The variability of the Sun is well established, as well as that of the Earth's climate. To what extent the two are connected, in the sense that solar variability drives climate, is the subject of considerable research and, in some cases, controversy. After an earlier workshop at the International Space Science Institute (ISS!) on Solar Composition and its Evolution, two ofthe participants came up with the idea to initiate a similar project on the topic of Solar Variability and Climate, a work shop aimed at obtaining an overview of the current knowledge of the variability of the Sun and of the Earth's Climate, and of their possible connections. A further, equally important objective was the strengthening of the interaction between the two, often diverse communities of solar physicists and climatologists. ISSI took up this idea and invited six convenors, E. Friis-Christensen, C. Froh lich, J. Haigh, J. Hansen, M. Schussler, and S. Solanki, who subsequently formu lated the aims and goals of the workshop, nominated a list of invitees, drafted a programme of introductory talks, and structured the workshop into three sections. For each section there was a concluding discussion session moderated by two co chairs. Moreover, there was a number of contributed poster papers for which there were two viewing sessions. The main intent of this format was to leave ample time for open, informal discussions, which is one of the principal aims of ISSI.

The discovery of chemical elements in celestial bodies and the first estimates of the chemical composition of the solar atmosphere were early results of Astrophysics - the subdiscipline of Astronomy that was originally concerned with the general laws of radiation and with spectroscopy. Following the initial quantitative abundance studies by Henry Norris Russell and by Cecilia Payne-Gaposchkin, a tremendous amount of theoretical, observa tional, laboratory and computational work led to a steadily improving body of knowledge of photospheric abundances - a body of knowledge that served to guide the theory of stellar evolution. Solar abundances determined from photospheric spectra, together with the very similar abundances determined from carbonaceous chondrites (where extensive information on isotopic composition is available as well), are nowadays the reference for all cosmic composition measures. Early astrophysical studies of the solar photospheric composition made use of atmosphere models and atomic data. Consistent abundances derived from different atmospheric layers and from lines of different strength helped to confirm and estab lish both models and atomic data, and eventually led to the now accepted, so-called "absolute" abundance values - which, for practical reasons, however, are usually given relative to the number of hydrogen nuclei.

The IAU Colloquium No. 143 "The Sun as a Variable Star: Solar and Stellar Irradiance Variations" was held on June 20 - 25, 1993 at the Clarion Harvest House, Boulder, Colorado, USA. The main objective of this Colloquium was to review the most recent results on the observations, theoretical interpreta tions, and empirical and physical models of the variations observed in solar and stellar irradiances. A special emphasis of the Colloquium was to discuss the results gained on the climatic impact of solar irradiance variability. The study of changes in solar and stellar irradiances has been of high interest for a long time. Determining the absolute value of the luminosity of stars with different ages is a crucial question for the theory of stellar evolu tion and energy production of stellar interiors. Observations of the temporal changes of solar and stellar irradiances - in the entire spectral band and at different wavelengths - provide an additional tool for studying the physical processes below the photosphere and in the solar- stellar atmospheres. Since the Sun's radiative output is the main driver of the physical processes with in the Earth's atmosphere, the study of irradiance changes is an extremely important issue for climatic studies as well. Climatic models show that small, but persistent changes in solar irradiance may influence the Earth's climate.

Measurements of solar irradiance, both bolometric and at various wavelengths, over the last two decades have established conclusively that the solar energy flux varies on a wide range of time scales, from minutes to the 11-year solar cycle. The major question is how the solar variability influences the terrestrial climate. The Solar Electromagnetic Radiation Study for Solar Cycle 22 (SOLERS22) is an international research program operating under the auspices of the Solar-Terrestrial Energy Program (STEP) Working Group 1: The Sun as a Source of Energy and Disturbances'. STEP is sponsored by the Scientific Committee of Solar-Terrestrial Physics (SCOSTEP) of the International Council of Scientific Unions (ICSU). The main goal of the SOLERS22 1996 Workshop was to bring the international research community together to review the most recent results obtained from observations, theoretical interpretation, empirical and physical models of the variations in the solar energy flux and their possible impact on climate studies. These questions are essential for researchers and graduate students in solar-terrestrial physics.

