Antarctic ice and future sea level rise: big questions
Image: NASA
By Scott K. Johnson | Published August 1, 2011 11:52 AM
There has been considerable angst and uncertainty about projections of the sea level rise that accompanies rising global temperatures. In fact, the last IPCC assessment settled on pretty conservative numbers due to that uncertainty. There are a lot of unknowns that make this one of the tougher variables to predict; Antarctica, in particular, has proven difficult to get a handle on.
Antarctica is split in two by the Transantarctic Mountains—for several reasons the ice sheet on the west side of the mountains is usually considered separately from the ice sheet to the east. While the East Antarctic ice sheet is easily the larger of the two, most sea level research is focused on its western counterpart, since it’s much more susceptible to large scale melting.
The West Antarctic ice sheet is inherently unstable because the continent beneath it forms a large basin that bottoms well below sea level. If the ice sheet melts back to the outer ring of the basin, you can pretty much float the whole ice sheet. Floating ice sheets aren't nearly as stable as those safely secluded on land in the coldest place on Earth.
There’s evidence for "collapses" of the West Antarctic ice sheet in the past, but the timing is very fuzzy. As the authors put it, the evidence "strongly suggests that the [West Antarctic ice sheet] largely disappeared, perhaps during the past few hundred thousand years and more confidently during the past few million years, in response to warming similar to or less than that projected under business-as-usual CO2 emission scenarios for the next few centuries." Since we don’t know the timing, we also don’t know how rapidly that melting took place, but it appears to have taken many centuries, at least. (But remember that the rate of warming events during that melting was much slower than is projected for the near future.)
Satellite data shows that the West Antarctic ice sheet is losing mass at a rate between 100 and 200 gigatons per year. That translates to 0.28 - 0.56 mm of sea level rise each year. That’s comparable to the amount of melting occurring in Greenland. Yet, sea level estimates from climate models have not been able to take Antarctica into account because they cannot adequately simulate the small-scale processes that drive mass loss there. This is one reason the IPCC’s 2007 report did not include upper bounds on sea level projections.
The vast majority of mass loss for the West Antarctic ice sheet occurs where the ice meets the Southern Ocean—either through calving off of icebergs or melting of the ice shelves. The physics here are critical, because the floating ice shelves actually act as buttresses that slow the flow of glacial ice toward the sea. It takes a lot of force to push all that water out of the way and advance the ice shelf—if you take it away, the ice sheet will flow much more quickly. Virtually all of the West Antarctic ice sheet is sensitive to this buttressing—that’s another reason why it’s so unstable.
Changes in melt rates of the ice shelves due to climate change are primarily indirect, caused by alterations to a mass of seawater called the Circumpolar Deep Water (the name says it all). This water is just warm enough to cause significant melting at the bottom of the ice shelves. The Circumpolar Deep Water has warmed by about 0.2°C, and changes in wind patterns have also driven more of it toward ice along the Amundsen Sea. This is responsible for a good portion of the current mass loss. The authors are quick to point out that it is not clear whether this wind pattern will persist or change with continued warming.
Atmospheric warming is especially important along the Antarctic Peninsula, where meltwater ponds on the surface of the ice have caused several recent breakups (including the Larsen B ice shelf [ http://earthobservatory.nasa.gov/Features/WorldOfChange/larsenb.php ]). The water in the crevasses can exert so much pressure that it fractures the ice, causing cracks of up to a kilometer long. Again, the authors note that the loss of ice shelves along the peninsula "have already produced increased outflow from many of the formerly buttressed glaciers."
The researchers conclude that we're unlikely to see a collapse of the West Antarctic ice sheet this century, but the uncertainty of how it will respond to warming is cause for concern. Answers to these questions will have to come from improved ice sheet models, which require an exceedingly detailed understanding of the local features and dynamics of the ice sheet itself.
The authors summarize the situation like this: "A collapse of the marine ice sheet in West Antarctica would raise sea level by more than three metres over the course of several centuries or less. Such an event seems possible, but improved understanding of the expected atmospheric and oceanographic forcing and the ensuing ice-sheet response is required to quantify its likelihood. Precisely understanding the vulnerability of the West Antarctic ice sheet to a warming climate remains a grand challenge for the ice-sheet and climate-modelling communities.”
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