Solar irradiance during the last 1200 years based on cosmogenic nuclides

Based on a quantitative study of the common fluctuations of C-14 and Be-10 production rates, we have derived a time series of the solar magnetic variability over the last 1200 years. This record is converted into irradiance variations by linear scaling based on previous studies of sun-like stars and of the sun's behavior over the last few centuries. The new solar irradiance record exhibits low values during the well-known solar minima centered at about 1900, 1810 (Dalton) and 1690 AD (Maunder). Further back in time, a rather long period between 1450 and 1750 AD is characterized by low irradiance values. A shorter period is centered at about 1200 AD, with irradiance slightly higher or similar to present day values. It is tempting to correlate these periods with the so-called little ice age and medieval warm period, respectively. An accurate quantification of the climatic impact of this new irradiance record requires the use of coupled atmosphere-ocean general circulation models (GCMs). Nevertheless, our record is already compatible with a global cooling of about 0.5 1 degrees C during the little ice age, and with a general cooling trend during the past millenium followed by global warming during the 20th century (Mann et al., 1999).