Reference
Usoskin, I.G., Schussler, M., Solanki, S.K. and Mursula, K. 2005. Solar activity, cosmic rays, and Earth's temperature: A millennium-scale comparison. Journal of Geophysical Research 110: 10.1029/2004JA010946.
What was done
The authors compared various sets of terrestrial temperature data with two parameters derived from 10Be and 14C data: (1) the sunspot number, which is related to total solar irradiance and UV flux, and (2) the flux of galactic cosmic rays impinging on the Earth's atmosphere.
What was learned
Usoskin et al. report that "sunspot numbers and cosmic ray fluxes reconstructed from records of the cosmogenic isotopes 10Be and 14C, respectively, show correlations and anticorrelations with a number of reconstructions of the terrestrial Northern Hemisphere temperature, which cover a time span of up to 1800 years." Of these two parameters, they say the cosmic ray flux seems to correlate better with the terrestrial temperature than the sunspot numbers. In addition, they note that "the positive correlation between the geomagnetic dipole moment and the temperature reconstructions provides further evidence favoring the cosmic ray influence on the terrestrial climate," because "the cosmic ray flux entering Earth's atmosphere is due to a combination of solar modulation and geomagnetic shielding, the latter adding a long-term trend to the varying solar signal."
What it means
In the words of the four solar scientists, their findings indicate that "periods of higher solar activity and lower cosmic ray flux tend to be associated with warmer climate, and vice versa," and that "the major part of this correlation is due to similar long-term trends in the data sets." In fact, both the sunspot number and cosmic ray parameters exhibit low-frequency variations that closely match the declining temperatures of the Roman Warm Period-to-Dark Ages Cold Period transition that was in progress at the start of the 1800-year records, the subsequent warming that produced the Medieval Warm Period, the following cooling that led to the Little Ice Age, and the most recent warming that produced the Current Warm Period. Hence, it would appear that the millennial-scale oscillation of Earth's climate that has given us these distinctive multi-century warm and cold periods was likely driven by a similar-scale oscillation of the solar-modulated cosmic ray flux, as is described in more detail in several of the Journal Reviews we have archived in the Extraterrestrial Climatic Effects section of our Subject Index.