Interview with Gordon Hamilton: glaciers, what changes?

© N.Pansiot/Tara Expéditions

As Tara pursues her mission in Greenland, we have wanted to know more about the transformations observed by scientists in this region of the globe. We are specifically interested in the studies carried out on the glaciers that generate multiple icebergs, between which the schooner had to navigate. Gordon Hamilton, glaciologist and associate research professor of Earth and Climate Sciences at the University of Maine answered our questions on the interaction between climate and ice cap.

©N.Pansiot/Tara Expéditions

Could you give us a glimpse of your work as a glaciologist?

I’ve been studying the ice sheet in Greenland for 15 years now. Thanks to data from field experiments or remote sensing -which allows us to observe from space- I am tracking the behavior and evolution of the ice cap over time. In the 1990s, minor changes were observed, of the order of a few centimeters per year. But, between 2003 and 2005, we suddenly noticed really rapid evolution, especially at the margins of the ice sheet. We have estimated, thanks to the water drained from the glaciers towards the ocean, that the melting rate of some of the big glaciers had doubled or tripled in just a matter of weeks.

This means that more ice is moving from the middle of the ice sheet towards the ocean, where it forms icebergs. Of course, the melting of the polar ice cap has huge implications on sea level rise. Therefore, we have been returning to Greenland very often -2 or 3 times a year- since these past 10 years in order to understand these sudden changes and see if they expand to other parts of Greenland, or not.

What are the explanations of these phenomena?

Of course, this is a difficult issue to solve! There are many variables to take into account, and for the moment we don’t completely understand how the ice sheet behaves. What should be borne in mind is that the ice cap interacts with climate in 2 ways: 1) through the atmosphere -and it’s well known that air temperatures are rising- and 2) through its contact with the ocean.

In fact, if you go to Greenland during the summer, the thing the more obvious that you’ll see is the presence of melt water, due to warm temperatures. Some of this melt water sits on top of the glaciers in ponds. They are called supraglacial lakes. These ponds can drain suddenly, all the water then flowing towards the bottom of the glacier, under several hundreds of meters of ice. The melt water sitting between the glacier and the bedrock then facilitates the sliding of the glacier. What we need to understand is to which extent increase in temperatures and glacier melt water accelerate the sliding of the major glaciers of Greenland. Some experiments have shown that glaciers actually move slightly faster on warm days, but only 4 or 5% faster than normal. There is therefore an effect, but it does not seem to be the main cause of the very dramatic change in Greenland glaciers.

We have observed more closely the parts of Greenland that are changing the fastest, that is to say these in contact with the ocean. Most of the ice sheet is drained by large outlet glaciers that move ice from the middle of the ice cap towards the coast, then flow into deep fjords. These glaciers dive down to 500 or 1000 meters depth and melt. Submarine melting is therefore the main and fastest cause of glaciers disappearance! In recent years, our major objective has been to understand how the ocean and its evolutions are affecting the big glaciers in Greenland.


N.Pansiot/Tara Expéditions

©N.Pansiot/Tara Expéditions

What have you observed?

Many interesting things! In particular because no research had previously been conducted on the oceanic waters of a typical fjord of Greenland. One of the first things we did was to perform analysis, from a small boat, using a typical oceanographic instrument: a CTD probe, that measures Conductivity, Temperature and water Depth. We were amazed to find that within about 20 km of the edge of the ice sheet, the water temperature reached 4°C between 200 and 1000 meters depth. This is a major reservoir of warm water sitting very close to the ice sheet, that can cause significant submarine melting, making ice unstable and subsequently accelerating its disappearance. I think this is the best explanation for the abrupt changes observed in Greenland over the past 15 years: they’ve all been caused by the ocean.

What are your expectations for the next decades?

The future of the Greenland ice sheet depends on many factors, including the climate and ocean changes that will occur in the coming decades. Parts of the ice sheet located in deep depressions of the bedrock, below sea level and submersible, are the most unstable. As the atmosphere warms up, the excess heat is absorbed by the ocean. Therefore, as long as the atmosphere gets warmer, the ocean temperatures will continue to increase. This means that if the ocean continues to warm up, outlet glaciers that flow into deep fjords will continue to melt.

This doesn’t sound like good news…

It’s bad news, of course. First, the increasing formation of icebergs cause sea level rise. Second, the melting of the ice cap adds fresh water into the salty ocean, and has major consequences on the marine ecosystem: fresh water stratifies the ocean and changes the vertical distribution of plankton and other organisms of low trophic levels. In turn, it affects the species of higher trophic levels, such as fishes and marine mammals. This supply of fresh water also disrupts the global ocean circulation. We are only beginning to realize how important the influence of icebergs on the ocean can be.

What do you expect of the upcoming Conference on Climate Change in Paris (COP21)?

I’m a scientist, not a politician! As a citizen, I hope some good will come out of it… As scientists, all we can do is continue to observe and communicate our results as we see and understand them. It is essential to perform experiments to explain why changes are occurring. I hope that the abundance of empirical evidence will convince not only the general public, but also decision makers and politicians that prompt action is necessary.

Interview by Noëlie Pansiot


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