The global climate of the Earth has significantly varied over the last Millennia. About 20000 years ago, a glacial age has left its way to a warmer climate, inducing dramatic changes in the ecological populations and presumably influencing the same evolution of human civilization, boosting the development of agriculture and favouring the creation of stable settlements.
On a regional scale, the climate has varied and does presently vary on many different time scales, leading to a continuously changing pattern of temperatures, humidity, precipitation, with important effects on the whole terrestrial biosphere.
As physicists, we are interested in understanding the mechanisms at work, by gathering data and properly analysing them, by building theoretical models and, if possible, making predictions on the future evolution of the system.
Along these lines, an important question is to understand the role of the solar forcing, in order to unravel the internal mechanisms of variability of the Earth's climate from the variable forcing of the Sun. On the other hand, we can learn about the past solar variability by reading into the terrestrial archives that provide us with proxy data on the history of both the Sun and the climate. Thus, realizing that the Sun and the Earth form a closely coupled system, where the variable properties of the former may affect in many subtle ways the behavior of the latter, is an important step toward the understanding of both.
The Course Past and Present Variability of the Solar-Terrestrial System: Measurement, Data Analysis and Theoretical Models is explicitly devoted to these issues. A solar cycle ago, in summer 1985, one of us (G. Cini) organized a similar school, in a time when this field was in a very early stage of development and definitely fewer high-quality measurements were available. After eleven years, the field has grown toward becoming a robust scientific discipline, new data have been obtained, and new ideas have been proposed by both solar physicists and climate dynamicists. For this reason, we felt that it was the right time to organize a new summer school, with the aim of formalizing the developments that have taken place during these years, and also for speculating and maybe dreaming of the new results that will be achieved in the upcoming years.
The papers of the lectures have now been collected in this volume.
First, in order to know what we are talking about, we need to obtain reliable data from terrestrial archives, and to properly date the records that have been measured. To these crucial aspects is devoted the first part of the book, dealing with various types of proxy data and with the difficult issue of the dating of the records.
Once obtained the data, we have to interpret them. This goal is nowadays relying upon a plethora of data analysis methods, that explicitly take into account the nonlinear nature of the system and try to elucidate the dynamics and the main processes active in the measured system. To the issue of data analysis and prediction is devoted the second part of the book.
Finally, once interpreted and analyzed the data, we have to build theoretical models describing the dynamics of the system considered. Due to the extreme complexity of the Sun/Earth system (as well as of its components, the Sun itself and the Earth's climate), we have to accept drastic simplifications in our modelling efforts, and we have to bear in mind that our models probably are nothing but a pale image of the real dynamics. To the theoretical and numerical modelling of the solar and climatic variability, and of their complex interactions, is devoted the third part of the book.
With this volume, we give an up-to-date view of the present state of this field. We hope that one solar cycle from now there will be someone willing to make a new progress report and continue the efforts we have made with this Course.
G. Cini Castagnoli and A. Provenzale