Glaciers, our largest fresh water source

Glaciers, our largest fresh water source

In a time of global warming, scientists pull the alarm every year about the consequences. One of the consequences that worry the most is the future of glaciers and what will happen if they keep loosing ice and disappearing. When asked what a glacier is, most of us easily picture this big mass of ice on top of mountains in high altitude or floating in the ocean at the pole. But most of us also don’t quite understand what is at stake. Let’s correct that!

Alpine glacier picture
Schlatenkees alpine glacier (source: Rafael Brix)

What is a glacier?

A glacier is a compact and dense mass of ice, constantly moving [1]. They flow down slopes the same way rivers do. Because of that, we can almost consider them as a fluid with high viscosity.

We classify glaciers in two categories: constrained and unconstrained.

Constrained glaciers (also known as alpine glaciers) are the ones formed depending on the topography, in mountains and thalwegs (line formed by the set of points with the lowest altitude in a valley).

For unconstrained glaciers (also known as continental glaciers), topography doesn’t constrain their extension. Therefore, they are considerably larger. Greenland and Antarctic ice sheets are in this category.

Antarctic ice sheet picture
Mount Sidley and Antarctic ice sheet (source: U.S. Navy photo)

Glaciers are not specific to our planet. They can also be found on Mars, at the pole, and on Ganymede, Callisto and Europa, three of Jupiter’s moons.

How is it formed?

Snow falls in high altitude or high latitude creating several layers. Under their own weight they get compacted creating a large mass of ice. It is this important weight that make it move forward. The snow provision needs to be more important than the melting, the sublimation (transition from solid to gaz) and the iced water slide down the slope.

A glacier has three zones. The accumulation zone where the snow turns into ice (usually on the most upstream part of alpine glacier). The transport zone, the thickest part, with low melting and very high erosion. The ablation zone where the glacier terminus is as a cliff-like shape.

They have different altitude formation depending on the latitude. It is a the sea level on the pole compared to the approximate 3,000 meters in the Alps.

A moving mass

When the glacier’s thickness exceed fifty meters, it gains in plasticity, in malleability [2]. This gives it the ability to move by itself because of its own weight. Doing so, the surface, not as malleable, breaks and creates crevasses and seracs (big column of ice between the crevasses).

Its speed depends on several characteristics: topography (orientation and inclination of a slope), its temperature (causing the presence of air and liquid water, lubricating the ground/glacier interface and increasing speed) and the smoothness of the ground (with more or less rocks, causing less friction and increasing speed).

Of course, the speed is not regular within the whole glacier. It reaches the maximum at the deepest of the accumulation zone and at the ablation zone surface while it reaches its lowest on the edges.

The average speed depends of the glaciers and is between a few centimetres and a few tens of centimetres per day. Although some of them can reach several meters per day, like in Greenland.

What do they provide?

Glaciers are the largest reservoir of fresh water. They are the source of some of the more important mainstem rivers in the world through meltwater, providing water for millions of humans, other animals and plants.

The high albedo (its capacity to reflect light) thanks to the ice, is also a determining factor. It helps regulating the amount of solar irradiance reaching the Earth therefore its global temperature.

By its movement, glaciers cause abrasion (erosion when two surfaces are rubbed against each other) and plucking (carrying rocks and boulders during its progression). This results respectively in the formation of cirques (concave valley with smooth ground) and moraines (accumulation of rocks).

Risk of disappearing

Now, about the elephant in the room… Because of global warming, glaciers are smaller and smaller. What consequences their diminishing or disappearing could have to us?

First, less glaciers and less ice sheets equals less albedo. This would results in an increase of the amount solar irradiance reaching the ground, then an even more warming, then more glaciers melting, etc. It would create a snowball effect impossible to stop.

Like pointed out before, they are the largest source of fresh water in the world. It will lead to dry out our main rivers, reducing the quantity of drinkable water and the water mandatory to irrigate crops.

In the end, the protection of our glaciers is important as well as reducing the global warming we are facing. Our survival depends on it.

References

[1] All About Glaciers | National and Ice Data Center, https://nsidc.org/cryosphere/glaciers

[2] Greve, R.; Blatter, H. (2009). Dynamics of Ice Sheets and Glaciers. Springer. doi:10.1007/978-3-642-03415-2

 

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