A new study advances the field of glacier study toward a longstanding goal, a universal slip law. Published in the journal Science last month, the study derives a glacier slip law that explains how friction operates to alter the velocity of glaciers, whether on rigid bedrock or on deformable beds, composed of sediment or other soft material. The study was authored by Lucas Zoet from the University of Wisconsin–Madison and Neal Iverson from Iowa State University.
The results of this study have important implications for improving sea level rise projections. The melting of glaciers and ice sheets contributes to sea level rise, one of the most important effects of anthropogenic climate change. Sea level rise is already displacing people and communities around the globe and is likely to continue to do so throughout this century. As an IPCC report published last year stated, “The key uncertainty in these calculations [of the Antarctic contribution to sea level rise] was found to come from the dependency on the relation between the sliding velocity and the friction at the ice-bedrock interface.” This dependency is precisely what the universal slip law describes.
Slip describes the movement of glaciers as they flow along their beds, either sliding over hard bedrock or deforming rock and sediment below them. This movement brings glaciers to lower elevations, where they melt, or break off directly into the sea. GlacierHub spoke to Neal Iverson, one of the authors of the study and a glaciologist at Iowa State University. Iverson explained “the faster glaciers move, the higher the flux of ice that gets shed into the ocean.” This movement is caused by glacier slip and usually occurs along deformable beds.
The contributions of the study were highlighted in a perspective piece about it in Science. The article by Minchew and Joughin contextualizes the Zoet and Iverson study and underscores the importance of a universal slip law. It also provides a broader, balanced view of the study. Minchew, a geophysicist at MIT, told GlacierHub that a slip law is a term that relates drag occurring along the bed of glaciers to the various factors that determine drag, including the speed of glacial sliding.
Zoet and Iverson conducted laboratory experiments using a device which studies glacier slip across various surfaces. This instrument works by rotating a ring of ice at a steady speed across a bed of sediment and measures the resulting shear stress. The results of their experiments yielded a slip relation for ice sheets and glaciers along deformable beds. Most importantly, the study revealed that the two forces governing glacial sliding operate at different glacier velocities. Iverson explained that at low speeds, glaciers slide along the bed, leaving the bed unchanged. In these circumstances, resistance is greater when glaciers move faster. Once slip speed passes a threshold, however, glaciers move fast enough to change the shape of their beds. At these faster velocities, glaciers move more easily, as they encounter less resistance. In other words, glacial sliding is dictated by friction, which acts differently when glaciers are moving fast or slow.
Minchew and Joughin note that the results of Zoet and Iverson’s study are significant because they show that a widely used slip law is applicable to glaciers on different bed types, suggesting that the law may be universally applied to all glaciers and ice sheets. A universal slip law would allow glaciologists to account for the rate at which glaciers move — something that is more important than ever due to climate change. It is also something that has “been a goal for glaciologists for over 50 years,” according to Leigh Stearns, a glaciologist at the University of Kansas. The results of the study suggest researchers can now use one model that varies by velocity rather than using different models for different glaciers.
Stearns explained why understanding the forces behind glacier slip is necessary for projecting sea level rise. “Some glaciers are very sensitive to the input of additional water in the spring [when snow and ice melt]—undergoing large-scale accelerations that coincide with changes in effective pressure and thus increased sliding,” she told GlacierHub. The results of the Zoet and Iverson study will lead to more accurate sea level rise predictions by improving the modeling of glacier slip, which feeds ice into the ocean.
While a universal sliding law could greatly improve the modeling of sea level rise, there remains debate on whether a universal glacier slip law will ever fully be achieved. While Minchew and Joughin note additional observational tests are needed to back up the results of Zoet and Iverson’s study, Minchew does believe a universal slip law can be defined. Expanding on his Perspectives piece, he told GlacierHub, “the primary goal of science is to simplify and provide manageable, falsifiable explanations for a broad range of observations, so in that regard, there are only a handful of key processes and parameters that could be important for including in a sliding law.” Minchew found Zoet and Iverson’s results were “important but not surprising.” He said the study “provided essential and valuable experimental observations that help illuminate the dynamics of subglacial till.”
Stearns, whose work has included the development of an alternative slip law, offered a different view on the article than Minchew. She believes a universal slip law is unlikely given the myriad conditions and factors that contribute to glacial sliding. She referenced Richard Alley, a geologist at Pennsylvania State University, noting that he “pointed out in the 1990s that the quest for a universal sliding law may be illusory.”
Although there remains some uncertainty over whether a universal glacier slip law will ever be fully accepted, experts agree that such a law would be a valuable contribution. Zoet and Iverson’s study represents a significant advancement towards this goal of a universal slip law. They revealed how the forces behind glacial sliding operate, which will undoubtedly improve the modeling of glacier movement and in turn, lead to more accurate predictions of sea level rise. This research will allow coastal communities to better prepare and adapt to rising sea levels. Further development in this field can be expected as Zoet and Iverson continue to advance this line of research.
