Sensitivity of Climate to Long-Term Variations of the Solar Output

Abstract

Fluctuations of solar irradiance can have a significant impact on terrestrial climate only if they are strong enough and last long enough to induce significant warming of the upper ocean. This is hardly true for the eleven year cycle, whose amplitude in terms of mean absorbed solar flux is less than 0.1 Wm ⁻². The Maunder Minimum case is somewhat different, as the solar luminosity in the second half of the XVIIth Century may have been less than its present value by as much as 0.4%, i.e. about 1 Wm ⁻². The Maunder Minimum is therefore a possible candidate for explaining the socalled Little Ice Age episode. It is, however, very difficult to really assess the relevance of such an hypothesis, as the chronology of the Little Ice Age itself is somewhat blurred, and we lack quantitative data on various possible other perturbation mechanisms that could have induced cooling, such as volcanic aerosols or a significant change in the deep ocean circulation. Concerning the Little Ice Age itself, the present data situation is not so good : XVIIth Century paleotemperature data are scarce, and the most abundant data concern glacier extensions, which cannot be directly used for climate model validation ; and we cannot expect fast improvement of this situation.