Climate change and solar variability: What's new under the sun?

The Sun has an obvious effect on climate since its radiation is the main energy source for the outer envelopes of our planet. Nevertheless, there is a long-standing controversy on whether solar variability can significantly generate climate change, and how this might occur. This is a crucial issue not only in the field of paleoclimatology, but also for predicting the future of the Earth's climate, which will be subject to perturbations by anthropogenic greenhouse gases. Indeed, if climate changes due to the Sun were large and rapid, this would make it more difficult to extract the anthropogenic effects from precise records of instrumental data over the past century. Hence, Sun-climate relationships have never been so controversial as today, forming a debate that often escapes the scientific arena. Here, we provide a review of this problem by considering changes on different time scales, from the last million years up to recent decades. In doing so, we also critically assess recent claims that the variability of the Sun has had a significant impact on global climate. The different studied records also illustrate the mulfi-disciplinary nature of this difficult problem, requiring knowledge in several fields such as astronomy and astrophysics, atmospheric dynamics and microphysics, isotope geochemistry and geochronology, as well as geophysics, paleoceanography and glaciology. Overall, the role of solar activity in climate changes - such as the Quaternary glaciations or the present global warming - remains unproven and most probably represents a second-order effect. Although we still require even more and better data, the weight of evidence suggests that solar changes have contributed to small climate oscillations occurring on time scales of a few centuries, similar in type to the fluctuations classically described for the last millennium: The so-called Medieval Warm Period (900-1400 A.D.) followed on by the Little Ice Age (1500-1800 A.D.). (c) 2006 Elsevier B.V. All rights reserved.