CR
Cryospheric Sciences
Violaine Coulon

Violaine Coulon

Violaine Coulon is a PhD student of the glaciology unit, at the Université Libre de Bruxelles (ULB), Brussels, Belgium. She is using a numerical ice sheet model to investigate the dynamics and stability of the Antarctic Ice Sheet for the past 1.5 million years. She also investigates the sensitivity of the Antractic Ice Sheet to the incorporation of lateral variability in the viscoelastic Earth structure across Antarctica.

Image of the Week – The solid Earth: softer than you might think!

Rebounding beach in the Canadian Artic [Credit : Mike Beauregard distributed by Wikimedia Commons]

Global sea level is rising and will continue to do so over the next century, as has once again been shown in the recent IPCC special report on 1.5°C. But did you know that, in some places of our planet, local sea level is actually falling, and this due to rising of the continent itself?! Where is this happening? In places where huge ice sheets used to cover the land surface during the last ice age, such as Scandinavia, Canada, or Siberia. Even though these ice sheets melted several thousands of years ago, the land that once lied under them is still rising in reaction to the release of their previous burden. This is what we call Glacial Isostatic Adjustment or GIA. Where does this adjustment come from? Our Earth is not as solid as you would think…


Our Image of the Week represents a layered beach located in Nunavut, in the Canadian Arctic. This specific landform is caused by the glacial rebound of the Arctic coastline resulting from the response of the lithosphere to the melting of the Laurentide Ice sheet, an ice sheet that used to cover the North American continent until less than 10 000 years ago.

Earth during Last Ice Age [Credit: Wikimedia Commons]

What is Glacial Isostatic Adjustment?

Imagine sitting on a very comfy couch, watching a movie. At the end of the movie, the couch has perfectly adapted to the shape of your body. Once you get up, you’re still able to see where you’ve been sitting, as the couch takes a little time to get back to its original form. Well… this is exactly what happens with the Earth’s crust and mantle. To understand this, you need to visualize the internal structure of our planet Earth, which is layered in spherical shells: under our feet lie the rocky tectonic plates, which constitute the Earth’s crust. These crusty plates – whose thickness varies between a few kilometers under oceans to a few tens of kilometers under the continents – are floating on a viscous layer, called the mantle. It is almost 3000 kilometers thick and actually slowly flows like a liquid, at a speed of a few centimeters per year.

Even though the Earth’s crust is a very strong material, the pressure applied by an ice sheet thick of several kilometers is so important that the crust will locally deform under the heavy ice mass, sinking down into the viscous mantle. That’s what happened over large areas of the Northern Hemisphere that were covered by ice masses during the last ice age, and what is still happening in the remaining ice sheets of Greenland and Antarctica, which have been depressing the Earth’s crust beneath them for thousands of years.

Just like for the couch in the example above, when the weight is removed, the mantle rebounds, carrying with it the overlying crust. Over the 20 000 years since the last glacial maximum, lands now relieved of their previous ice-burden are gradually rebounding. The Earth’s delayed response to the variation of mass on its surface is explained by the viscous character of the Earth’s mantle.

Glacial Isostatic Adjustment [Credit: Wikimedia Commons]

Why is it important to take it into account?

Even though the Siberian, Scandinavian and Laurentide ice sheets melted several thousands of years ago (causing a rise in global sea level), these regions that were previously glaciated are still locally emerging to compensate the loss of their overlying weight. The level of the coastline relative to the local sea-level thus increases. One says that the “relative sea level” is falling, and this at a rate that is essentially determined by the rate of the post-glacial rebound (which can exceed 1 cm/year in some areas, as shown in the figure below!). The rates of relative sea level can be influenced even at sites that are quite far away from the centres of the last glaciation, although it is much less significant.

Rate of the post-glacial rebound [Credit: NASA, Wikimedia Commons]

A good understanding of glacial isostatic adjustment is important to distinguish the different components and contributors to a local sea-level evolution: what part is due to the uplift of the land? And what part is due to the rising of global sea-level?
In addition, glacial isostatic adjustment also impacts the behaviour of  modern Greenland and Antarctic ice sheets. By influencing the geometry of the underlying bedrock, it will impact the sensitivity of the ice sheet to global warming and thus the glacial isostatic adjustment itself: this is a vicious circle!

The problem is that glacial isostatic adjustment also depends on the local properties of the Earth’s crust and mantle, which are not constant at the Earth’s surface. A lot of work is still needed to understand all of this properly. Luckily, since NASA launched GRACE – a satellite mission that maps variations in the Earth’s gravity field –  in 2002, scientists have observations they can use to constrain their models and improve their understanding of this complicated matter.

