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).
Obviously, at least one of these records must also be influenced by changes in climate. Since 14C is influenced by completely different geochemical behavior than 10Be, the investigation of 14C records can help to solve the contradictions. Since carbon cycle models allow us to understand past changes in atmospheric CO2 and 13C concentrations it is also possible to use these models to infer the 14C production rate based on measured 14C concentrations in tree rings. Until 1950 AD, when significant amounts of anthropogenic 14C were released into the atmosphere by the nuclear weapons tests, we can calculate the variations in the 14C production rates and infer the solar magnetic modulation from these records. There are uncertainties in connecting the 14C production rate to recent instrumental measurements of solar magnetic modulation (see figure). However, regardless of these uncertainties the conclusions by Usoskin et al. (2003) and Solanki et al. (2004) cannot be confirmed by the analysis of the 14C records (Muscheler et al., 2005). The 14C tree ring records indicate that today’s solar activity is high but not exceptional during the last 1000 years.
Figure 1: Reconstructions of the (11-year smoothed) solar magnetic modulation based on 14C data from tree rings combined with instrumental measurements (Muscheler et al.,2005). The purple curve shows the results according to a normalization using balloon-borne measurements. The black and green curves show results using an alternative normalization and the northern hemisphere (black) and globally averaged (green) 14C data. The red curve is the group sunspot number.
Are there additional proxies that could shed light on the solar history? First of all the sunspot record represents a good proxy for solar variations during the last 400 years. While the group sunspot record indicates high solar activity during the last 60 years compared to the preceding 350 years, the Wolf sunspot record reached today’s values for a short period around 1780 AD (with the exception of the very strong sunspot maximum around 1957 AD). Aurora observations also provide information about the history of the sun. However, there are no systematic observations and therefore the conclusions are uncertain. 44Ti measurements in meteorites have the potential to provide high quality estimates of past changes in solar magnetic modulation but, until now, the data are sparse and exhibit large errors (Castagnoli et al., 2003). 36Cl is another useful cosmogenic isotope, but that too has data quality and bomb contamination problems in the modern period. Therefore, it is not yet possible to draw firm conclusions about solar activity changes during the last 400 years based on these data. To conclude, depending on the applied 10Be record different histories of the sun have been inferred. The 14C tree ring record does not confirm the 10Be record from Southern Greenland and the assertion that recent solar activity levels are exceptional. Alternative estimates of the history of the Sun are not yet good enough to be used as independent checks.
Therefore, in the view of the uncertainties and the conflicting data it doesn’t seem to be appropriate to make uncritical and sensational claims about the history of the sun. As long as the differences between the 10Be records are not understood, conclusions based on only one of these records should be treated with caution. Atmospheric 14C concentrations, on the other hand, are much less sensitive to a climate influence during the last 1000 years and, therefore, can provide good estimates of the history of the sun. However, the disagreement between 14C-based solar activity and group sunspot number (Muscheler et al., 2005) should remind us that the variations of the solar activity are not yet completely understood.
Regardless of any discussion about solar irradiance in past centuries, the sunspot record and neutron monitor data (which can be compared with radionuclide records) show that solar activity has not increased since the 1950s and is therefore unlikely to be able to explain the recent warming.
Bard, E., Raisbeck, G.M., Yiou, F., Jouzel, J., 2000. Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus 52B, 985-992.
Beer, J., Blinov, A., Bonani, G., Finkel, R.C., Hofmann, H.J., Lehmann, B., Oeschger, H., Sigg, A., Schwander, J., Staffelbach, T., Stauffer, B., Suter, M., Wölfli, W., 1990. Use of 10Be in polar ice to trace the 11-year cycle of solar activity. Nature 347, 164-166.
Cini Castagnoli, G.C., Cane, D., Taricco, C., Bhandari, N., 2003. GCR Flux Decline during the last Three Centuries: Extraterrestrial and Terrestrial Evidences. In: T. Kajita, Y. Asaoka, A. Kawachi, Y. Matsubara, M. Sasaki. (Eds.), GCR Flux Decline during the last Three Centuries: Extraterrestrial and Terrestrial Evidences, Universal Academy Press, Inc., pp. 4045-4048.
Raisbeck, G.M., Yiou, F., 2004. Comment on ”Millennium Scale Sunspot Number Reconstruction: Evidence for an Unusually Active Sun Since the 1940s”. Physical Review Letters 92, DOI: 10.1103/PhysRevLett.92.199001.
Usoskin IG, Solanki SK, Schussler M, Mursula K, and Alanko K., Millennium-scale sunspot number reconstruction: evidence for an unusually active sun since the 1940s. Phys Rev Lett. 2003, 91(21):211101.