For the 40 percent of the world’s population who live within 100 kilometers of the coastline, sea-level rise is more than just a mathematical calculation, it’s a survival challenge. Although scientists are confident about the impacts of accelerated glacier melting and ice flow on rising sea levels, projections for future ice loss remain at a fairly early stage. Developing better predictions for how glaciers melt and flow in the future remains a daunting task for glacier modelers.
A new analysis published in the Journal of Science argues that the “largest uncertainty” in ice sheet models used to predict future sea-level rise originates from our limited understanding of underwater processes at the ice-bed interface. These ice-bed processes beneath water involve interactions among the weight of the ice, water pressure, and the roughness of the bedrock. One of the major consequences, of these underwater interactions and a cause of sea-level rise is basal sliding, when the glacier slides over the bed as a result of meltwater between the ice and the bed acting as a lubricant.
To address the uncertainties of ice sheet models, the paper analyzed 140 wet-based glaciers in Greenland. Wet-based glaciers are known to have a thin layer of water between the ice and the rock bed. In contrast, glaciers found in the frigid Antarctic lack such a layer and are frozen to the end.
Scientific research on glaciers began in the early 18th century and developed more fully later on. Although glaciers seem static, their waning and waxing over time has long been recognized. Several theories have been proposed for this characteristic, including the Weertman formula, named after scientist Johannes Weertman. The Weertman formula states that the speed a glacier moves at its bed beneath the water is determined by both the friction and the amount of water surrounding the bed. Withstanding some bickering between Weertman and other scientists during the 1950s, the Weertman model has been widely accepted since then. An array of sea-level rise prediction models have built on this theory, with the latest study challenging the findings of the Weertman formula.
One of the two authors of the study, Leigh Stearns, a scientist at the Center for Remote Sensing of Ice Sheets from the University of Kansas, spoke to GlacierHub about her research on the topic. “We found that the commonly-used model for basal sliding (the Weertman model) does not apply to all 140 Greenland glaciers that we analyzed,” she said.
Instead, the researchers found that subglacial water pressure, the water pressure difference between the ice sheet end and the hard bed underwater, dominates the speed of glacier flow.
Intrigued by their initial observations of the 140 overlooked mountain glaciers in Greenland, Stearns and her university colleague C. J. van der Veen found the effect of friction on glacier sliding speed to be “virtually non-existent,” which implicitly defers the Weertman notion. As a result, they spent a long time trying to figure out what other factor correlated better with glacier speed, according to Stearns.
This analysis involved a closer study on the subglacial water pressure in Greenland. Stearns and van der Veen believe this aspect has been largely overlooked by the glaciological community to date. They started their observations by calculating water pressure from the thickness of the ice and then calculating the effective pressures under the water. Stearns and van der Veen paired these findings with the latest observational data about glacier flow speed and found that the two are highly related.
— NASA ICE (@NASA_ICE) May 1, 2018
However, Stearns also discussed the limitations of her study with GlacierHub. “We don’t understand all the mechanics for why the relationship between sliding velocity and effective pressure are so good, and why the relationship between sliding velocity and basal drag is so bad,” she said.
Recognizing these uncertainties, the paper focused on current models of sea-level rise, which are based on the strong relationship between sliding speed and the roughness of the bed.
“Hopefully it will allow them to constrain their sea-level rise prediction models better, so uncertainties of future ice sheet mass balance are reduced,” Stearns added.
The paper notes that it is “imperative for the ice sheet modeling community to explore the impact that this new relationship may have on sea-level rise prediction.” With that said, the consequences of the researchers’ new and challenging theory are still unfolding and could be highly significant.
GlacierHub contacted other scientists who built their work on the Weertman theory for feedback on Stearns and van der Veen’s latest findings, but these scientists did not respond to GlacierHub’s request for comment.