Background
The authors write that the galactic cosmic ray (GCR)-climate theory involves a solar forcing of the climate that significantly amplifies the forcing owing to solar irradiance." Noting that "the GCR flux is also modulated by earth's magnetic field," they thus state that "if the GCR-climate theory is correct, one would expect not only a relatively strong solar-climate link, but also a connection between earth's magnetic field and climate."

What was done
In a test of this supposition, Knudsen and Riisager "compare a new global reconstruction of the Holocene geomagnetic dipole moment (Knudsen et al., 2008) with proxy records for past low-latitude precipitation (Fleitman et al., 2003; Wang et al., 2005)," the first of which proxy records is derived from a speleothem δ¹⁸O record obtained from stalagmite Q5 from Qunf cave in southern Oman, and the second of which is derived from a similar record obtained from stalagmite DA from Dongge cave in southern China.

What was learned
The two researchers report that the various correlations they observed over the course of the Holocene "suggest that the Holocene low-latitude precipitation variability to some degree was influenced by changes in the geomagnetic dipole moment." In particular, they say that the general increase in precipitation observed over the past 1500 years in both speleothem records "cannot be readily explained by changes in summer insolation or solar activity," but that it "correlates very well with the rapid decrease in dipole moment observed during this period," which relationship is explained by the fact that "a higher dipole moment leads to a lower cosmic ray flux, resulting in reduced cloud coverage and, ultimately, lower precipitation."

What it means
Knudsen and Riisager say that "in addition to supporting the notion that variations in the geomagnetic field may have influenced earth's climate in the past," their study also provides support for a link "between cosmic ray particles, cloud formation, and climate, which is crucial to better understand how changes in solar activity impact the climate system."

References
Fleitmann, D., Burns, S., Mudelsee, M., Neff, U., Kramers, U., Mangini, A. and Matter, A. 2003. Holocene forcing of the Indian monsoon recorded in a stalagmite from southern Oman. Science 300: 1737-1739.

Knudsen, M.F., Riisager, P., Donadini, F., Snowball, I., Muscheler, R., Korhonen, K. and Pesonen, L.J. 2008. Variations in the geomagnetic dipole moment during the Holocene and the past 50 kyr. Earth and Planetary Science Letters 272: 319-329.

Wang, Y.J., Cheng, H., Edwards, R.L., He, Y.Q., Kong, X.G., An, Z.S., Wu, J.Y., Kelly, M.J., Dykoski, C.A. and Li, X.D. 2005. The Holocene Asian monsoon: Links to solar changes and North Atlantic climate. Science 308: 854-857.