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Figure 7. Comparison of the reconstructed solar modulation function $ of Figure 6 with ice rafted debris (IRD) retrieved from a sediment core in the North Atlantic (Bond etal, 2001). In this figure, the $ record has been low-pass filtered with a cut-off frequency of 1/(900 years) and is plotted inversely compared to Figure 6. The ice rafted debris consists of glass particles originating from volcanic eruptions in Iceland. Incorporated into icebergs, they drift much further south during periods of low solar activity.

Karlen, 1973) and the water level of lakes in the French Jura (Holzhauser, 1997). These are just a few examples of a rapidly growing number of publications in this field.

It is well known that the Sun plays the fundamental role as our energy source. However, it is still an open question what role the Sun plays in climate change. Direct measurements of the total (TSI) and the spectral (SSI) irradiance using satellite born radiometers show relatively small changes in accordance with the solar magnetic activity. To answer this question, we have (1) to understand the physical mechanisms responsible for the irradiance changes, (2) to reconstruct the solar variability over the past millennia, (3) to derive from this variability record, reliable estimates of the TSI and SSI changes, and (4) to determine the response of the climate system using appropriate models. As far as the second topic is concerned we can rely on a variety of solar activity proxies based on direct observations (sunspots, aa-index, aurorae). However, all of them are limited to periods ranging from a few decades to a few centuries. To date, the only proxy providing information about the solar variability on millennial time scales are cosmogenic radionuclides stored in natural archives such as ice cores. They clearly reveal that the Sun varies significantly on millennial time scales and most likely plays an important role in climate change.

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