We regularly hear in the news that the polar ice caps are melting with increasing speed and scientists all over the world are concerned about the rapidity of the ice retreat. Their concerns are entitled, considering that Greenland contains an amount of ice that would raise global sea level by approximately 10 m, while the Antarctic ice sheet contains enough ice to raise global sea level by around 60 m (Huybrecht, 2002). That means the loss of both ice caps would raise global sea level by almost 70 m and many major cities that are build around the ocean would be endangered: e.g. Sydney, Melbourne, Bangkok, New York, Miami, Tokyo, Shanghai, Mumbai, Buenos Aires, Dubai, Athens or Barcelona, just to name a few.
Now, we don’t know if that scenario will occur or how strong sea level will rise eventually, but we have to consider that centimetres of sea level rise are already threatening low lying areas, which is why extensive research is necessary.
However, the problems we have with these huge ice sheets are the harsh climatic conditions that exacerbate the sampling of scientific data and it is therefore difficult to obtain enough information on ongoing ice mass loss especially in the interior of the ice sheets. To overcome those problems modern space geodetic techniques can help by providing new information.
But before I can actually explain the satellite missions, it is important to know a few basics about the response of the Earth’s crust to ice load.
The Earth’s crust floats on viscous mantle material, which allows tectonic plates to move. Now, if we add a lot of weight, say a huge ice sheet, we have the same effect as the floating boat on a lake: the boat sinks into the water, pushing the water aside. Here we have the surface load on top of the Earth’s crust, forcing the crust to bend into the mantle material by pushing the viscous mantle material aside. Once the weight gets removed, the mantle material will start to flow back and the Earth’s crust slowly lifts up again. The adjustment of the Earth’s crust due to mantle material distribution is a visco-elastic deformation, called Glacial Isostatic Adjustment (GIA), and is still ongoing in regions that were covered in much more ice during the last ice age.
Alongside the slow process of the visco-elastic deformation, the Earth’s crust itself also reacts to surface loads. This happens on a much shorter time scale and is the elastic response of the crust.
GRACE – Gravity Recovery And Climate Experiment
The twin satellite of the GRACE space mission delivers measurements about gravitational changes on Earth by varying the distance between two satellites. At an altitude of ~450 km the twin satellites orbit the Earth in a tandem arrangement, separated by ~220 km. The Earth’s gravitational field influences the speed of the satellites with a stronger gravity forcing the satellite to speed up. That causes a change in distance between the two satellites, which is detected by a very sensitive microwave ranging system that even recognizes a change as small as 10 micrometers.
Now you might wonder what gravity has to do with ice mass changes but the idea behind all that is that gravity is directly related to mass, hence, more gravity indicates more mass, while less gravity means less mass. Detecting gravitational changes over time therefore delivers valuable clues about changes in mass, which in turn can tell us a story about ice mass changes.
Unfortunately, it never is that easy…
A detected change in mass over an ice-covered region can have different causes. It could be simply due to variation in climate, changing the surface load (ice mass), which is what we are interested in, but it could also be caused by GIA.
The problem scientist are facing is that GRACE is not able to separate between the different sources and we have to do so by hand, with the knowledge we have about climate and GIA.
ICESat – Ice, Cloud and land Elevation Satellite
The ICESat mission is one of many that is used to obtain observations on surface elevation changes by measuring the time it takes a laser pulse to return from the Earth’s surface to the satellite. Over time these measurements deliver information about surface height changes and ice height changes. Knowing ongoing changes in ice elevation means knowing ongoing ice gain or loss, however, to make life not too easy a change in height can not only be caused by ice thickness changes but also by variations in the Earth’s crust and the compaction of snow.
As mentioned earlier, the adjustment of the Earth’s crust due to surface loads has an affect on mass variations and with that on elevation, as the crust lifts up or bends down. The second factor that has to be considered here is the compaction of snow. When fresh snow falls we have these nice little snowflakes covering the surface; once there is further snow accumulation these snowflakes are buried and trapped under the new layer. Due to overburden pressure and temperature the snowflakes get compressed more and more with increasing load until the state of dense glacier ice has been reached. That process involves a loss in thickness (due to compaction) but not in the amount of ice, which needs to be considered in detected elevation changes.
So far that sounds easy, doesn’t it? Unfortunately, it’s not.
The compaction of snow depends on the amount of overburden pressure, temperature and melting/refreezing events and is not well known for the big ice caps due to the lack of measurements and in-situ observations. Some models have been established under identified snow and climate conditions for Greenland and Antarctica, providing an indication of how snow compaction might look under those conditions, but unfortunately they are still unreliable.
GPS – Global Positioning System
Less fancy than the previous two missions and involving a bit less trouble are measurements with GPS stations.
Here we are talking about GPS stations that are installed on rocks in ice-covered regions. The stations have to be installed on ice-free bedrock, where they are used to observe any ongoing change in the vertical movement of the Earth’s crust. In polar regions we are especially interested in possible uplift rates, as that would give us information about ongoing GIA. Unfortunately, the network of GPS stations throughout the ice sheet is rather sparse due to the general coverage of ice, leaving us once more with just enough information, mainly from coastal regions, to make assumptions about general ongoing changes.
Nevertheless, all of these missions are very helpful tools, delivering new information that, combined with in-situ observations and measurements, can help scientist to better understand the current situation of the polar ice caps.
The hope that came with the GRACE and ICESat mission was to combine the results of elevation changes, mass changes and crustal movement, which turned out to be more difficult than expected and therefore has not been explicitly done yet. Focusing on Antarctica the idea of my research is to find a solution on how to combine the satellite missions, including all the problems that are involved, and to obtain a better understanding on the stability of the Antarctic ice sheet.