An issue of increasing importance in solar physics and climatology is solar variability (Sofia and Fox, 1994). It seems that most solar type stars show some magnetic variability at the surface (Baliunas and Jastrow, 1990). To understand the underlying processes and to model them is a big challengein solar physics.

When satellites began to observe the sun outside the disturbing atmosphere it soon became clear that the solar irradiance is also variable and seems to be related to the 11- year Schwabe cycle (Willson, et al., 1986), (Foukal, this volume). This result is of great importance because the sun is the engine which drives the climate system on earth. Although the variations are small (0.1-0.2%) there is clear evidence from observational data of solar type stars (Baliunas and Jastrow, 1990) and from theoretical considerations as well that much larger fluctuations over longer time scales potentially could occur.

This raises the question what role the sun plays in today’s climate change and possibly in the near future. There are basically two approaches to address this question. 1. The solar irradiance and the climate have to be monitored continuously with high precision in order to detect changes and to understand how they are caused. 2. Since there are reasons to believe that the sun also exhibits long-term changes which cannot be detected during short periods of direct observations, one has to investigate the past by looking for connections between climate and solar variability.

To reconstruct past climate changes is comparatively simple. There is a large amount of climatic observations available for the last 2-3 centuries. The climatic conditions of earlier times can be reconstructed quite reliably based on isotopic ratios, pollen assemblages, and many other parameters measured in natural archives.