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Variability of Insolation at Earth's Surface: The Other Sun-Climate Connection
Volume 8, Number 41: 12 October 2005

Widespread measurements of the flux of solar radiation received at the surface of the earth have been made since the late 1950s, with nearly all of the measurements revealing a sizeable decline in the surface receipt of solar radiation that was not reversed until the mid-1980s (Wild et al., 2005).  During this time, there was also a noticeable dip in earth's surface air temperature, after which temperatures rose at a rate and to a level of warmth that climate alarmists claim were both without precedent over the past two millennia, which phenomena they attribute to similarly unprecedented increases in greenhouse gas concentrations, the most notable, of course, being CO2.

This reversal of the decline in the amount of solar radiation incident upon the earth's surface, in the words of Wild et al., "is reconcilable with changes in cloudiness and atmospheric transmission and may substantially affect surface climate."  In what way?  Wild et al. say that "whereas the decline in solar energy could have counterbalanced the increase in down-welling longwave energy from the enhanced greenhouse effect before the 1980s, the masking of the greenhouse effect and related impacts may no longer have been effective thereafter, enabling the greenhouse signals to become more evident during the 1990s."

Qualitatively, this scenario sounds reasonable; but when the magnitude of the increase in the surface-received flux of solar radiation over the 1990s is considered, the statement is seen to be rather disingenuous.

Over the range of years for which high-quality data were available to them (1992-2002), Wild et al. determined that the mean worldwide increase in clear-sky insolation averaged 0.68 W m-2 per year, which increase they found to be "comparable to the increase under all-sky conditions."  Consequently, for that specific ten-year period, these real-world data suggest that the total increase in solar radiation received at the surface of the earth should have been something on the order of 6.8 W m-2, which is not significantly different from what is implied by the satellite and "earthshine" data of Palle et al. (2004), although the satellite data of Pinker et al. (2005) suggest an increase only about a third as large for this period.

Putting these numbers in perspective, Charlson et al. (2005) report that the longwave radiative forcing provided by all greenhouse gas increases since the beginning of the industrial era has amounted to only 2.4 W m-2, citing the work of Anderson et al. (2003), while Palle et al. say that "the latest IPCC report argues for a 2.4 W m-2 increase in CO2 longwave forcing since 1850."  Consequently, it can be readily appreciated that the longwave forcing of greenhouse gases over the 1990s would have been but a fraction of a fraction of the observed increase in the contemporary receipt of solar radiation at the surface of the earth.

To thus suggest, as Wild et al. do - i.e., that the increase in insolation experienced at the surface of the earth over the 1990s may have enabled anthropogenic greenhouse gas signals of that period to become more evident - seems just a tad incongruous, as their suggestion implies that the bulk of the warming of that period was due to increases in greenhouse gas concentrations.  This incongruity is made all the worse by the fact that methane concentrations rose ever more slowly over this period, apparently actually stabilizing near its end (see Methane (Atmospheric Concentrations) in our Subject Index).  Consequently, a much more logical conclusion would be that the primary driver of the global warming of the 1990s was the large increase in global surface-level insolation.

Sherwood, Keith and Craig Idso

Anderson, T.L., Charlson, R.J., Schwartz, S.E., Knutti, R., Boucher, O., Rodhe, H. and Heintzenberg, J.  2003.  Climate forcing by aerosols - a hazy picture.  Science 300: 1103-1104.

Charlson, R.J., Valero, F.P.J. and Seinfeld, J.H.  2005.  In search of balance.  Science 308: 806-807.

Palle, E., Goode, P.R., Montanes-Rodriguez, P. and Koonin, S.E.  2004.  Changes in earth's reflectance over the past two decades.  Science 304: 1299-1301.

Pinker, R.T., Zhang, B. and Dutton, E.G.  2005.  Do satellites detect trends in surface solar radiation?  Science 308: 850-854.

Wild, M., Gilgen, H., Roesch, A., Ohmura, A., Long, C.N., Dutton, E.G., Forgan, B., Kallis, A., Russak, V. and Tsvetkov, A.  2005.  From dimming to brightening: Decadal changes in solar radiation at earth's surface.  Science 308: 847-850.