Two new papers, in Science, and in Geophysical Research Letters, demonstrate that Thwaites Glacier in Antarctica has reached a point of instability, which, at some point in the future, will lead to the collapse of that part of the West Antarctic Ice Sheet (WAIS), and, eventually, the rest of it as well. Over a period of some centuries, sea level rises will be catastrophic. The video above is by NASA, discussing some of the key points.
Various reports on this subject have traced work on this question back to geologist John Mercer (1922-1987), either to a well-known paper he published in Nature in 1978 linking WAIS collapse to anthropogenic global warming, or to a more obscure paper he published in 1968, in which he posited that WAIS was the source of higher sea levels during the Sangamon interglacial 120,000 years ago. That earlier paper also mentioned possible future danger from “industrial pollution,” but only tangentially within a larger focus on Antarctica’s glacial history.
Having published on the history of this subject, I’d like to develop the available narrative somewhat, both to expand on its roots, but also to discuss some of the twists and turns that have led us from an initial suspicion that WAIS could rapidly collapse to the disquieting conclusion at which glaciologists have now arrived.
WAIS’s instability is predicated on its “marine” topography, i.e., the fact that it rests on bedrock that is below sea level. That topography was discovered by seismically sounding the ice sheet on traverses undertaken at part of the International Geophysical Year of the late 1950s.
(I interviewed one of the leaders of this effort, Charles Bentley, in 2008. The transcript is here; it is very long, but we discuss the traverse pretty early on.)
Shortly following the discovery of WAIS’s marine nature, the propensity of such an ice sheet to disappear was suggested by John Hollin, and separately by Raymond Adie (1925-2006) and Gordon Robin (1921-2004). These suggestions were not well elaborated upon, and were small parts of larger speculations concerning the glacial history of Antarctica as a whole. However, they were the basis of Mercer’s suggestion about the source of Sangamon-era sea levels a few years later.
A much more elaborate case for the instability of West Antarctica was made in 1972 by Mercer’s younger colleague at Ohio State, Terry Hughes, in a privately circulated “bulletin” straightforwardly titled, “Is the West Antarctic Ice Sheet Disintegrating?” Hughes’s argument was modified and published in the Journal of Geophysical Research in 1973. Hughes later discussed the relationship between his research and Mercer’s in his obituary for Mercer (pdf) in the Journal of Glaciology.
We don’t have room to go into the full structure of Hughes’s argument here. Suffice it to quote Hughes’s obituary for Mercer, where he refers to it as an “elaborate chain of speculation.” Very quickly he expanded on this chain by using WAIS instability as a means of explaining how the earth’s long-term cycles of warming and deglaciation reversed themselves—a case he made in detail in another bulletin-turned-article, “The West Antarctic Ice Sheet: Instability, Disintegration, and Initiation of Ice Ages,” published in 1975.
However, once Hughes moved to the University of Maine, where he would spend the rest of his career, he, in collaboration with his colleague, geologist George Denton, and Russian geologist Mikhail Grosswald, repurposed the prospect of the instability of marine ice sheets as a means of explaining how the Laurentide Ice Sheet in North America disintegrated. (This work was part of the multi-institutional CLIMAP project.)
The critical point to take home from all of this is that, by the late 1970s, the strongest evidence for marine ice sheet instability was that such instability made sense as a part of an explanation of the natural process of deglaciation. Within this framework, WAIS collapse was simply a future event in the same process that had already resulted in the destruction of the Laurentide Ice Sheet, but not the Greenland Ice Sheet (which is not a marine ice sheet). This was essentially the main point of a 1981 book that Hughes and Denton edited called The Last Great Ice Sheets.
Meanwhile, though, Northwestern University’s glaciological theorist Hans Weertman (Hughes’s first mentor in glaciology) had published a paper in 1974, “Stability of the Junction of an Ice Sheet and an Ice Shelf,” (pdf) developing a rudimentary two-dimensional model of the physics of marine ice sheets to establish the plausibility of their instability.
This physical plausibility established, it became important to determine whether the theoretical instability of WAIS was, in fact, manifesting itself in an actual collapse, which could only be determined by empirically studying the present state and movement of the ice sheet.
The first observations were made during the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS), which ran through the mid-1970s, by the project glaciologist, Bob Thomas. More ice drains out of West Antarctica through that sector than any other, so it was a logical place to begin work. While Thomas felt that the ice sheet could drain rapidly if the Ross Ice Shelf were suddenly removed, the existence of pinning points holding the ice shelf in place made it implausible (pdf) that it would disappear rapidly, if indeed the ice shelf was not actually in equilibrium. (See my 2009 interview with Thomas.)
