Earth's cryosphere is particularly susceptible to climate change. Rising temperatures are certain to result in profound and widespread changes at high latitudes, where the ground remains frozen all year. Approximately one quarter of the Northern Hemisphere contains permafrost – an area so vast it can only be regularly and comprehensively monitored through satellite remote sensing. The Lena Delta in Siberia lies in such a permafrost zone; it consists of a number of islands and river channels that are covered by ice in winter. A part of this river delta was imaged by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) TanDEM-X satellites in October 2014. The colours correspond to the different signals that the satellites transmit and receive. For instance, the ice floes covering the river channels appear in blue, whereas the frozen ground takes on a greyish colour. Yellow patches can be seen in the midst of this frozen ground, which correspond to shallow bodies of water.
Bodies of water
Such bodies of water are abundant in many high-latitude regions, including the Lena Delta. The yellow colour that they have in the radar image corresponds to a particular kind of interaction between the ice cover and the microwaves transmitted by the satellite. By analysing such images, researchers can estimate the physical properties of the ice cover. Particularly, such microwave data provides: "Valuable information on whether these abundant shallow bodies of water in the Arctic completely freeze to the bottom or not, which affects permafrost conditions under these lakes, fish habitats, and hydrologic connectivity between bodies of water in winter," says Guido Grosse from the Alfred Wegener Institute (AWI). In future, such data could be used operationally to monitor long-term changes in thermokarst lake ice conditions, resulting in a better understanding of climate warming impacts on lake systems, underlying permafrost and the potential release of the greenhouse gases methane and carbon dioxide in the Arctic.
It is not just these bodies of water that are in constant flux; the surrounding land is also subject to changes, and some of these are expected to become more pronounced as the climate continues to warm. The soil contains ice – and with increasing temperatures – more and more of this ice is likely to melt. The thawing permafrost soils can become unstable, leading to slope failures and coastal erosion, as well as posing a risk to human infrastructure.
These are among the many questions undertaken within the Helmholtz research alliance 'Remote Sensing and Earth System Dynamics', whose members include Sonya Antonova (AWI) and Simon Zwieback (ETH Zürich). Specifically, their research addresses the question of whether TanDEM-X data can be processed to show every summer's seasonal ground subsidence due to the melting of ice within the uppermost soil layer. "The magnitude of the subsidence can be used to derive the ice content within the active layer," explains Antonova. "This quantity is of the greatest interest for permafrost modelling." The subsidence can be assessed using the Differential SAR Interferometry (DInSAR) method, which requires acquiring at least two SAR images at different times over the same location. According to Zwieback: "This estimation is – in many regions, such as the Lena delta – hampered by the impact of additional surface processes, such as changes in the moss moisture content or vegetation growth, and it will be important to characterise the uncertainties that these processes cause. Subsidence measurements obtained on site help us to validate the interferometry results."
Sonya Antonova and Julia Boike have installed several on site measurement stations for determining subsidence in the Lena Delta. These observations are one instance of the long-term measurements of the state of permafrost at Svalbard and in the Lena Delta at the Samoylov Research Station. This station is operated by the Russian Academy of Science in collaboration with AWI, and hosts many international research projects. "We need to monitor the surface indicators to identify the hot spots for changes in local hydrology, energy flux and moisture balances," says Boike. Detecting and observing the ongoing changes – such as thawing permafrost and the dynamics of bodies of water in a warming Arctic – have become critical to quantify the impacts of climate change. Recently developed space-based monitoring techniques provide key tools to estimate the global impacts of the changes in vast Arctic regions on all of us.
The success of TanDEM-X forms the basis for the development of innovative radar technologies. Researchers at DLR and at the Helmholtz Alliance 'Remote Sensing and Earth System Dynamics' are already working on a new mission proposal with an innovative digital radar antenna – Tandem-L. A significantly higher imaging capability could be achieved by means of this new technology; it will exceed that of TanDEM-X by a factor of 100. While TanDEM-X only enables one global image of Earth to be acquired per year, Tandem-L will image Earth's entire landmass at a higher resolution twice a week. Hence, Tandem-L will be able to capture dynamic changes on Earth's surface with the necessary imaging frequency to provide information urgently needed for answering current scientific questions regarding the biosphere, geosphere, cryosphere and hydrosphere. Such a mission could be launched in 2020.