Ocean Conversation with

H.G.

Space

It is the home of H.G., an apartment on the ground floor of an apartment building built in the 50s, in the center of Brussels. After entering, one passes the office and the living room, and enters the brightly lit dining room at the back. This is where we sit. It is next to the kitchen. Through the large windows, one can see the big garden with a stone fountain in the middle. Throughout the conversation, H.’s daughter, who is sick and is therefore skipping school, sits quietly behind me on a big couch, reading a book.The second part of the conversation takes place in H.G.’s office at the Université Libre de Bruxelles. It’s a small office filled with maps of the Antarctic ice sheet of different sizes and periods. The biggest one is also the oldest one, which is separated in four paper sheets that are already turning yellow. The walls of the space are also quite a warm white. H.’s desk is towards the back of the space, near the window. The blind is closed. At the front of the room, directly to the right of the door is another desk. No one is using it, but the fact that it is there creates the impression that a colleague of

M.M.What’s your relation to the ocean?

H.G.Do you mean my professional or personal relation?

M.M.Maybe we can start with the professional.

H.G.Well, I started working on the ocean during my PhD. In a sense I am a trained oceanographer. I studied ocean–atmosphere interactions.

I am a modeler. I make and use computer models of different parts of the Earth system. My research focus now is primarily on glaciers + ice sheets. Because I worked with models → there was no real need for me to actually, physically, go to the ocean. There is often a disconnect: between the scientists that do the observations in the field + the modelers. I use the data that they provide to improve and validate the models.

My professional relationship to the ocean is a relation of a researcher to a subject of scientific interest, so I’m treating it as such. Some people have a strong relationship to the system they are studying, others don’t. My work did not necessarily bring me close to the ocean. You get to know the ocean better when you are doing field work, e.g. on a research ship.

I’m mainly going there on holiday. I enjoy swimming.

So, back to what I am working on at the moment: I make models of ice sheets → mainly the ice sheets on Greenland and Antarctica. The models I develop try to describe the physical processes of those systems. In the model, the area of the ice sheet is divided into small grid cells (this process is called discretization). On that grid, the model solves mathematical equations that describe e.g. the conservation of energy, mass and momentum.

The ice sheet model I am using determines the rate of mass gain/loss at the top of the ice sheet and calculates how the ice is flowing under the force of gravity to the margins.

Snow is deposited at the top of the ice sheet. The snow that falls on the ice sheet is compressed into ice. The ice is solid but still flows under its own weight.

Ice moves to lower regions of the ice sheet and towards the margins where it melts or flows into the sea. So the ice flows into the ocean as water or as ice. There, it can cause a rise in sea level.

M.M.Are ice sheets mainly produced on land?

H.G. You could say that, but as they flow to the margins they can get in contact with the ocean. The ice sheet in → Greenland is predominately land-based. But its outlet glaciers can have a strong interaction with the ocean. The ocean is melting the ice from underneath + the side. In contrast → a large part of the Antarctic ice sheet is grounded below sea-level, meaning that the ice is in contact with ocean water.

There are also large ice shelves in Antarctica → which are extensions of the ice sheet that are floating on the water. They can be several hundred meters thick.

Apart from ice sheets and ice shelves, there is also sea ice, which is essentially created by freezing sea water. Sea ice can be several tenths of meters thick.

As we move from the center of an ice-covered continent to the margins, we have first the land-based ice sheet → then we have floating ice shelves → and then sea ice and open ocean.

At the moment, there are only a few Greenland → outlet glaciers that have ice shelves. The rest just have an ice cliff in the water and in front is a soup of water, sea ice + iceberg parts.

M.M. What kinds of data do you need to make the model?

H.G.

The model will also consider feedback mechanisms. For example: When the ice sheet grows → it changes the climate. The higher the ice sheet gets, the colder the climate gets on top of it.
→ Conversely, the lower the ice sheet, the warmer the climate.

Since we have satellite missions, there exists a huge amount of data for the recent past, but less before that and much less before humans came to look. At other times in the past or in the future and in places with less or no data, models allow for a physically consistent → mathematical description, which can be used to fill gaps in the data and increase our scientific knowledge.

When enough observational data is available, a process called "data assimilation" can be used that combines observations with the physics of the model. For the past or the future where no data is
available, the model is the only way to extend our knowledge**, based on data from the present.

For example, the last glacial maximum was a period 18,000 years ago with much larger ice sheets than what we have today. As there is limited data from that time, we can use a model to create a physically consistent description of the climatic conditions and ice sheet state at that time. The model solves the equations at discrete points on the grid.

M.M. How do you characterize the information that is simulated by the model?

