Melting ice – flooded shores

In the scientific community, they are affec­tionately referred to as Tom and Jerry, because one seems to always be chasing the other but never actually catches it. Tom and Jerry are two satellites that circle the Earth at an altitude of 450 kilometres; they com­plete a full orbit in just 90 minutes. Deployed in 2002, Tom and Jerry form the core of the German-American satellite mission GRACE, which stands for ‘Gravity Recovery And Cli­mate Experiment‘. As the name implies, the two satellites’ task is to measure Earth’s gravitational field – every month.

For AWI geophysicist Dr Ingo Sasgen, the GRACE data is essential. He is currently working to determine the status of the major ice sheets on Greenland and in the Antarctic – and ­above all, how quickly they will retreat due to climate change, causing the sea level to rise. “Changes in the ice masses in Greenland and the Antarctic are having a notice­able effect on the Earth’s gravitational field,” says Sasgen. “When the ice melts, the field becomes weaker. The GRACE readings can tell us whether and where, the ice shields are generally growing or shrinking.”

As we all know, every planet produces an attracting force: its gravitational field. The strength of the field depends on the mass of the planet. But, unlike a billiard ball, the mass of the Earth is not equally distributed in its interior. And on the planet’s surface, too, masses are constantly being redistributed – e.g. the seawater is moved by the tides. Tom and Jerry are capable of measuring all these differences. The two satellites fly together at a distance of roughly 200 kilometres, using a microwave radar system to accurately track their distance.

When the first satellite flies over an area with stronger gravity, the field exerts an extra pull; this temporarily increases its speed, and the distance between it and the second satellite grows larger. This deviation can tell us how strong the gravitational field fluctuation is in the respective area. According to Ingo Sasgen, “Despite the considerable distance from the Earth, GRACE delivers very precise readings. In fact, we can even tell how much the gravitational field of the Amazon basin is strengthened by the additional weight of the rainwater during the rainy season.”

As such, it’s hardly surprising that Greenland’s ice masses also have a distinct ­gravitational fingerprint. After all, roughly 500 billion tonnes of snow fall there every year. In the past, much of the snow remained on the ground, even on the outskirts. It then became compressed and added to the ice. “But over the past 20 years, we’ve seen a clear trend: Greenland is losing significantly more ice than it’s gaining through new snowfall – most recently, roughly 250 billion tonnes per year,” explains Ingo Sasgen. That’s troubling news, because if Greenland’s ice melted completely, the global sea level could rise by up to seven metres. “That being said, the seven metres are based on the total volume of Greenland’s ice sheet. In past interglacial periods, Greenland always retained some ice. As such, we assume that it won’t lose the entire mass,” stresses the AWI researcher. “ In addition, the rise in sea level can differ substantially from region to region.” Here, too, the Earth’s gravitational field is an important factor: the seawater is currently being pulled at by the gravitational field of the major ice masses in Greenland and the Antarctic, similar to how the moon drives the tides. Should the ice melt, this pull would weaken; as a result, the sea level in both regions would actually drop in comparison to the global level. “These regional phenomena are often overlooked in discussions about the rising sea level,” claims Ingo Sasgen. 

To better understand today’s situation, the AWI researcher is taking a look back in time and investigating the formation and loss of glaciers during past glacial periods. He is also a specialist for post-glacial rebound, a long-term phenomenon produced by the Earth’s glacial history: “For example, during the last glacial period, roughly 20,000 years ago, North America was covered by a three to four-­kilometre-thick ice shield, and its tremendous weight caused the entire landmass under it to sink. The melting of these ice masses alone produced a global sea-level rise of approximately 80 metres,” says Ingo Sasgen. Today, North America is rising by circa one centimetre per year, because the land mass is still adjusting to the loss of its ice shield.

When, where and how quickly the sea level changed in the past is something that researchers can now glean from sediment samples, because in regions covered by water, the symbiotic communities of freshwater organisms were replaced by those consisting of saltwater organisms – and their remains can still be found in the soil today. But the most interesting thing about the loss of glaciers back then: the melting of ice masses in the Northern Hemisphere produced rising sea levels, especially in the Southern Hemisphere. “These remote effects depend in part on gravitational field changes during the melting,” says Ingo Sasgen. “With the melting of the glaciers, the gravitational pull over North America became weaker. As a result, the water masses were essentially redistributed to other bodies of water, covering great distances.”

