The dramatic increase of the Earth’s average surface temperature during the last 50 years has indicated that a human induced contribution to climate variations may now be significant. For this reason, climate change is no longer regarded as just a scientific issue.  The public discussion on climate change is now so much focused on the effect of the increasing atmospheric content of CO2 that the existence of natural climate variations seems to be neglected.

But the Earth’s climate has always been changing. This is fully documented in both the geological and in the historical records. During the last 150 years a sufficient number of temperature measurements have been available to estimate climate change on a global or at least hemispheric scale. Also on this time scale clear changes in our climate have taken place.

Scientific studies have indicated that the varying activity of the Sun is the largest and most systematic contributor to natural climate variations. However, it has been difficult to identify the responsible physical mechanism because the most obvious mechanism, a change in total solar irradiation, does not seem to be sufficiently effective to account for the observed variations in temperature. Therefore, among other mechanisms, it has been hypothesised that the cosmic ray flux into the atmosphere could affect the formation of aerosols and thereby the formation of clouds. Recently such a mechanism connecting atmospheric ionisation with aerosol formation has indeed been found experimentally in the laboratory. Since the cosmic ray flux is modulated by solar variability, such a physical mechanism would imply a significant effect on the cloud cover and hence on climate.

Such indirect effects of solar variability are not at all included in today’s climate models, but a realistic prediction of future climatic must be based on a thorough assessment of the effect of solar variations on climate change.