Issue 35

Watching Antartica from Space

“Has it all melted yet?”

People often ask that when I tell them I’m a polar scientist, using satellites to study the Greenland and Antarctic ice sheets from space.

Luckily, the ice sheets are over 3km thick so there is still quite a bit of ice left. It’s amazing to use satellites to watch how quickly things are changing at the poles.

Lots of other questions crop up when I’m talking to colleagues, students, friends and family, so I’ve tried to answer the most popular ones here, and also said a little about some really exciting work I’ve been involved in recently.

What is an ice sheet, and why are they so important?

Ice sheets are a large, thick area of ice covering the land or bedrock beneath it. There are two ice sheets on the planet at the moment – Antarctica is in the south with the penguins, and Greenland is to the north with the polar bears in the Arctic. Together, they hold more than 99% of the world’s freshwater – if the whole Antarctic ice sheet melted, sea levels would rise by about 58m! Although that will not happen in our lifetime – sea levels are currently rising at a rate of 3.4mm per year – we’re definitely seeing changes occurring faster than ever before which is why we need to monitor these important regions.

Together the Antarctica and the Artic hold more than 99% of the world’s freshwater.

So it really is melting?

Yes! Satellite data has shown us that Antarctic ice sheet melt alone has caused sea levels to rise by 7.6mm since 1992, which is when our records go back to, but 3mm of this has happened in the last five years alone, so things are definitely ramping up. This shows how important it is for us to keep monitoring the ice sheets to find out how much ice it is losing, and how this will affect sea levels in future, especially for vulnerable coastal cities and communities. Other sea level contributors include glaciers and ice caps, thermal expansion of the oceans, and land water storage in rivers and lakes.

The ice sheets are complex (which is one reason why my job is so interesting), and different sectors of Antarctica are losing ice faster than other regions. The graph below shows how West Antarctica is currently contributing the most to global sea levels.

In some parts of West Antarctica, the ice is 122m thinner than it was in 1992. And it’s now thinning up to five times faster, as the glaciers increasingly lose more ice than they can gain through snowfall.

At the edges, warm ocean water is also eating away at the ice shelves – the ice that floats from the continental ice sheet out to sea – so that it melts from underneath. The ice shelves hold back the ice further inland, so if they break off we often see the glaciers speeding up and flowing out to sea more quickly.

A graph showing the increase in sea level contribution and the mass of icecaps over time

A sketch of a satelitte going round the globe

^ Antartica in 3D

How do satellites help you to monitor something as huge as the Antarctic ice sheet?

At 14 million square kilometres, Antarctica is not only huge, it also has an incredibly harsh climate and a landscape with rugged mountains as well as the ice itself. So it’s difficult to measure things from the ground, especially when you live in Leeds like I do! Satellites allow us to take a massive step back, and view the entire continent from space instead.

At CPOM, we use data from different satellite missions, but we focus on two main techniques:

Synthetic Aperture Radar, or SAR, uses satellites that return to the same area every few days, actively sending radar waves down to the ground surface and measuring the energy reflected back. The fact that radars are active sensors, as opposed to passive sensors that rely on light from the Sun, means that we can take pictures of Antarctica all year round, even during the polar winter when it is dark 24 hours a day. Another great thing about radar is that it is in a different part of the electromagnetic spectrum to visible light (ie light that we see with our eyes), which means it can ‘see’ through clouds. This is really important when you think about what Antarctic weather is like, as it can often be cloudy.

^ Ice velocity in West Antarctica. Some areas are moving more than 3,000m per year. Processed by A.E. Hogg/CPOM

With two or more radar images of the same area, we can detect changes at the Earth’s surface – small changes on the ground, for example, as a glacier moves. We can use visible features, such as crevasses, to track how fast the ice is moving. This image shows ice velocity at West Antarctica’s Pine Island and Thwaites Glaciers, measured by the Copernicus Sentinel-1 satellite mission launched by the European Space Agency (ESA). The pink areas are the fastest flowing, and the cyan blue regions are much slower.

