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Does Earth's Geomagnetic Field Strength Modulate Surface Air Temperature?
Volume 9, Number 31: 2 August 2006

In our Editorial of 19 July 2006, we reviewed the findings of Idso (1998) and Shaviv (2005), which suggest that the increase in the air's CO2 content over the 20th century only provided some 15-20% of the observed warming of that period, with the remainder coming from increased solar activity, which enhanced the weak magnetic field that is carried by the solar wind, which provided more shielding of the earth from low-energy galactic cosmic rays, which led to a reduction in cosmic ray-induced ion production in the lower atmosphere, which resulted in fewer condensation nuclei being created there, which led to fewer low-level clouds, which resulted in more solar radiation reaching the surface of the earth and, finally, an increase in near-surface air temperature. In this Editorial, we review a report that considers an additional factor - earth's geomagnetic field strength - that also affects the cosmic ray flux and influences air temperature.

Usoskin et al. (2005) introduce their study of this intriguing subject by stating that "the variation of 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," while further noting that "the existence of a geomagnetic signal in the climate data would support a direct effect of cosmic rays on climate."

The four researchers began their analysis, which was intended to evaluate this latter proposition, by reproducing 1000-year reconstructions of two notable solar-heliospheric indices derived from cosmogenic isotope data, i.e., the sunspot number and the cosmic ray flux (Usoskin et al., 2003; Solanki et al., 2004), and by creating a new 1000-year air temperature history of the Northern Hemisphere by computing annual means of six different thousand-year surface air temperature series: those of Jones et al. (1998), Mann et al. (1999), Briffa (2000), Crowley (2000), Esper et al. (2002) and Mann and Jones (2003).

In comparing these three series (solar activity, cosmic ray and air temperature), Usoskin et al. found that they "indicate higher temperatures during times of more intense solar activity (higher sunspot number, lower cosmic ray flux)." In addition, they report that three different statistical tests "consistently indicate that the long-term trends in the temperature correlate better with cosmic rays than with sunspots," which suggests that something in addition to solar activity must have been influencing the cosmic ray flux, in order to make the cosmic ray flux the better correlate of temperature.

Noting that earth's geomagnetic field strength would be a natural candidate for this "something," Usoskin et al. compared their solar activity, cosmic ray and temperature reconstructions with two long-term reconstructions of geomagnetic dipole moment that they obtained from the work of Hongre et al. (1998) and Yang et al. (2000). This effort revealed that between AD 1000 and 1700, when there was a substantial downward trend in air temperature associated with a less substantial downward trend in solar activity, there was also a general downward trend in geomagnetic field strength. As a result, Usoskin et al. suggested that the substantial upward trend of cosmic ray flux that was needed to sustain the substantial rate of observed cooling (which was more than expected in light of the slow decline in solar activity) was likely due to the positive effect on the cosmic ray flux that was produced by the decreasing geomagnetic field strength.

After 1700, the geomagnetic field strength continued to decline; but air temperature did a dramatic turnabout and began to rise. The reason for this "parting of company" between the two parameters, according to Usoskin et al., was that "the strong upward trend of solar activity during that time overcompensate[d] [for] the geomagnetic effect," leading to a significant warming. In addition, as we demonstrated in our 19 July 2006 Editorial, a minor portion of the warming of the last century or so (15-20%) may have been caused by the concomitant increase in the air's CO2 content, which would have complemented the warming produced by the upward trending solar activity and further decoupled the upward trending temperature from the declining geomagnetic field strength.

In their totality, these several observations tend to strengthen the hypothesis that cosmic ray variability was the major driver of changes in earth's surface air temperature over the past millennium, and that this forcing was primarily driven by variations in solar activity, modulated by the more-slowly-changing geomagnetic field strength of the planet, which sometimes strengthened the solar forcing and sometimes worked against it. Once again, however, there is room for only a small impact of anthropogenic CO2 emissions on 20th-century global warming.

Sherwood, Keith and Craig Idso

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Crowley, T.J. 2000. Causes of climate change over the past 1000 years. Science 289: 270-277.

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Hongre, L., Hulot, G. and Khokhlov, A. 1998. An analysis of the geomagnetic field over the past 2000 years. Physics of the Earth and Planetary Interiors 106: 311-335.

Idso, S.B. 1998. CO2-induced global warming: a skeptic's view of potential climate change. Climate Research 10: 69-82.

Jones, P.D., Briffa, K.R., Barnett, T.P. and Tett, S.F.B. 1998. High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with general circulation model control-run temperatures. The Holocene 8: 455-471.

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Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.

Shaviv, N.J. 2005. On climate response to changes in the cosmic ray flux and radiative budget. Journal of Geophysical Research 110: 10.1029/2004JA010866.

Solanki, S.K., Usoskin, I.G., Kromer, B., Schussler, M. and Beer, J. 2004. Unusual activity of the Sun during recent decades compared to the previous 11,000 years. Nature 431: 1084-1087.

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.

Usoskin, I.G., Solanki, S., Schussler, M., Mursula, K. and Alanko, K. 2003. A millennium scale sunspot number reconstruction: Evidence for an unusually active sun since the 1940's. Physical Review Letters 91: 10.1103/PhysRevLett.91.211101.

Yang, S., Odah, H. and Shaw, J. 2000. Variations in the geomagnetic dipole moment over the last 12,000 years. Geophysical Journal International 140: 158-162.