Solar irradiance during the last 1200 years based on cosmogenic nuclides

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  (Maunder). Further back in time, a rather long period between 1450 and 1750  is characterized by low irradiance values. A shorter period is centered at about 1200 , 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°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 data are thus limited to the last 2 decades. This is A positive relationship is now well established rather unfortunate, as a direct solar influence has between the 11-year cycle of solar magnetic activbeen invoked to explain slow variations of several ity and the output of the sun (Willson and Hudson, climatic parameters such as sea level atmospheric 1988; Willson, 1997). Over the last two 11-year pressure (Kelly, 1977), sea surface temperatures cycles (#21 and #22), the amplitude of these flucta(Reid, 1987), equatorial wind patterns (Labitzke tions has remained of the order of 0.1% for the and Van Loon, 1988), land air temperatures (Friistotal radiative output. Willson (1997) even proChristensen and Lassen, 1991) and strength of posed that the irradiance increased by 0.04% cyclogenesis (Tinsley, 1994). Several authors even between the last 2 minima of solar activity, proposed that the solar variability is responsible although this fact has been questioned recently for a significant part of the high-frequency com(Fröhlich and Lean, 1998). ponent of the natural variability of climate (Eddy, Such small fluctuations can only be measured 1976; Lean and Rind, 1994; Lean et al., 1995; Crowley and Kim, 1996; Mann et al., 1998, 1999) and should thus be taken into account when * Corresponding author. e-mail: [email protected] extracting the anthropic influence over the last Tellus 52B (2000), 3