Ice is melting, seas are rising—how scientists are tracking the changes

Will the sea rise by 20 cm or 3 meters by 2100? This is obviously an interesting question to answer. Predicting the sea level in 75 years requires precise calculations and correct models of the melting of the ice from, e.g., Greenland. To do so, researchers need valid data—and photos, too. All of this can be obtained by space technology.

For almost 20 years, this is exactly what Professor Shfaqat Abbas Khan from DTU Space has used in connection with the numerous research articles published by him and his colleagues on the melting of the Greenland ice sheet. On several occasions, the researchers have gained new knowledge about the extent of ice melting, the speed at which it occurs, the acceleration of the melting, and the changes in the landscape that it causes.

How much of this knowledge would have seen the light of day without space technology?

“Nothing,” is the short answer from Khan. “We are deeply dependent on space technology to be able to observe changes in ice, water, and land,” the professor elaborates.

GPS monitoring of mountains

In 2023, Khan was appointed to head up the Center for Ice-Sheet and Sea-Level Predictions (CISP), which brings together researchers from the Geological Survey of Denmark and Greenland, the University of Copenhagen, and Dartmouth College, U.S.

Until 2029, the researchers will use many different space observations to study the melting of ice, not only in Greenland, but across the globe, including the melting from both poles and mountain glaciers. With this knowledge, the researchers will develop new models that predict sea level rise.

A core technology in Khan’s and his colleagues’ research into the melting of the Greenland ice sheet has for many years been the GNET network, which consists of 61 GPS stations located along Greenland’s coasts. GNET is owned by the Danish Agency for Climate Data under the Danish Ministry of Climate, Energy and Utilities.

The GPS stations use signals from GPS satellites, and the stations’ location on the Greenlandic mountains makes it possible to monitor changes in the height of the mountains with a precision of as little as 1/10 millimeter.

This has led to studies that have shown how much the Greenlandic mountains rise, which is interesting to follow, because when the ice sheet on top melts, the pressure on the mountains is relieved, and they thus rise. At the beginning of 2024, new results showed that—in some places—there was a rise of up to 20 cm over the 10-year period 2013–2023.

Mountains rising anyway

However, it is not possible to translate the rise of the mountains directly to how much of the ice sheet is melting these years. This is because land areas that were weighed down by large ice caps during the last ice age are still rising—a phenomenon called postglacial land uplift.

Senior Researcher Valentina Barletta—a colleague of Khan and a research staff member at CISP—has tried to take account of this process in her studies of Greenland.

“The Earth behaves a bit like a memory foam mattress: When you get up from the bed, you leave an imprint, and it takes some time before the material has found its shape again. On Earth, this means that we still have land masses that are ‘straightening out’ after the last ice age,” explains Barletta.

This means that if you want to use measurements of the mountains’ uplift as an expression of how much ice is melting right now, you need to correct those measurements with an estimate of the post-glacial land uplift that would raise the mountains in any case.

Daily ice monitoring

Barletta got the idea of using the entire network of Greenlandic GPS stations as one instrument to register the upward movement of all mountains. You could say that the researchers thus “transformed” the entire Greenlandic bedrock into a kind of scale that weighs the ice it loses.

Since it is already possible to monitor the melting of the Greenland ice sheet using data from the GRACE satellites, the researchers were able to compare data from GRACE and the GPS stations, respectively. This has enabled them to isolate the uplift of the mountains caused by the post-glacial uplift alone.

Now it is possible to see how many millimeters the mountains then rise as a result of the current ice melting. As a result, the researchers have been able to report that every time the mountains (as a total land area) rise 1 mm, it corresponds to 54 billion tons of the ice sheet having melted. The researchers have also been able to calculate that Greenland is currently losing about 5 km³ of ice per week.

“By using the GPS stations in this way, we can monitor the melting of the Greenland ice sheet day by day, thus allowing us to better monitor the loss of the ice mass all year round. This will help us calculate more precise estimates of how much meltwater the Greenland ice cap contributes to global sea level rises,” says Barletta.

Four ways to monitor the ice

Satellite altimetry: Satellites can measure the height of a surface, a method called altimetry. You can measure the height of both the ice sheet and the sea. This is done by a satellite sending a radar or laser signal down to Earth and then measuring the time it takes for the signal to be returned. Knowing the speed of the signal, the measured time is translated into a distance between the satellite and the surface.

GPS: Using GNET’s 61 GPS stations on the Greenlandic mountains, all the way around Greenland, researchers can monitor how much the mountains rise. This can be converted to how much of the ice sheet is melting. The DTU researchers have installed additional GPS stations, including 200 km into the ice sheet in Northeast Greenland.

Gravity field measurements: The GRACE satellites measure the Earth’s gravitational field. Gravity varies on Earth, and the ice melting in the polar regions causes such large changes in local gravity that it can be detected by the GRACE satellites.

Photos: With satellite photos, the movements of the ice can be monitored, for example in the glaciers of Greenland. By determining the velocity, you can calculate how much ice is flowing into the ocean and whether this speed is stable or accelerating.