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Did Increasing Solar Activity Drive 20th-Century Global Warming?
Volume 9, Number 29: 19 July 2006

Throughout the 1980s and 90s, one of us (SBI) published several papers wherein he analyzed a number of what he called "natural experiments" in a multifaceted quest to quantify earth's near-surface air temperature response to perturbations of the planet's surface radiative balance (Idso, 1998). Subsequently, Nir J. Shaviv of the Hebrew University of Jerusalem's Racah Institute of Physics took up the identical challenge, similarly deriving a number of pertinent results (Shaviv, 2005).

Knowing that variations in solar activity correlate closely with climatic variations, but that climatic changes attributable to changes in solar activity are much larger than can be explained by changes in solar irradiance, Shaviv realized that an amplifier of some sort must be involved in the solar/climate relationship. What he and many other researchers have suggested, in this regard, is that when solar activity increases and the weak magnetic field that is carried by the solar wind intensifies (providing more shielding of the earth from low-energy galactic cosmic rays), there is a reduction in cosmic ray-induced ion production in the lower atmosphere that results in the creation of fewer condensation nuclei there and, hence, less low-level cloud cover, which allows more solar radiation to impinge upon the earth, increasing surface air temperature (and vice versa throughout).

Shaviv next identified six periods of earth's history (the entire Phanerozoic, the Cretaceous, the Eocene, the Last Glacial Maximum, the 20th century, and the eleven-year solar cycle as manifest over the last three centuries) for which he was able to derive reasonably sound estimates of different time-scale changes in radiative forcing, temperature and cosmic ray flux. From these sets of data he derived probability distribution functions of whole-earth temperature sensitivity to radiative forcing for each of the six time periods and combined them to obtain a mean planetary temperature sensitivity to radiative forcing of 0.28C per Wm-2. Then, noting that the IPCC (2001) suggested that the increase in anthropogenic radiative forcing over the 20th century was about 0.5 Wm-2, Shaviv calculated that the anthropogenic-induced warming of the globe over this period was approximately 0.14C (0.5 Wm-2 x 0.28C per Wm-2). This result harmonizes perfectly with the temperature increase (0.10C) that was calculated by Idso (1998) to be due solely to the 20th-century increase in the air's CO2 concentration (75 ppm), which would have been essentially indistinguishable from Shaviv's result if the warming contributions of the 20th-century concentration increases of all greenhouse gases had been included in the calculation.

Next, based on information that indicated a solar activity-induced increase in radiative forcing of 1.3 Wm-2 over the 20th century (by way of cosmic ray flux reduction), plus the work of others (Hoyt and Schatten, 1993; Lean et al., 1995; Solanki and Fligge, 1998) that indicated a globally-averaged solar luminosity increase of approximately 0.4 Wm-2 over the same period, Shaviv calculated an overall and ultimately solar activity-induced warming of 0.47C (1.7 Wm-2 x 0.28C per Wm-2) over the 20th century. Added to the 0.14C of anthropogenic-induced warming, the calculated total warming of the 20th century thus came to 0.61C, which was noted by Shaviv to be very close to the 0.57C temperature increase that was said by the IPCC to have been observed over the past century. Consequently, both Shaviv's and Idso's analyses, which mesh well with real-world data of both the recent and distant past, suggest that only 15-20% (0.10C/0.57C) of the observed warming of the 20th-century can be attributed to the concomitant rise in the air's CO2 content.

In light of these real-world-based observations, plus the multitude of studies that indicate most climate changes of the past were clearly associated with changes in solar activity (see Solar Effects in our Subject Index), the case for anthropogenic CO2 emissions playing anything more than a minor role in contemporary global warming would appear to be fading fast.

Sherwood, Keith and Craig Idso

Hoyt, D.V. and Schatten, K.H. 1993. A discussion of plausible solar irradiance variations, 1700-1992. Journal of Geophysical Research 98: 18,895-18,906.

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

Intergovernmental Panel on Climate Change. 2001. Climate Change 2001. Cambridge University Press, New York, NY, USA.

Lean, J., Beer, J. and Bradley, R. 1995. Reconstruction of solar irradiance since 1610 - Implications for climate change. Geophysical Research Letters 22:3195-3198.

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. and Fligge, M. 1998. Solar irradiance since 1874 revisited. Geophysical Research Letters 25: 341-344.