How researchers are using the latest Earth observation data – Part three
Part three of this blog series looks into our dynamic Earth: How is it changing, at what speed and with what consequences? The satellite missions TerraSAR-X and TanDEM-X offer new perspectives, which inspire scientists to take new approaches.##markend##
Earth is constantly moving – in some places suddenly and violently, in other places barely noticeable and subtly - the more information we have about the mechanisms behind landslides and other geohazards, the better we can take preventative action and protect ourselves from potential danger. Geophysicists like Bernhard Rebus and Jayson Eppler from the Simon Fraser University have been keeping a watchful eye on changes in ground surface and terrain structures for this purpose. They are supported by the TerraSAR-X satellite. The radar satellite can acquire images of the same area with the same settings every 11 days – time series measurements. TerraSAR-X can detect subsidences and displacements of the Earth's surface of less than half a metre. The high temporal and spatial resolution of the data gave Rebus and Eppler another idea: the scientists developed a special process, 'Demodulation Unwrapping', in order to use old datasets in a new and better way – based on TerraSAR-X data.
Very vulnerable giant
Steady movement is also seen in the perpetual ice. The dynamics are subject to a fine balance. Whether solid ice, ocean-covering sea ice or drifting ice, the massive masses are highly sensitive to changes. The decline in sea ice is among the strongest signs of climate change on Earth. To predict the rise in sea level and other developments more reliably in future, scientists will need detailed information on the dynamic processes in and under the ice. "TanDEM-X and TerraSAR-X offer unique possibilities to determine the flow processes and surface mass balance of glaciers in detail," says Helmut Rott from the Environmental Earth Observation (ENVEO) IT GmbH. With the new 3D elevation world map generated by the TanDEM-X mission, for the first time, a dataset is available that represents the polar regions completely and with great accuracy.
The Antarctic Peninsula is one of the most vulnerable glacier regions in the world. The high-resolution radar data reveal the fragile conditions of the area around the Larsen-A ice shelf, which catastrophically collapsed in 1995 due to an abrupt rise in temperature. At the Science Meeting in Oberpfaffenhofen, Rott presented evaluations of TanDEM-X elevation images showing an average surface decrease of 1.99 metres per year in 2011 and 2013. This results in an annual loss of 4.59 cubic metres of glacier volume and a massive decline of 4.21 gigatons. At present, the flow velocity on the glacier front is five times higher than it was 20 years after the collapse. The speed fluctuations in recent times indicate that the glacier is highly unstable. The analysis of the TanDEM-X and TerraSAR-X data was carried out in collaboration with colleagues Jan Wuite, Thomas Nagler and Stefan Scheiblauer, as well as DLR scientists Dana Floricioiu and Erling Johnson.
Another great worry for polar research lies in the northern hemisphere. The Jakobshavn Isbrae Glacier is considered to be the fastest glacier in Greenland – its calving front continues to retreat inland at very high speeds. It is one of the world's most studied glaciers. Jakobshavn has also become a tourist attraction, since it is thought to have produced the iceberg that sunk the Titanic. From space, the serious considerations arising from the rapidly changing glacier are visible. "In the time series acquired by TerraSAR-X and TanDEM-X, it is noticeable that both the dynamics and flow velocity of the Jakobshavn Isbræ are subject to severe seasonal fluctuations, with record values in the summer," explains glaciologist Ian Joughin from the University of Washington at the Science Meeting. In the summer of 2012 alone, it retracted 12 kilometres. This corresponds to a flow rate of over 46 metres per day. Since the late 1990s, its pace has more than doubled, and flatter glacier areas have thinned by more than 200 metres. As a direct consequence, the global sea level has risen by more than one millimetre during this time. The radar data are therefore crucial for future research into ice and glacier dynamics and the protection of these vulnerable giants.
Miniscule wave, far-reaching effects
We are taking another look at the Arctic – and this time not just at, but also into, the ice. A team of researchers from the University of Alaska, Fairbanks has coaxed a new capability out of TanDEM-X data for observing sea ice and its movements. At the Science Meeting, they demonstrated this using an example from the coastal region of Barrow at the northernmost tip of Alaska. The terrain image from January 2015 shows the nearer expanses of drift ice on the open sea in the expected high resolution – the slight horizontal deformations and vertical movements of the sea ice can be seen to an accuracy of centimetres. But closer analysis revealed a surprise: The up-and-down movements of the drift ice are caused by what are known as infragravity waves (picture detail C). We already knew that these waves spread out through the fast ice, but had not been able to measure the effect so far, for the simple reason that the wave is miniscule, with an amplitude of about 1.2 to 1.8 millimetres. Even more astonishing is the fact that the waves appear to originate in the North Atlantic – meaning they have travelled a distance of over 3000 kilometres. Taking part in the study were Dyre Oliver Dammann, Franz Meyer, Andrew Mahoney, Mark Johnson and Hajo Eicken from the University of Alaska, Fairbanks. The TanDEM-X mission data are now providing us with further insights on the influence of infragravity waves and deformations within the expanses of drift ice. This means a change not only in the form of the Earth, but also in our way of following its movements.