H.G.It’s the model output. Regarding the relation of data vs. model output → you should be able to compare this output to contemporary data and thus test the accuracy of the model. This can be done by using a so-called hindcast simulation, so the opposite of a forecast. When producing a hindcast: you should limit yourself regarding the amount of data that you use to optimise the model. Because otherwise you won’t have enough “unused” or “free” data to compare and test your model output with.

M.M. How did you start working in this field?

H.G. Well, initially I was working on automatic speech recognition. But then this didn’t seem like something I wanted to be busy with day-to-day. So I started to become interested in climate and Earth system science.

A typical ice sheet model has a resolution of between 1.000m and 10.000m. Essentially, when you want to double the resolution of the model, you need to invest x10 the computing power. Often you need a super computer to run the model in its fullest capacity. Because a single processor on even the fastest computer is not fast enough, we have to distribute the computation across several computers. We cannot make more powerful processors, so we have to use multiple processors simultaneously instead to achieve the needed processing capacity.

M.M. So in order to study the effects of global warming, for example, you contribute to global warming, by using all this electricity and not-always-sustainable technology?

H.G.You could say that, yes. However, the footprint of the climate modelling community is probably small compared to that of the industry.

M.M. What was your educational parcours before your PhD?

H.G. In my MA I focused on automatic speech recognition. I chose that subject because I have a great personal interest in music. I had studied physics (in Berlin) and wanted to apply my knowledge to the field of musicand acoustics. So I ended up working in a group on improving hearing aids. What we did was basically to study how the ear works → in order to develop better speech recognition technologies.

But when I started thinking about what I wanted to do in the long term, I realized that it was not interesting for me to continue working in this field and develop ever better speech recognition software for some tech company.

→ I was brought up very ecologically
→ And was always very sensitive to climate related issues. So I chose to change my orientation and focus on something that seemed to me more relevant in the long term.

M.M. Is there an element of the ocean that is particularly relevant or even crucial for you?

H.G.The interaction between the ocean + the ice sheets. → The biggest uncertainty concerning future changes in sea level is related to how the ocean would influence the ice sheets, especially under the current conditions of global climate change.

The element I would therefore like to name, is the interface where the ocean meets the ice.

It is important to understand how heat is transported by the ocean currents to the ice and how the changing temperature of the water influences ice melting. → How does warm water intrusion into the fjords in Greenland and under the ice shelves in Antarctica affect the outlet glaciers and ice shelves? How do the currents evolve as a result?

→ How exactly does the interaction between the two systems (ice sheets + ocean) play out? This interaction can be understood as a third agent that arises from the “meeting” between the two systems. How does this interaction affect the ice sheet on one hand and the temperature of water on the other hand? And how does this interaction, in turn, affect the ocean circulation around the ice sheet?

M.M. Do you think it would be possible to hear the movement of the ice in the warmer ocean water?

H.G. No I don’t think so. But there’s a huge sound when the ice breaks off when a glacier calves.

Conversation part 2

While looking at the maps of Antartica on the walls:

M.M.Why don’t you use the meridians to make the grid for the models?

H.G.Because the meridians are nearer to each other towards the poles and further away from each other towards the equator; so this would lead to higher resolution at some parts of the model + lower resolution in others. However, the model requires an equal resolution across the different points of the glacier.

M.M.Are the two different values on this map referring to the heights of the glacier at those points? Why are they different, are they the measurements from two different expeditions?

H.G. Yes, probably. But I wonder how it can be that the differences are so considerable?

M.M.How are the measurements made? By drilling?

H.G.No, this would take too much time + be too expensive. They are typically done by radar measurements.

We take a tour of the lab and look at the ice cores. Cores from the Antarctic are placed in cardboard cylinders and stored in regular household deep freezers. The lab consists of three adjacent rooms. In the first room, there are about five large deep freezers, in which the cores are stored. In the second space, the ice is melted and analyzed by help of purifying machines and computers. The third space is cooled to -25°C. By wearing uniforms to keep themselves warm under those conditions, researchers work with the samples of the ice cores themselves, analyzing their structure under a light sensitive filter.

M.M.Do ice cores resemble geological cores, which make visible the different geological epochs?

H.G.Yes. But ice also traps air molecules. Therefore, when analyzing the core, you can have a sample of the atmosphere as it was hundreds of thousands of years ago.

M.M.Is the escalating carbon dioxide content of the air in the last 50-100 years ingrained in the ice as it is on the soil?

H.G.Yes. In fact there has not been a sharp increase in CO2 as we had in the last 50 years during the last 800.000 years. We have no ice core record from before.