Rignot, Mouginot, Morlighem, Seroussi and Scheuch map the observed retreat of these grounding glaciers, and over the past twenty years, and yes, once they let go, there is nothing holding the ice shelf back from lurching into the Southern Ocean. They conclude
Using two decades of ERS-1/2 data, we document a continuous and rapid retreat of the grounding lines of Pine Island, Thwaites, Haynes, Smith and Kohler glaciers, which drain a large sector of West Antarctica on a retrograde, submarine bed, a configuration deemed unstable by ice sheet numerical models (e.g. Favier et al., 2014, Katz and Worster, 2010; Parizek et al., 2012) unless normal and tangential ice shelf buttressing could increase significantly (Gudmundson, 2013), which is unlikely. The retreat is proceeding along fast-flowing, accelerating sectors that are thinning, become bound to reach floatation and un-ground from the bed. We find no major bed obstacle upstream of the 2011 grounding lines that would prevent further retreat of the grounding lines farther south. We conclude that this sector of West Antarctica is undergoing a marine ice sheet instability that will significantly contribute to sea level rise in decades to come.The process is an avalanche, with the ungrounding of the blocking Twaites and Pine Island Glaciers leading to rapid collapse. From the paper by Joughin, Smith and Medley appearing in Science,
When simulated losses exceed 1 mm/yr sle, much greater losses generally follow within a few years. Using our basin-scale model, however, such rapid collapse is difficult to model, especially since interaction with other basins becomes increasingly important. Thus, we take 1 mm/yr sle to be a threshold that, once crossed, marks the onset of rapid (decades) collapse as the grounding line reaches the deepest regions of the marine basin.Now other bunnies will say this better, and already have, for example, Suzanne Goldenberg and those to be named later, what Eli is looking at is how the Mysterious Mr. Revkin appears, to tweet in glee and error at Ms. Goldenberg
Awful misuse of "Collapse" in headlines on centuries-long ice loss in W. Antarctica. See rates in papers. Same as '09 http://t.co/kk48ztySvZThe link goes to a post at his Dot Earth blog at the NY Times, where our hero manages to miss the entire point about the rapid (decades) collapse once the melt exceeds the threshold. Our Mr. Revkin starts with a quote from the abstract
— Andy Revkin (@Revkin) May 12, 2014
The *, of course, is the Pielke *Except possibly for the lowest-melt scenario, the simulations indicate that early-stage collapse has begun. Less certain is the time scale, with the onset of rapid (>1 mm per year of sea-level rise) collapse in the different simulations within the range of 200 to 900 years.To translate a bit, that means sometime between 200 and 900 years from now the rate of ice loss from this glacier could reach a volume sufficient to raise sea levels about 4 inches (100 millimeters) a century. At that point, according to the paper, ice loss could pick up steam, with big losses over a period of decades.* But in a phone conversation, Joughin said the modeling was not reliable enough to say how much, how soon.
Update, 8:03 p.m. | * At the asterisk above, I added a phrase indicating that the modeling showed abrupt and large ice losses at the discussed stage some centuries from now.And, of course, some centuries could be one or two centuries if we are not lucky.
Well, who picks this up but our friend Willard Tony