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Abstract

Climate, the statistical description of weather including the probabilities for anomalies, changes continuously, since both the interaction among the climate system components (atmosphere, ocean, biosphere, cryosphere, and lithosphere) and the irradiance of the sun vary on all timescales up to billions of years. On timescales which are of most interest for mankind, i.e. up to centuries, major factors causing climate variability are: changing composition of the atmosphere; spectral solar irradiance variations; systematic changes of ocean-atmosphere-land interaction, if resulting in major changes of ocean circulation; volcanic eruptions injecting mass into the low latitude stratosphere (>20 km height); and land cover change modifying the surface energy budget. Two of these influencing factors are now largely anthropogenic, namely changing atmospheric conditions and land cover change. Thus, the climate discussion has become to a large extent one about anthropogenic influence. At different periods, different influencing factors have been more important or dominant. Since the complex interactions are only partly understood, it is difficult to attribute observed climate parameter changes or trends to particular causes. In this brief status report on global climate change, present knowledge is reviewed, mainly based on the second full assessment report of the Intergovernmental Panel on Climate Change. Born near Berchtesgaden in Bavaria, Hartmut Grassl took his Diploma in physics (1966) followed by a PhD in meteorology (1970) at the University of Munich. He then moved on to the University of Mainz, winning the Young Scientist Award of the German Meteorological Society in 1971. He received his habilitation in meteorology in 1978 from the University of Hamburg. He is currently Director of the Joint Planning Staff for the World Climate Research Programme in Geneva. His research interests include observation of the earth from space, aerosols and climate, and lidar and radar remote sensing of the lower atmosphere. From 1988 to 1994 he was a member of the Intergovernmental Panel on Climate Change; he has also contributed to the German Parliament's Commission of Enquiry on 'Protection of the earth's atmosphere' (1989-94) and from 1992-94 chaired the Global Change Advisory Board to the German Government. He is a scientific member of the Max Planck Society and a member of the Academia Europaea, and in 1995 was appointed to the German Order of Merit (First Class).