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Solar Forcing of Climate: Is It Significant?
Reference
Frohlich, C. and Lean, J. 2002. Solar irradiance variability and climate. Astronomische Nachrichten 323: 203-212.

Background
A number of different spacecraft have monitored total solar irradiance (TSI) for the past 23 years, with at least two of them operating simultaneously at all times. In addition, TSI measurements made from balloons and rockets supplement the satellite data. From this wealth of information, a composite TSI record has been developed that spans two 11-year solar cycles.

What was done
The authors compare the composite TSI record with an empirical model of TSI variations, based on known magnetic sources of irradiance variability, such as sunspot darkening and brightening. They then describe how "the TSI record may be extrapolated back to the seventeenth century Maunder Minimum of anomalously lower solar activity, which coincided with the coldest period of the Little Ice Age." This exercise, they say, "enables an assessment of the extent of post-industrial climate change that may be attributable to a varying Sun, and how much the Sun might influence future climate change."

What was learned
In the words of the authors, "warming since 1650 due to the solar change is close to 0.4C, with pre-industrial fluctuations of 0.2C that are seen also to be present in the temperature reconstructions."

What it means
From this study, it would appear that solar variability can explain a significant portion of the warming experienced by the earth in recovering from the global chill of the Little Ice Age, with a modicum of positive feedback accounting for the rest. With respect to the future, however, the authors say that "solar forcing is unlikely to compensate for the expected forcing due to the increase of anthropogenic greenhouse gases which are projected to be about a factor of 3-6 larger." The magnitude of that anthropogenic forcing, however, is computed by many different approaches to be much smaller than the value employed by the authors in making this comparison (Idso, 1998). Likewise, the anticipated rise in the air's CO2 content may also be much smaller than what is specified by the set of scenarios employed by the authors, due to simultaneous CO2-induced increases in biospheric carbon sequestration (Idso, 1991a,b). Hence, while past temperature changes seem reasonably well explained by solar radiation variations, the future - as always - is a much more murky matter.

References
Idso, S.B. 1991a. The aerial fertilization effect of CO2 and its implications for global carbon cycling and maximum greenhouse warming. Bulletin of the American Meteorological Society 72: 962-965.

Idso, S.B. 1991b. Reply to comments of L.D. Danny Harvey, Bert Bolin, and P. Lehmann. Bulletin of the American Meteorological Society 72: 1910-1914.

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


Reviewed 27 November 2002