Did the Sun hit record highs over the last few decades?

Guest commentary by Raimund Muscheler

[note: this is a restore (8/8/05) of an article from August 3, 2005 that was accidentally deleted due to a technical glitch. Unfortunately, most of the comments could not be retrieved. We sincerely apologize to our readers!]

The solar influence on climate is a controversial topic in climate research (see previous posts here and here). The irradiance changes are assumed to be relatively small and the importance of potential amplifying mechanisms is still a matter of current debate. One reason for these uncertainties is that there are only approximately 25 years of satellite-based observations of the solar irradiance. Sunspot observations for the last 400 years clearly indicate that current levels of solar activity are very different from the state of the sun during the Maunder minimum (from approx. 1645 to 1715 AD) where almost no sunspots could be observed.

Cosmogenic radionuclides (such as 10Be and 14C) records are the most reliable proxies to extent solar activity reconstructions beyond the period of direct observations of the sun. They are produced in the atmosphere by the interaction of galactic cosmic rays with the atoms of the atmosphere. High solar shielding of the galactic cosmic rays during periods of high solar activity decreases the radionuclide production rates and vice versa for low solar activities. Cosmogenic radionuclide production rates are also influence by the geomagnetic field. Similarly to the solar magnetic modulation, high geomagnetic field intensity decreases the flux of galactic cosmic rays and radionuclide production rates and the opposite for low geomagnetic field intensity.

The processes responsible for the radionuclide production are well known and can be modeled quantitatively. The biggest uncertainties lies in the interpretation of the radionuclide records that can be measured in natural archives such as ice cores in the case of 10Be or tree rings in the case of 14C. This is due to the fact that changes in atmospheric transport and deposition in the case of 10Be or changes in the carbon cycle in the case of 14C can influence the measured concentrations. Unidentified climatic influences lead to errors in the reconstruction of solar activity changes based on these records. This problem is illustrated by two alternative reconstructions of past changes in solar activity based on ice core 10Be records. Based on a 10Be record from Antarctica Bard et al. (2000) conclude that currents levels of solar activity were also reached or exceeded around 1200 AD. By contrast, Solanki et al. (2004) conclude that solar activity during recent decades is exceptionally high compared to the past 8000 years. Their method seems to be confirmed by a 10Be record from Southern Greenland (Dye 3, Beer et al, 1990). However, the two 10Be records from Antarctica and Greenland exhibit big disagreements for the last 55 years which is the main reason for these very different conclusions (Raisbeck and Yiou, 2004).

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