Further reading

Edited by Clara Burgard and Sophie Berger

Image of the Week – Will Santa have to move because of Climate Change?

Santa Claus on the move [Credit: Frank Schwichtenberg, CC BY 3.0, Wikimedia Commons]

Because of global warming and polar amplification, temperature rises twice as fast at the North Pole than anywhere else on the planet. Could that be a problem for our beloved Santa Claus, who, according to the legend, lives there? It appears that Santa could very well have to move to one of its second residences before the end of this century. But even if he moves to another place, the smooth running of Christmas could be in jeopardy…


But…. Where does Santa live?

The most famous of Santa’s residence is in Lapland, Finland, at Korvatunturi. But since this area is a little isolated, Finns then moved it near the town of Rovaniemi. For Swedes, it’s in Gesunda, northwest of Stockholm. The Danes, them, are convinced that he lives in Greenland while according to the Americans, he lives in the town of North Pole, Alaska. In Norway, there is even disagreement within the country: some Norwegians believe he lives in Drøback, 50 km south of Oslo while other believe he lives in the Northernmost inhabited town in the world: Longyearbyen, Spitsbergen, where Santa even has its own postbox!
Even in the southern hemisphere, Christmas Island claims to be Santa’s second home.

Santa’s huge postbox in Longyearbyen, Spitsbergen [Credit: Marie Kotovitch] and Rovaniemi, Finland: the self-proclaimed “official hometown of Santa Claus” [Credit: Pixabay]

It seems that Santa Claus has many places to stay.. But according to the legend, Santa’s real permanent residence is in fact the true North Pole. However, as shown by the Arctic Report Card 2018, the Arctic sea-ice cover continues its declining trends, with this year’s summer extent being the sixth lowest in the satellite record (1979-2018). The latest IPCC 1.5°C warming special report states that “ice-free Arctic Ocean summers are very likely at levels of global warming higher than 2°C” relative to pre-industrials levels. Considering that the world is currently on course for between 2.6 to 4.8°C of warming relative to pre-industrial levels by 2100, Santa’s home is projected to sink into the Arctic Ocean before the end of the current century. It appears it would be time for Santa to start thinking about which one of his second residences he will choose to move to…

Will Santa have to find a new home? [Credit: Pixabay]

Rudolf might be in trouble…

Of course, if he moves away from the melting North Pole, Santa still needs snow at Christmas to be able to take off his sled. But, actually, this could become a problem.
This year, there was still no snow in Rovaniemi, Finland, the self-proclaimed “official hometown of Santa Claus”, by the end of November, making the local tourist attractions very worried. Luckily, it has now snowed there since, but how does this look like for the years to come? According to the latest Arctic Report Card, the long-term trends of terrestrial snow cover are negative.

Another problem which might complicate Santa’s work was underlined in a study published in 2016. This study showed that reindeers are getting smaller because of warmer Arctic temperatures. How come? During the long winter, reindeers are usually able to find their food (which consists of grasses, lichens and mosses) by brushing aside the snow that covers it. But because of the warmer temperatures, rain now falls on the existing snow cover and freezes. The animals’ diet is thus locked away under a layer of ice. As a result, reindeers are hungry and lose their babies or give birth to much leaner ones. The Arctic Report Card 2018 states that the population of wild reindeer in the Arctic has decreased by more than half in the last two decades.

All this is not going to get better, as Arctic temperatures for the past five years (2014-18) all exceed previous records. According to the Danish Meteorological Institute, in November 2016, Arctic temperatures were reaching an incredible peak at around -5°C while average temperature at this period usually is around -25°C.

Climate change also affects reindeers [Credit: Photo by Red Hat Factory on Unsplash]

Christmas trees also at risk!

You may say that Santa is Santa and that he will be able to find a solution to all these problems. Let’s hope you’re right! But another problem is looming on the horizon: you might soon not be able to welcome Santa in your own home as it should with a beautiful Christmas tree.

Indeed, this summer’s heat waves have strongly affected Christmas tree crops everywhere in Europe. Moreover, a 2015 study shows that native Scandinavian Christmas trees are also affected by climate change, and more specifically by reduced snowfall. The latter acts like an insulation layer which protects the roots from the cold winter.

We hope that this post has made you realize the urgency of the fight against global warming! However, in the meantime, don’t forget that the most important to spend a nice Christmas is the Christmas spirit! We wish you all a very merry Christmas and a wonderful new year!

As a little Christmas gift..

  • If you want to find out the truth about Santa’s real home, you can always check it by yourself by using the Santa Tracker by Google to follow Santa’s Christmas Eve trip and check where he comes back at the end of the night…
  • The highlights of the Arctic Report Card 2018 are summarized in this video.

Further reading

Edited by Clara Burgard