Scientific Requirements

For a long time, the role of the forest with respect to climate change was underestimated. Today, we know that forests are an important factor for climate conditions. In modern climate models, the interaction of the forest and the atmosphere, hydrosphere and geosphere play a central role. This is because the forests are predominantly responsible for the carbon exchange between the terrestrial ecosystems and the atmosphere. But forests also influence the albedo of the earth’s surface and, hence, the radiation balance, and are an important factor for the atmospheric water balance. As forests represent climatically extremely well-tuned ecosystems, climate changes have considerable influence on their distribution, the constituent species and the growth. Climatic conditions influence the forest directly through temperature, precipitation and wind, and indirectly through insect and disease infestation. Climate change further increases the pressure on forests.

Hydrosphäre

The hydrosphere is part of the geosphere and encompasses the above-ground and subterranean water reserves. Seen from space, the earth is a blue planet, illustrating the large area of water: three quarters of the earth’s surface is covered by water. The water cycle and the climate are closely correlated and mutually dependent. Scenarios of the future climate show numerous uncertainties, particularly with regard to variations in the water cycle. The global warming of almost 0.8°C over the last 100 years should result in an intensification of the water cycle. Almost all climate models show that an increase of temperature at the surface by 1°C causes an increase in precipitation by 2-3%, due to the increased evaporation, particularly over the oceans. Warming of 1°C will, theoretically, raise the water vapour capacity of the atmosphere by 7%. In general, it is assumed that the hydrological cycle will be intensified by global warming, due to greenhouse gases.

However, it is very difficult to confirm such a tendency. This has two principal reasons. Firstly, precipitation and evaporation strongly vary from place to place, and sufficiently precise time series of data only exist for a few regions. On the other hand, in recent decades, not only has the concentration of greenhouse gases in the atmosphere increased, but also that of aerosols, which lead to cooling and a fall in precipitation. According to recent model computations, the hydrological cycle can react stronger to changes in the aerosol concentration as to changes in the concentration of greenhouse gases. Since aerosols have a short lifetime in the atmosphere, their influence on the areas where they are created is limited. The global effect on the hydrological cycle is, therefore, relatively unclear. In the period 1951-2005, there is no clear global trend for precipitation over land. However, regional trends are often clearly visible.

Another climate change effect very probably coming our way is already making itself evident, i.e. the accelerated thawing of snow, ice and glaciers that act as natural water reservoirs, and the increase in rain and decrease in snowfall with increasing temperatures. This leads to increased run-off in winter and early spring and leaves less water available for summer and the dry season, when the need is the greatest. While it has to be assumed that precipitation will increase globally with higher temperatures, in drier regions it can get even drier, due to reduced precipitation, changes in the rainy seasons and higher evapotranspiration. The relationship between changes in precipitation into water availability is non-linear. Small falls in precipitation can be amplified in the hydrological system leading to considerably larger reductions in ground water production with possible consequences for the water quality, if the withdrawal of ground water increases simultaneously. Falling ground water levels in coastal regions often lead to saltwater intrusion that can render the water useless. The increase in sea levels as a consequence of global change further accelerates this process.

With the intensification of the water cycle due to warming, one has to expect more frequent and severe extreme events, even if observations don’t generally confirm this trend.

In order to understand the influence of the climate and man-made incursions on the water cycle better, it is necessary to have knowledge of the components of the hydrosphere, in order to determine the current status and the variability. Particularly important are the parameters, near-surface soil moisture and surface roughness, as well as the water levels in rivers and wetlands together with the river flow speeds.

Further applications of Tandem-L for the investigation of processes in the hydrosphere, particularly also in the area of oceanography, are:

For parts of the earth, only coarse, inconsistent or incomplete elevation models exist, which have been put together from numerous different data sources using different surveying methods. TanDEM-X will close these gaps and generate a global, homogeneous elevation model, as an indispensable basis for numerous scientific and commercial applications. The TanDEM-X DEM is specified to achieve a spatial resolution of 12 m x 12 m and a vertical accuracy of 2 m. For data acquisition, TanDEM-X employs an interferometric radar system in X-band, which only penetrates vegetation to a small amount, due to the short wavelength of 3.1 cm. Hence, TanDEM-X delivers a digital elevation model of the surface of the earth, i.e. the upper surface of strongly vegetated areas (forest) and of urban areas (roof surfaces). In contrast, a digital elevation model describes the topographic variation under the vegetation. With a wavelength of 23.6 cm, Tandem-L penetrates vegetation and, using polarimetric SAR interferometry, can not only measure the surface topography, but also the topography under the vegetation. Hence, Tandem-L will complement TanDEM-X to provide additional information on the earth’s topography.