To be clear, in the 1970s, all glaciologists who were interested in possible WAIS collapse regarded it as a natural, rather than an anthropogenically triggered, process. And it was beginning to become clear that such a natural collapse was unlikely to happen in the near-term. Thus, John Mercer used his 1978 Nature paper, “West Antarctic Ice Sheet and CO2 Greenhouse Warming: A Threat of Disaster,” to argue that what might not happen naturally might well happen as a consequence of human action.
Beyond Mercer’s concerns about global warming, Terry Hughes continued to view the question at hand not to be whether a collapse would occur, but what was to stop it from doing so. Looking at a map of Antarctica, he and Denton saw that, while the Ross Ice Shelf did inhibit the flow of ice through that outlet, there was little to inhibit the flow of ice through the Thwaites and Pine Island Glaciers into the Amundsen Sea. In a 1981 letter to the Journal of Glaciology, he argued that that sector was “the weak underbelly of the West Antarctic Ice Sheet” (pdf), which could collapse naturally or be accelerated by anthropogenic global warming. And, indeed, this is the sector now regarded as having passed a point of no return.
So, if all the basic ideas concerning our current state of affairs were on the table by 1981, why did it take a further 33 years for the conversation to reach the point that it has?
According to Justin Gillis and Kenneth Chang’s report on the latest results at the New York Times, Mercer was “assailed” when he presented his ideas. Elizabeth Kolbert at The New Yorker‘s website suggests “scientists” are overly cautious in pressing their concerns in order to avert accusations of alarmism by climate-change denialists. However, I don’t think such narratives help us to understand the evolution of the discourse surrounding the WAIS issue.
Mercer’s paper did, in fact, receive a great deal of positive attention, both in the public and scientific press. A full cultural history of this media reaction to Mercer’s work is called for, but, as a first step, one can look at the early and frequent mention of his assertion in Nature‘s coverage of global warming (see here, here, here, and here). Additionally, the American Association for the Advancement of Science and the U. S. Department of Energy, among other important organizations, immediately took a strong interest in the issue.
Concerns about alarmism came, in part, from scientific insiders who were concerned that both public and scientific media were overreacting to an issue that certainly demanded attention, but was one where researchers had yet to really even scratch the surface.
At that time, the only evidence for WAIS collapse on any timeframe was Weertman’s rudimentary model of marine ice-sheet instability, and Hughes’s self-consciously radical speculation concerning the role of marine ice sheets in driving changes in the paleoclimate, and the unfinished state of the last planetary deglaciation. Meanwhile, the likelihood that the main body of Antarctica would remain below freezing even with global warming, and the apparent equilibrium of ice flow across the ice sheet and its ice shelves, spoke against the need for urgency.
And new evidence pointing to collapse was a long time in coming. So, as time passed, many scientists interested in assessing the impacts of future climatic change proved reluctant to include a possible WAIS collapse in their projections. In 1989, for example, climate modeler Hans Oerlemans acknowledged the “great deal of public attention” that had surrounded sea-level rise since Mercer’s 1978 paper, but asserted, albeit not conclusively, that there was little reason to believe that WAIS would be a major contributor to sea-level change in the near future.*
As late as a 1997 article in Science, Charles Bentley—who had begun an intensive program of study of the glaciology of WAIS following Mercer’s paper—argued that the evidence spoke against any near-term danger of collapse, either as a natural or anthropogenically triggered process. He pointed specifically to the unlikelihood that deep warm-water currents could melt ice-sheet grounding lines, and to the apparent slowness of the glaciers flowing into the Amundsen Sea. Both these reassuring points would be overturned within the following few years, leading to a renewed and more widely shared concern over the potential of WAIS collapse, culminating in the present moment.
We will address the crucial recent history of WAIS collapse research in Pt. 2.
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*Oerlemans was co-author of the chapter on sea-level rise of the first IPCC assessment report, which appeared in 1990. The IPCC process has been criticized for not paying proper heed to the risk of WAIS collapse, but it should also be pointed out that that process is statutorily constrained to project only over the next 100 years. Virtually all assessments, up to nearly the present, agreed that WAIS contribution to sea-level rise in that short timeframe will be minimal.