The changing climate is just one factor contributing to the melting and thinning of the slow-moving rivers of ice that terminate in spots such as Greenland’s fjords. Each glacier’s geometry—including its thickness and how steeply it flows—also makes a difference. Now glaciologists, in a recent Nature Geoscience paper, offer a simple way to predict the dynamics of a flowing ice “stream,” and thus offer a means of determining which ones might become the biggest contributors to sea level rise.
These dynamics, not previously taken into account, confound forecasts about the disappearance of Greenland’s ice sheet: By 2100, it could elevate Earth’s oceans by 46 mm-538 mm, according to varying estimates. “The whole goal is to be able to predict what’s going to happen in the future,” says study author Denis Felikson, a graduate student at the University of Texas at Austin. To do that, he reasons, glaciologists must first be able to explain changes that already occurred.
Glaciers can shrink in two ways, he explains. Warming temperatures melt away water at the surface, accounting for approximately half of the mass disappearing from in the Greenland ice sheet. But a bit of that water can trickle below the glacier and lubricate it, speeding its flow and causing it to thin out. That leads to more icebergs calving off the end of glaciers and falling into the sea, accounting for the other half of lost mass.
To study the contribution of thinning, the researchers compared aerial photographs taken of West Greenland in 1985, and again between 2012 and 2015. Because the photographers captured each spot from two different angles, they could use the photos, along with calculations of the land shape beneath the ice, to determine ice thickness and how steeply the frozen rivers flowed.
According to a longstanding theory, if a bit of a glacier begins to thin out, there are two possible outcomes. One is that the thinning spreads both up- and downstream, and the glacier shrinks. The other is that the thin section will move downstream, pushed along by thicker sections of ice, and the glacier mass will stay about the same.
Felikson and colleagues could predict how far inland the thinning would creep by time of the recent photographs by calculating the Péclet number, which is based the ice surface slope and thickness, from the 1985 images. In some glaciers, thick ice runs along a fairly flat surface (low Péclet number), and in that case, any thinning spreads in both directions. Other glaciers are thinner, but steep (high Péclet number). In those, the thinning was less likely to spread. Instead, the thinned part simply flowed out the end of the glacier into the sea, being replaced by thicker sections coming from upstream.
Therefore, the Péclet number predicts how far inland the thinning of a glacier will extend, and thus how much it might shrink in the future. For example, the Jakobshavn Glacier has thinned to 120 kilometers inland, putting it at high risk to lose mass by iceberg calving.
“This doesn’t revolutionize our view of Greenland melting, but it’s an important step to be able to predict it,” says Josh Willis, a climate scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. Willis, who was not involved in the study, is also principal investigator of the space agency’s Oceans Melting Greenland project. Climate scientists will still be guessing about how much of Greenland’s ice will disappear, he says, but “it’s definitely going to be a better guess than we had before.”
The Péclet calculations will be useful as scientists decide which glaciers to monitor with limited resources, adds Mary Albert, a mechanical engineer, not involved in the study, who researches ice physics at Dartmouth College in Hanover, New Hampshire. Climate modelers can also use the calculations to determine which glacial streams to include, simplifying their calculations, says Felikson.
Felikson says the method could be applied to predict the extent of thinning in any glaciers that terminate in water. However, Albert suggests that the calculations are most relevant for ice streams like the ones in West Greenland, which are all above sea level. “There are many glaciers like that, but there are some glaciers with different bed conditions that may yield surprises,” she says.