We also monitor glaciers on the Greenland ice sheet – you can find out more on the CPOM data portal. This has maps of ice velocity for key glaciers on both the Antarctic and Greenland ice sheets in near real time, so you can see how fast they are flowing right now. If you want to do a dedicated project in school, why not take a look at the IRIS Melt project which is specially designed for use in schools.

We also use another type of data called radar altimetry, which is where the satellites measure the height of the ice surface below by registering the time interval between sending and receiving very short electromagnetic pulses to the Earth surface from the satellite, 780km up in space! ESA’s CryoSat-2 mission has a radar altimeter that measures changes in ice sheet elevation, and we’ve been able to see how the ice surface of West Antarctica’s Pine Island and Thwaites Glaciers are lowering, whilst some parts of East Antarctica are thickening.

What’s your favourite satellite?

Definitely CryoSat!

ESA’s CryoSat-2 is an Earth Explorer satellite, and was the first altimetry mission specially designed to measure sea ice and land ice. It was launched in 2010, and since then has been heroically measuring the ice sheets and sea ice thickness. The mission was only meant to last three years, but nine years later CryoSat is still going strong!

CryoSat is really good at measuring the edges of the ice sheet, where icebergs calve out to sea, as well as tiny variations in the height of the ice. This satellite has been absolutely fundamental in allowing us to measure the ice losses from Antarctica and Greenland, and their impact on global sea levels mentioned earlier.

Sea levels are currently rising at a rate of 3.4mm per year.

What else can you see from space?

Using satellite data, we can monitor the huge cracks that appear along ice shelves before icebergs calve. This Landsat 8 image shows the growth of two large cracks in Antarctica’s Brunt ice shelf. With only a 5km bridge remaining we are still waiting to see when this huge iceberg will break off! Check out our CPOM data portal to keep an eye on the cracks yourself.

The InSAR data made it much easier to see where and how the crack was growing, and to predict when the huge iceberg, called A-68 and nearly four times the size of London, would eventually break off.

Although iceberg calving happens naturally, it can allow the ice left behind to flow out to sea more easily. We are using the Sentinel-1 radar satellite to keep a close watch on the Brunt ice shelf throughout the Antarctic winter to learn more about what happens when such a huge berg breaks off.

Have you ever been to Antarctica? How cold was it?

It was pretty cold! I’ve been lucky enough to go to both Greenland and Antarctica a few times now. In 2013/14 I was part of a team travelling across West Antarctica’s Pine Island Glacier, where I was collecting field data with a radar that operates at the same frequency as CryoSat. These ground measurements help us to improve how we calculate ice losses from the area from the satellite data. For three months we camped out on the ice sheet, travelling 900km in total with snow tractors towing all our equipment and our ‘caboose’ – a converted shipping container with a cooking and living space!

Then, in early 2018, I went back to Antarctica as part of the CPOM/NERC/ESA land ice CryoVex campaign. I’ve also carried out fieldwork in Greenland where temperatures got as cold as -42ºC! it was freezing. Last time, in April 2019, our field site was next to Jakobshavn Isbrae, the fastest glacier in the world, which satellites have shown is thinning at a rate of over 20m per year – the height of a five-storey building. You can find out more about my field campaigns by watching this video I made for ESA.

Anna operating the surface radar on the iSTAR traverse. Credit: Jan De Rydt

^ Anna operating the surface radar on the iSTAR traverse. Credit: Jan De Rydt

What’s the most exciting thing you’ve done lately?

I put forward a proposal for seven areas of fast-flowing ice to be named after some of the satellites we use to study them, and they’ve now been approved by the UK Antarctic Place-names Committee. I’d been studying ice flow on the Antarctic Peninsula in a way that wouldn’t have been possible without satellite observations, and thought that the satellites, and the international teams behind the missions, needed a bit more recognition for their efforts. They are definitely my satellite heroes!

The names have been added to the British Antarctic Territory Gazetteer and can be used on all maps, charts and in all future publications. Result!

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Dr Anna Hogg
Centre for Polar Observation and Modelling (CPOM) at the University of Leeds

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