctica could actually contribute to sea level fall if snowfall increases from enhanced evaporation of the warmer surrounding ocean. It seems however that increased snowfall occurs now on East Antarctica but increased outflow of ice in West Antarctica and the Antarctic Peninsula overrides the positive accumulation on East Antarctica – mass was lost towards the end of the 20th century. Modeling predicts that eventually ice accumulation will exceed outflow and the mass of ice on Antarctica will increase slightly or stay the same. However, the IPCC analyses do not account for dynamic changes in outflow of ice due to a warming ocean – for example speeding up of glaciers. Those dynamics of outflow are still a work in progress in the research community.

The IPCC concludes that the processes controlling Antarctic Mass Balance predict a small or negligible contribution to sea level rise by 2100. However, their report does recognize the potential for collapse of marine-based parts of the WAIS starting now or in the next few decades. This raises the possibility of an added contribution to sea level rise of a few tenths of a meter (490-785 mm; 1-2 feet or more)2. That number would be added to the totals above, that is, not 1-4 feet but 2-6 feet of rise by 2100. Their report was written before the results on the nascent marine-based WAIS collapse were published this spring, but they do recognize that possibility.

What’s the bottom line about Antarctic ice and global warming at this point in time?

In the words of IPCC2,
First, “Projections suggest a substantial increase in 21st century Antarctic snowfall, mainly because a warmer atmosphere would be able to carry more moisture into polar regions. Regional changes in atmospheric circulation probably play a secondary role. For the whole of the Antarctic ice sheet, this process is projected to contribute between 0 and 70 mm to sea level fall [emphasis added].”
Second, “There are strong indications that enhanced outflow (primarily in West Antarctica) currently outweighs any increase in snow accumulation (mainly in East Antarctica), implying a tendency towards sea level rise.”
Third, “Before reliable projections of outflow over the 21st century can be made with greater confidence, models that simulate ice flow need to be improved,…”
Last, “Sea level could rise if the effects of marine instability [collapse of marine-based portions of the ice sheets] become important, but there is not enough evidence at present to unambiguously identify the precursor of such an unstable retreat.”

Regardless, it is clear that even with present imperfect information and understanding of Antarctic ice sheet stability societies around the world need to prepare for several feet of sea level rise over the next 86 years.

1 Fretwell, P., Pritchard, H. D., Vaughan, D. G., Bamber, J. L., Barrand, N. E., Bell, R., Bianchi, C., Bingham, R. G., Blankenship, D. D., Casassa, G., Catania, G., Callens, D., Conway, H., Cook, A. J., Corr, H. F. J., Damaske, D., Damm, V., Ferraccioli, F., Forsberg, R., Fujita, S., Gim, Y., Gogineni, P., Griggs, J. A., Hindmarsh, R. C. A., Holmlund, P., Holt, J. W., Jacobel, R. W., Jenkins, A., Jokat, W., Jordan, T., King, E. C., Kohler, J., Krabill, W., Riger-Kusk, M., Langley, K. A., Leitchenkov, G., Leuschen, C., Luyendyk, B. P., Matsuoka, K., Mouginot, J., Nitsche, F. O., Nogi, Y., Nost, O. A., Popov, S. V., Rignot, E., Rippin, D. M., Riviera, A., Roberts, J., Ross, N., Siegert, M. J., Smith, A. M., Steinhage, D., Studinger, M., Sun, B., Tinto, B. K., Welch, B. C., Wilson, D., Young, D. A., Xiangbin, C., and Zirizzotti, A., 2013, Bedmap2: improved ice bed, surface and thickness datasets for Antarctica: The Cryosphere, v. 7, p. 375-393.

2 Church, J.A., P.U. Clark, A. Cazenave, J.M. Gregory, S. Jevrejeva, A. Levermann, M.A. Merrifield, G.A. Milne, R.S. Nerem, P.D. Nunn, A.J. Payne, W.T. Pfeffer, D. Stammer and A.S. Unnikrishnan, 2013: Sea Level Change. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

3 These numbers are equivalent values that do not take into account isostatic adjustments.

4 Shepherd, A., Ivins, E. R., A, G., Barletta, V. R., Bentley, M. J., Bettadpur, S., Briggs, K. H., Bromwich, D. H., Forsberg, R., Galin, N., Horwath, M., Jacobs, S., Joughin, I., King, M. A., Lenaerts, J. T. M., Li, J., Ligtenberg, S. R. M., Luckman, A., Luthcke, S. B., McMillan, M., Meister, R., Milne, G., Mouginot, J., Muir, A., Nicolas, J. P., Paden, J., Payne, A. J., Pritchard, H., Rignot, E., Rott, H., Sørensen, L. S., Scambos, T. A., Scheuchl, B., Schrama, E. J. O., Smith, B., Sundal, A. V., Angelen, J. H. v., Berg, W. J. v. d., Broeke, M. R. v. d., Vaughan, D. G., Velicogna, I., Wahr, J., Whitehouse, P. L., Wingham, D. J., Yi, D., Young, D., and Zwally, H. J., 2012, A Reconciled Estimate of Ice-Sheet Mass Balance: Science, v. 338, p. 1183-1189.

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