Closing remarks: astronomical

Over the past two days, we have covered many facets of the basic interactions between the solar activity and the Earth’s climate. As an astronomer, I should perhaps first comment on the fact that solar activity is not the only astronomical or astrophysical phenomenon to influence physical conditions in the biosphere. Over a very long timescale of thousands of millions of years the evolution of the Sun from a pre-main-sequence star to a star of G type has not only fundamentally controlled the physical and chemical processes in the formation of the planets but has controlled their surface physical characteristics. Over timescales an order of magnitude less, the location of the Solar System in the Galaxy may have influenced life on Earth. For example it has been noted that when the Sun crossed the spiral arms of the Galaxy and their dense dust clouds, some catastrophies might have resulted; the disappearance of the dinosaurs could be accounted for by such phenomena, as was once suggested by Sir William McCrea, F.R.S.; but nearby supernovae, grazing comets, and on large meteorites might very well have played a decisive role in the evolution of species and of our Earth. On a smaller timescale, a million years, the variation in solar energy falling on the Earth, due to secular changes in the terrestrial orbit parameters (Milankovitch-Berger theories), would have caused climatic changes and have been shown to account for the successive ice ages of the Quaternary. While bearing this in mind the role of solar activity on the timescale of recent millennia, but also on shorter timescales, is of obvious importance to society and, as we have seen in this meeting, is only now being properly investigated.