The Sun-Climate Connection Over the Last Millennium: Facts and Questions

Some general qualities of the Sun are described. Definitions of solar and geomagnetic activity, space weather as well as climate are introduced. Highlights of the history of solar-climate research since the beginning of the 17th century are shortly discussed. In addition, the main problems and shortcomings of modern helioclimatology are presented.

The main manifestations of solar activity, such as sunspots, floccules and faculae are described. A short history of research on the Sun’s activity from Galileo and Scheiner to modern times is presented. The main instrumental solar indexes - Wolf number, group sunspot number, sunspot area - and their general statistical laws (Gnevyshev-Ohl rule, Waldmeier rule etc.) are described. Up-to-date modern knowledge of major solar periodicities (cycles of Schwabe, Hale, Gleissberg etc.) and global extremes of solar activity (Maunder and Spörer minima, Medieval maximum etc.) are discussed. Moreover, some major phenomena related to space weather (solar wind, aurora borealis etc.) and other terrestrial manifestations of solar activity are considered. Modern methods of statistical analysis of non-stationary time series, including Fourier, wavelet, and singular spectrum analyses, long-range correlation analysis and multi-fractal approach are introduced. A method used for the evaluation of significance in details of wavelet spectra is presented. Furthermore, some enduring problems are shortly described.

In addition to a historical overview and general information, major advances and problems of solar paleoastrophysics are described, including the data obtained by means of cosmogenic isotopes (¹⁴C, ¹⁰Be), nitrates (NO₃^- ion), and historical observations. The most recent proxy-based paleoreconstructions of sunspot numbers and sunspot area, covering time intervals from 150 to 10 000 years, are presented. Possible sources of time variations of the concentration of ¹⁴C, ¹⁰Be and NO₃^- ion in terrestrial archives are considered.

A general introduction, history as well as major recent advances and problems of paleoclimatology are presented, including paleoclimatology of stable isotopes (¹³C, ¹⁸O), dendroclimatology, and the use of other climate proxies. An estimation of advantages and shortcomings of various sources of the climatic information is made. Moreover, the divergence issue between different proxies is discussed. Long-term climatic oscillations, established by means of paleoclimatology are also described.

Modern concepts of the physical mechanisms providing a potential link between solar as well as cosmic factors and climate are described. Possible climatic effects of solar luminosity, fluxes of galactic and solar cosmic rays, UV and microwave radiation are considered. A survey of the most important experimental evidence of the reality of solar-cosmic influence on climate is presented.

Possible factors contributing to global warming during the last 100 years - the greenhouse effect, natural climatic variability, variations of solar activity, and cosmic ray fluxes as well as anthropogenic changes in land-cover are described. It is shown that currently it is very difficult to estimate even the relative role of each of these factors in global warming due to our still inadequate knowledge of them as well as discrepancies between different key data sets. It is shown that despite some successes and tremendous efforts in the fields of both climatology and paleoclimatology our knowledge of the Earth’s past climate is still lacking.

A paleoastrophysical forecast of solar activity throughout the 21^st century is presented. It is shown that due to our insufficient knowledge of the origin and character of global warming it is hard to predict accurately the next changes in the global climate for this century. Possible scenarios of the future of climate evolution are considered.