A first world atlas to estimate glacier water volumes

The evolution of mountain glaciers is a major issue: in many countries they serve as reservoirs of drinking water, have an economic impact through tourism in particular and participate in the rise in sea levels. This evolution was until then little known. We just published a world atlas measuring the flow velocities and thicknesses of more than 200 glaciers, as well as a scientific article in the journal Nature Geoscience.

Despite their small size (727 km²) compared to the combined size of the two large ice caps, Antarctica (000 million km²) and Greenland (14 million km²), the melting of mountain glaciers has contributed to 30% of sea level rise since the 1960s.

Beyond this global impact, the role of glaciers and their evolution is essential at the local level, so their future is a source of growing concern for mountain areas and their foothills.

Little-known glaciers

Despite this fundamental role of glaciers, we have only a very limited idea of ​​the volumes of ice stored in glaciers. This is due in particular to the fact that glaciers are distributed at all latitudes, in regions that are often difficult to access. Working directly in the field is therefore very complex. Thus, measurements of ice thickness currently only exist over barely more than1% of glaciers on Earth's surface (outside the Greenland and Antarctic ice caps).

Because of this lack of observations, scientists have developed indirect methods to estimate the amounts of ice on Earth. These methods were first based on the area of ​​glaciers, easily mapped from aerial photos or satellite images.

From the 2000s, methods based on the surface slope of the glacier emerged, when numerical models of the Earth's surface began to be available on a global scale.

Beyond the slope, the speed at which the glacier is flowing is even more relevant information for estimating the distribution of ice thickness. Indeed, glaciers flow under the effect of their own weight; an image often used is that of a highly viscous fluid, such as honey. Therefore, mapping the speed at which the glacier is flowing is essential to better estimate the distribution of ice thicknesses and therefore the volume of glaciers.

However, field observations of these flow velocities are, again, very limited, but the massive amount of imagery taken from satellites has opened up tremendous opportunities for measuring the flow of all Earth's glaciers.

Satellites to the rescue

To quantify the flow velocity of all the glaciers in the world, researchers from the Institute of Environmental Geosciences in Grenoble and Dartmouth College (USA) used more than 800 pairs of satellite images. These images were acquired between 000 and 2017 by NASA's Landsat-2018 satellites and the European Space Agency's (ESA) Sentinel-8 and Sentinel-1 satellites. This new generation of satellites constitutes a revolution for the observation of glaciers, with images of all emerged land acquired systematically at regular time intervals (from 2 to 5 days). For example, the Sentinel-16A and 2B satellites acquire an image of each point on the earth's surface every 2 days, with the possibility of observing objects with a size of a few tens of meters. Thus, between two consecutive images, the displacement of a glacier is clearly visible in these images.

Flow of glaciers in the Cordillera Blanca in the Andes of Peru. The estimation of the runoff made it possible to highlight in this region lower volumes of ice than in the previous estimates, thus impacting the availability of water.

Author provided 

Several million hours of calculations on the servers of the Grenoble Alpes University were necessary for us to assemble a unique atlas of the flow of more than 200,000 glaciers around the Earth.

One of the main contributions of this atlas is the coverage of a great diversity of glaciers, ranging from small Andean glaciers only a few kilometers long, to ice caps in the Canadian Arctic or ice fields in Patagonia. , flowing over areas of several thousand square kilometres. These maps thus make it possible to better understand the way in which glaciers flow.

They illustrate the wide variety of behaviors, with glaciers flowing at a few tens of meters per year (like some glaciers in the Alps), and others whose flow speeds reach several kilometers in a single year (for example some Patagonian glaciers). This unique database allows researchers to better constrain the representation of glaciers in models, and thus better estimate their future evolution.

Flow of the Upsala Glacier, one of the largest glaciers in the Southern Patagonian Icefield. Red colors indicate a flow velocity of more than 1,5 km per year.

Author provided 

Furthermore, it is this exhaustive atlas of ice flow velocities that has enabled our team to re-estimate the mapping of the distribution of ice thickness and therefore the volume of glaciers. Indeed, by combining information on the surface flow velocity of glaciers with that of the surface slope, in a numerical model simulating the way in which the ice slides and deforms, we have thus proposed a new representation of the glacier geometry.

In many regions, the results of this work provide estimates that are significantly different from the previous ones, with important consequences on the availability of drinking water for consumption, but also for example for agriculture or hydroelectric production. For example, in the basins of the Indus and Chenab, located in the Himalayas, the water stocks contained by the glaciers would be 30% greater than previous studies. Conversely, in the tropical Andes of South America, the new estimates are more alarming, with glacial water stocks almost a quarter lower, thus increasing the pressure on water resources in these regions.

Videos representing the ice thicknesses of the Barpu and Chogo glaciers in the Karakoram (Himalayas).

Beyond a new inventory of the volume of glaciers, this study makes it possible to redefine the three-dimensional geometry of glaciers with greater precision and in accordance with the mechanics of ice flow. This is crucial to better simulate the future evolution of glaciers and, in particular, to identify the sectors where the glaciers will disappear and those where they should persist, at least until the end of the century, although in markedly more reduced.

This study marks a major improvement in the quantification of the distribution of ice thicknesses. However, the estimation of the volume of glaciers remains subject to significant uncertainties, especially in regions of the world where populations are highly dependent on glaciers. To minimize these uncertainties and improve the results in these regions, it is essential to be able to have some field observations in order to better constrain the calibration of the thickness modelling. This calibration step is all the more important as glaciers are varied objects, subject to multiple environmental conditions. Consequently, using in models behavior laws established on the basis of observations made on a few glaciers is necessarily a source of uncertainty.

Romain Millan, Post-doctoral fellow in glaciology, Grenoble Alpes University (UGA) et Antoine Rabatel, Glaciologist, Physicist at the University of Grenoble Alpes, Grenoble Alpes University (UGA)

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.

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