Forschungsinteressen
Übersetzung ist in Arbeit
Since my dissertation, I have been fascinated by the interaction between land and atmosphere, ranging from turbulence processes in the near-surface layer of the atmosphere to global-scale interaction between land surface and the climate. When, for the first time, studying the dynamics of a coupled atmosphere – vegetation model, I surprisingly found that the atmosphere – vegetation system can reveal multiple equilibria: depending on how the model started, the Sahara appeared to be a rather green savanna, mainly in the western part, or the desert as it is today. Obviously, vegetation dynamics can lead to “tipping points” in the climate system.
A “green Sahara” existed several thousand years ago during the so-called Holocene climatic optimum. While the retreat and expansion of the desert was very likely triggered by changes in the Earth orbit around the sun, it turned out that the changes in the extent of the Sahara could not be explained when ignoring the strong feedback between atmosphere, vegetation and land surface in West Africa. Furthermore, my colleagues and I predicted that the highly non-linear biogeophysical interaction should have caused a, in comparison with orbital forcing, rather fast retreat and expansion of the Sahara. Some reconstructions support this prediction, others are at variance. Hence, the dynamics of the Sahara are still an interesting topic.
(e.g. 
Claussen et al. 1999; Claussen 2008, Brovkin and Claussen 2008)
And the story continues: it has become evident that vegetation change affects climate dynamics globally via changes in surface conditions (absorption of solar radiation, roughness, transpiration, …) and changes in the carbon cycle. Roughly speaking, tropical forests tend to cool, and boreal forests tend to warm the climate. Therefore one could assume that changes between glacials and interglacials should have been amplified by vegetation dynamics which were strongest in high northern latitudes. A number of modelling experiments seem to support this conjecture. Nonetheless, this question is far from being solved.
(e.g. Claussen 2009)
If historic and current climate change is analysed, then anthropogenic land use and land cover change have to be taken into account. Our modelling studies suggest that humans have significantly perturbed the natural carbon cycle by deforestation already in the 16th century. The impact on climate was, however, much smaller – actually not detectable against the background of natural climate variability - because of the competing effects of warming associated with CO2 emissions from deforestation and the cooling associated with changing the reflectivity, the roughness and the transpiration of land surfaces. Only in the last century the warming presumably became the dominant process. How strong these effects will be in the future, and whether anthropogenic land cover change could be used to purposely modify the climate, remains a challenge.
(e.g. Pongratz et al., 2009)