The role of the sun in climate forcing

The Sun is by far the most important driving force of the climate system. H owever, only little is known how variable this force is acting on different time scales ranging from minutes to millennia and how the climate system r eacts to changes in this forcing. Changes of the global insolation can be r elated to the nuclear fusion in the core of the Sun, the energy transport t hrough the radiative zone and the convection zone, the emission of radiatio n from the photosphere, and the distance between Sun and Earth. Satellite b ased measurements over two decades show a clear correlation between the sol ar irradiance and the Ii-year sunspot cycle. The irradiance amplitude is ab out 0.1%. This is too small to affect significantly the climate. However, t here are indications that, on longer time scales, solar variability coluld be much larger. The analysis of cosmogenic nuclides stored in natural archi ves provides a means to extend our knowledge of solar variability over much longer time periods. The response of the climate system to solar forcing depends not only on the amount of radiation, but also on its spectral composition (e.g. UV contrib ution), seasonal distribution over the globe, and feedback mechanisms conne cted with clouds, water vapour, ice cover, atmospheric and oceanic transpor t and other terrestrial processes. It is therefore difficult to establish a quantitative relationship between observed climate changes in the past and reconstructed solar variability. However, there is growing evidence that p eriods of low solar activity (so called minima) coincide with advances of g laciers, changes in lake levels, and sudden changes of climatic conditions. These findings point to an active role of the Sun in past climate changes beside other geophysical factors, internal variability of the climate syste m, and greenhouse gases. In fact a non-linear regression model to separate natural and anthropogenic forcing since 1850 is consistent with a solar con tribution of about 40% to the global warming during the last 140 years. (C) 1999 Elsevier Science Ltd. All rights reserved.