|“Ice Sheets and Sea Level Rise – Why Worry?”|
by Christina Hulbe, Ph.D., Geology Professor, Portland State University
On May 20, 2010, we had 35 attend this technical meeting, in Cramer Hall at PSU in downtown Portland. OR-AMS President Bobby Corser welcomed folks and gave the opening remarks.
Sea-Level Rise (SLR) is now 3 mm (+/- 0.5 mm) per year, whereas the average rate was 1.7 mm per year during the latter half of the 20th Century. The global tide gage records and satellite observations of sea surface height are the datasets used (CSIRO; Church & White, Geophysical Research Letters 2006).
SLR is not uniform, due to regional effects of changes in temperature and circulation. Thermal expansion accounts for about half the recent SLR, most of the rest is from melting terrestrial ice. The highest rises are in the western Pacific, based on TOPEX and Jason-1 data.
The Fourth IPCC Report suggests that future SLR could be 50-500 mm, according to emission scenarios A2, A1B, and B1, which offer +2 to +4 degC increases in global temperature during 2000-2100. However, Prof. Hulbe is not confident of such projections, as they fail to account for rapid changes in ice sheet dynamics (the rate of ice flow). Even at the lowest scenarios, tens of millions of people are threatened by SLR due to increased coastal development and infrastructure in the U.S., Europe, and the developing world.
The models used for the Fourth IPCC Report project ice loss from Greenland (positive SLR) and ice gain in Antarctica due to additional snowfall (negative SLR), so the net impact from polar ice sheets would be near zero. Greenland, a territory of Denmark, has researchers concerned though because the ice sheet is responding to global warming more than those models indicate. The changes, 50 cm per year, are largest on the west and southeast coasts, where large outlet glaciers are speeding up.
What are the computer models missing? The processes responsible for rapid change on outlet glaciers are not reproduced well by the models. Those processes all involve marine margins, places where glaciers flow to the sea.
An example of detailed modeling of rapid glacier change after collapse of the Larsen “B” Ice shelf, on the Antarctic Peninsula, was shown. The Larsen “B” Ice-shelf made news headlines in Nov. 2001, when the world observed a rapid well documented collapse of the ice-shelf. There, a process called basal sliding was the key to correct simulation of glacier speed up. This is possible where liquid melt water at the base of the ice allows it to slide over its bed. Basal sliding is represented in some ice sheet models but there are a few different methods and it is not clear yet which one is correct. The glaciers of west Antarctica have warmed 0.25 degC per decade (1957-2006).
Ice flow is driven by (1) Gravity (i.e., stress-gradient on ice), (2) Lateral shear (i.e., planes of glacial ice slipping over each other), (3) Basal Slip (i.e., frozen vs. thawed bed). Certain parts of the glacier can speed up by stretching and thinning of the ice upstream. Greenland glaciers, with sliding, could change 80-200 m in elevation by 2100, especially on the north and east coastlines. Surface ponds can rapidly drain down to the bed, but varies due to seasonal timing and proximity to a coast.
Ice sheet modelers are working together with climate modelers to improve the representation of ice sheets in the models used to make future projections. Complications are from different approaches to representing processes like basal sliding and from lack of information about the shape of the land surface beneath the ice.
In 200 years, SLR could go up 31 mm to 72 mm if the Antarctic ice-sheets melt and retreat, the grounding-line (sea-bed) retreats, and/or a substantial glacier thickness change occurs. In recent years, research has shown that glacial build-up is slow, but melt and retreat can be rapid. Many questions were discussed from the audience.
Note-taker: Kyle Dittmer, 2009-2011 Oregon-AMS Secretary