Mountain glaciers could contain less ice due to global warming

Mountain glaciers are essential sources of water for almost a quarter of the world’s population. But figuring out how much ice they contain — and how much water will be available as glaciers shrink in a warming world — has been notoriously difficult.

In a new study, scientists mapped the speed of more than 200,000 glaciers to get closer to an answer. They found that widely used estimates of glacier ice volume could be off by around 20% in terms of the contribution of land-based glaciers outside the Greenland and Antarctic ice sheets to sea level rise. Mathieu Morlighem: Professor of Earth Sciences, Dartmouth College

Mathieu Morlighem, professor of earth sciences at Dartmouth College, a leader in ice sheet modeling and co-author of the study, explains why the new results are a warning for regions that rely on seasonal meltwater glaciers, but barely register in the big picture of rising seas.

If mountain glaciers contain less ice than previously believed, what does this mean for people who depend on glaciers for their water?

Globally, nearly 2 billion people depend on mountain glaciers and snowpack as their main source of drinking water. Many also depend on water from glaciers for hydropower generation or agriculture, especially during the dry season. But the vast majority of the world’s glaciers are losing more mass than they gain over the course of the year as the climate warms, and they are slowly disappearing. This will deeply affect these populations.

These communities need to know how long their glaciers will continue to provide water and what to expect when the glaciers disappear so they can prepare. In most places, we found total ice volumes significantly lower than previous estimates.

In the tropical Andes, from Venezuela to northern Chile, for example, we found that glaciers had about 23% less ice than previously thought. This means that people downstream have less time to adapt to climate change than they would have expected. Even in the Alps, where scientists have many direct measurements of ice thickness, we found that glaciers may contain 8% less ice than previously thought.

The big exception is the Himalayas. We calculated that there could be 37% more ice in these remote mountains than previously estimated. This buys time for the communities that depend on these glaciers, but it does not change the fact that these glaciers are melting with global warming.

Policy makers should consider these new estimates to revise their plans. We don’t provide new predictions of the future in this study, but we do provide a better description of what glaciers and their water supplies look like today.

How do these results affect estimates of future sea level rise?
First, it is important to understand that melting glaciers are only one contributor to sea level rise as the climate warms. About a third of current sea level rise is due to thermal ocean expansion – as the ocean warms, the water expands and takes up more space. The other two-thirds come from shrinking mountain glaciers and ice caps.

We found that if all the glaciers, not including the large ice caps of Greenland and Antarctica, were to melt entirely, sea levels would rise about 10 inches instead of 13 inches. It might seem like a big difference, considering the size of the ocean, but you have to put things into perspective. Complete disintegration of the Antarctic ice sheet would contribute 190 feet to sea level and the Greenland ice sheet would contribute 24 feet. The 3 inches we talk about in this study does not challenge current sea level rise projections.

Why was it so difficult to determine glacier ice volume, and what did your study do differently?

You might be surprised how unknown some of the basic characteristics of remote mountain glaciers are. Satellites have transformed our understanding of glaciers since the 1970s, and they provide an increasingly clear picture of the location and extent of glaciers. But satellites cannot see “through” the ice. In fact, for 99% of the world’s glaciers, there is no direct measurement of ice thickness. Scientists have spent more time mapping the Greenland and Antarctic ice sheets and the terrain below, and we have much more detailed volume measurements there.

NASA, for example, devoted an entire airborne mission, Operation IceBridge, to collecting ice thickness measurements in Greenland and Antarctica. Scientists have developed various techniques to determine the volume of glaciers, but the uncertainty for distant mountain glaciers is quite high.

We did something different from previous studies. We used satellite imagery to map the speed of glaciers. Glacier ice, when thick enough, behaves like thick syrup. We can measure how far the ice has traveled using two satellite images and map its speed, which ranges from a few feet to about 1 mile per year. Mapping the movement of over 200,000 glaciers was no easy task, but it created a dataset that no one had seen before. We used this new information on ice speed and simple ice deformation principles to determine the ice thickness at each pixel in these satellite images. In short, the speed of the ice that we observe from space is due to the sliding of the ice on its bed and also to its internal deformation.

The internal deformation depends on its surface slope and the thickness of the ice, and the slipperiness of its bed depends on the temperature of the ice at its base, the presence or absence of liquid water and the nature of the sediments or rocks below. Once we were able to calibrate a relationship between ice speed and slip, we were able to calculate ice thickness. To map the flow velocity of all these glaciers, we analyzed 800,000 pairs of images collected by the European Space Agency and NASA satellites.

Of course, as with any indirect method, these are not perfect estimates and will be further improved as we collect more data. But we have made a lot of progress in reducing overall uncertainty.

Teresa H. Sadler