Solar irradiance during the last 1200 years based on

Based on a quantitative study of the common fluctuations of 14C and 10Be 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 circula-tion models (GCMs). Nevertheless, our record is already compatible with a global cooling of about 0.5–1°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). 1. Solar irradiance variations above the atmosphere with spacecraft and dat