This book provides a comprehensive presentation of Earth s energy flows and their consequences for the climate. The Earth s climate as well as planetary climates in general, are broadly controlled by three fundamental parameters: the solar irradiance, the planetary albedo and the planetary emissivity. Space measurements indicate that these three quantities are remarkably stable. A minor decrease inplanetary emissivity is consistent with theoretical calculations. This is due to the ongoing increase of atmospheric greenhouse gases making the atmosphere more opaque to long wave terrestrial radiation. As a consequence radiation processes are slightly out of balance as less heat is leaving the Earth in the form of thermal radiation than the incoming amount of heat from the sun. Present space-based systems cannot yet satisfactorily measure this imbalance, but the effect can be inferred from the measurements of the increase of heat in the oceans. Minor amounts of heat are also used to melt ice and to warm the atmosphere and the surface of the Earth.
The book brings to fore the complexity of feedback processes of the Earth s climate system and in particular the way clouds and aerosols affect the energy balance both directly and indirectly through feed-back loops driven by the dynamics of atmospheric, ocean and land surface processes. The book highlights recent scientific progress as well as remaining challenges.
Previously published in Surveys in Geophysics, Volume 33, Nos. 3-4, 2012"

The variability of the Sun is well established, as well as that of the Earth's climate. To what extent the two are connected, in the sense that solar variability drives climate, is the subject of considerable research and, in some cases, controversy. After an earlier workshop at the International Space Science Institute (ISS!) on Solar Composition and its Evolution, two ofthe participants came up with the idea to initiate a similar project on the topic of Solar Variability and Climate, a work shop aimed at obtaining an overview of the current knowledge of the variability of the Sun and of the Earth's Climate, and of their possible connections. A further, equally important objective was the strengthening of the interaction between the two, often diverse communities of solar physicists and climatologists. ISSI took up this idea and invited six convenors, E. Friis-Christensen, C. Froh lich, J. Haigh, J. Hansen, M. Schussler, and S. Solanki, who subsequently formu lated the aims and goals of the workshop, nominated a list of invitees, drafted a programme of introductory talks, and structured the workshop into three sections. For each section there was a concluding discussion session moderated by two co chairs. Moreover, there was a number of contributed poster papers for which there were two viewing sessions. The main intent of this format was to leave ample time for open, informal discussions, which is one of the principal aims of ISSI.

The discovery of chemical elements in celestial bodies and the first estimates of the chemical composition of the solar atmosphere were early results of Astrophysics - the subdiscipline of Astronomy that was originally concerned with the general laws of radiation and with spectroscopy. Following the initial quantitative abundance studies by Henry Norris Russell and by Cecilia Payne-Gaposchkin, a tremendous amount of theoretical, observa tional, laboratory and computational work led to a steadily improving body of knowledge of photospheric abundances - a body of knowledge that served to guide the theory of stellar evolution. Solar abundances determined from photospheric spectra, together with the very similar abundances determined from carbonaceous chondrites (where extensive information on isotopic composition is available as well), are nowadays the reference for all cosmic composition measures. Early astrophysical studies of the solar photospheric composition made use of atmosphere models and atomic data. Consistent abundances derived from different atmospheric layers and from lines of different strength helped to confirm and estab lish both models and atomic data, and eventually led to the now accepted, so-called "absolute" abundance values - which, for practical reasons, however, are usually given relative to the number of hydrogen nuclei.

Measurements of solar irradiance, both bolometric and at various wavelengths, over the last two decades have established conclusively that the solar energy flux varies on a wide range of time scales, from minutes to the 11-year solar cycle. The major question is how the solar variability influences the terrestrial climate. The Solar Electromagnetic Radiation Study for Solar Cycle 22 (SOLERS22) is an international research program operating under the auspices of the Solar-Terrestrial Energy Program (STEP) Working Group 1: The Sun as a Source of Energy and Disturbances'. STEP is sponsored by the Scientific Committee of Solar-Terrestrial Physics (SCOSTEP) of the International Council of Scientific Unions (ICSU). The main goal of the SOLERS22 1996 Workshop was to bring the international research community together to review the most recent results obtained from observations, theoretical interpretation, empirical and physical models of the variations in the solar energy flux and their possible impact on climate studies. These questions are essential for researchers and graduate students in solar-terrestrial physics.

This book provides a comprehensive presentation of Earth's energy flows and their consequences for the climate. The Earth's climate as well as planetary climates in general, are broadly controlled by three fundamental parameters: the solar irradiance, the planetary albedo and the planetary emissivity. Space measurements indicate that these three quantities are remarkably stable. A minor decrease in planetary emissivity is consistent with theoretical calculations. This is due to the ongoing increase of atmospheric greenhouse gases making the atmosphere more opaque to long wave terrestrial radiation. As a consequence radiation processes are slightly out of balance as less heat is leaving the Earth in the form of thermal radiation than the incoming amount of heat from the sun. Present space-based systems cannot yet satisfactorily measure this imbalance, but the effect can be inferred from the measurements of the increase of heat in the oceans. Minor amounts of heat are also used to melt ice and to warm the atmosphere and the surface of the Earth. The book brings to fore the complexity of feedback processes of the Earth's climate system and in particular the way clouds and aerosols affect the energy balance both directly and indirectly through feed-back loops driven by the dynamics of atmospheric, ocean and land surface processes. The book highlights recent scientific progress as well as remaining challenges. Previously published in Surveys in Geophysics, Volume 33, Nos. 3-4, 2012