The patterns of this distribution are a bit like a fingerprint – they can be used to identify the waters’ source regions. As a consequence, he expects to see similar phenomena in the event that the ice in Greenland or major parts of the Ant­arctic melts: the melting in Greenland would most likely contribute to rising sea levels in the Southern Hemisphere, while the loss of ice in the Antarctic would predominantly ­affect the Northern Hemisphere.

But that’s only part of the story. “There are so many factors that influence the sea level that making reliable predictions is still extremely difficult,” says Ingo Sasgen. His AWI colleague Dr Klaus Grosfeld agrees, adding: “Another aspect is the fact that minor changes can have major repercussions.” For example, if the temperature of ocean currents in the Antarctic Ocean changed by only a few tenths of a degree, it could produce widespread reactions in Antarctic ice masses. The glaciers’ flow speed would then increase due to melting iceshelves, and more inland ice would be transported out to sea, as indicated by both climate models and historical climate data.

For Western Antarctica, a current hotspot for ice loss, this interrelation has already been soundly verified with the help of satellite observations. In response, Ingo Sasgen and Klaus Grosfeld are now working with their colleagues to develop powerful mathematical climate models that attach more importance to this type of phenomena. Their goal: a high-resolution regional forecast of how much the sea level will actually rise along various coastlines.

Since the most important contributing factor will naturally be the melting glaciers, it’s especially important to estimate this melting as accurately as possible. Accordingly, Ingo Sasgen is not solely relying on the data produced by GRACE; he and AWI glaciologists are also drawing on readings from ESA satellites like CryoSat-2, which uses radar to precisely measure the height of ice-covered areas. For example, if the surface of the Greenland ice sheet sinks considerably, it tells us that the sheet has lost ice.

He also integrates the results of climate simulations, which e.g. use meteorological data to determine how much snow and ice form in the polar regions, and how much melts. “Every method has its own strengths and weaknesses, which is why we combine all three,” says Ingo Sasgen. And this approach has paid off! In 2012, Ingo Sasgen and his international colleagues were the first-ever working group to publish an article comparing the use of all three methods for individual regions in Greenland. What they found: the methods dovetail nicely, with each helping to compensate for the weaknesses of the others.

For example, GRACE can only determine whether there is ultimately more ice or less ice; it can’t tell researchers whether a massive loss of ice is due to a lack of precipitation or a warm air intrusion from the Subtropics. Further, a geological phenomenon from the past has to be taken into account when analysing the GRACE data: as mentioned above, heavy glaciers can cause landmasses to sink. Once the ice melts, the landmass begins to rise again – a process that can continue for hundreds or thousands of years after the ice has disappeared. That’s what we’re now observing in North America, which is also strengthening the local gravitational field.

But in Greenland and the Antarctic, too, the land is rising on a major scale, since the ice sheets are now substantially smaller than in the last glacial period. Accordingly, this post-glacial rebound phenomenon also has to be taken into account when calculating the melting on Greenland and in the Antarctic. Plus, a second problem arises in connection with analysing the CryoSat-2 data: depending on the respective layers of ice and snow, the radar signals penetrate to different depths, which introduces an element of uncertainty to the data on the ice’s height.

And mathematical models are only ever as good as the data fed into them. “By combining all three methods, we can now analyse the ice mass balance for Greenland and the Antarctic much better than we could just a few years ago,” says Ingo Sasgen.

But that’s not all: in 2018 the World Climate Research Programme will launch a new proj­ect in which researchers from around the globe will painstakingly analyse and compare GRACE readings and data from the radar satellites. They will also investigate how much each region contributes to rising sea levels, and what the role of post-glacial rebound is in the overall balance. The timing is good, since NASA and the German Research Centre for Geosciences (GFZ) has scheduled a GRACE successor mission for spring 2018. The AWI has helped cover the costs of the launch vehicle and is currently collaborating with the GFZ to optimise the data.

An important technical innovation is the improved system for measuring the distance between the satellites; a laser connection will allow the gravitational field to be measured more precisely. The goal of the participating AWI researchers is clear: Ingo Sasgen and Klaus Grosfeld want a tool that perfectly combines all three measuring methods, and one that allows them to monitor the status of the ice in real-time – in other words, essentially a remote diagnostics programme for the polar regions that also supplies the meteorological or climatic causes of changes as they occur. “The plan is to also make it freely accessible,” explains Klaus Grosfeld, “Whoever want to, can check in at any time to see the current state of the ice mass balance in the polar regions, and what effect it has on the sea level.”