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Solar Control of Climate: Lessons from the Tropical Andes
Volume 9, Number 37: 13 September 2006

Working with data - biogenic silica, magnetic susceptibility, total organic carbon (TOC), total nitrogen (TN), δ13CTOC, δ15NTN and C/N ratios - derived from sediment records of two Venezuelan watersheds that they obtained from cores of Lakes Mucubaji and Blanca, along with ancillary data obtained from other studies that had been conducted in the same general region, Polissar et al. developed continuous decadal-scale histories of glacier activity and moisture balance in that part of the tropical Andes (the Cordillera de Merida) over the past millennium and a half, from which they were able to deduce contemporary histories of regional temperature and precipitation. So what did they learn?

The international (Canada, Spain, United States, Venezuela) team of scientists write that "comparison of the Little Ice Age history of glacier activity with reconstructions of solar and volcanic forcing suggest that solar variability is the primary underlying cause of the glacier fluctuations," because (1) "the peaks and troughs in the susceptibility records match fluctuations of solar irradiance reconstructed from 10Be and δ14C measurements," (2) "spectral analysis shows significant peaks at 227 and 125 years in both the irradiance and magnetic susceptibility records, closely matching the de Vreis and Gleissberg oscillations identified from solar irradiance reconstructions," and (3) "solar and volcanic forcing are uncorrelated between AD 1520 and 1650, and the magnetic susceptibility record follows the solar-irradiance reconstruction during this interval." In addition, they write that "four glacial advances occurred between AD 1250 and 1810, coincident with solar-activity minima," and that "temperature declines of -3.2 1.4C and precipitation increases of ~20% are required to produce the observed glacial responses."

In discussing their findings, Polissar et al. say that their results "suggest considerable [our italics] sensitivity of tropical climate to small [our italics] changes in radiative forcing from solar irradiance variability," which is something many people have had difficulty accepting in prior years. Hopefully, this paper will help them to "see the light" on this subject a little clearer. Not to be purveyors of too much good news, however, the six scientists say that their findings imply "even greater probable responses to future anthropogenic forcing," and that "profound climatic impacts can be predicted for tropical montane regions."

With respect to these latter ominous remarks, we note that whereas Polissar et al.'s linking of significant climate changes with solar radiation variability is a factual finding of their work, their latter statements with respect to hypothesized CO2-induced increases in down-welling thermal radiation are but speculations that need not follow from what they learned; for the two types of radiative forcing operate quite differently from each other, and there are many contemporaneous biological consequences of atmospheric CO2 enrichment that may produce powerful cooling effects on earth's climate. Hence, there is no guarantee there will be any net impetus for warming as the atmosphere's CO2 content continues to rise.

Another point worth noting in this regard is Polissar et al.'s acknowledgement that "during most of the past 10,000 years, glaciers were absent from all but the highest peaks in the Cordillera de Merida," which indicates that warmer-than-present temperatures are the norm for this part of the planet, and that any significant warming that might yet occur in this region (as well as most of the rest of the world) would only mark a return to more typical Holocene (or current interglacial) temperatures, which have themselves been significantly lower than those of all four prior interglacials. What is more, atmospheric CO2 concentrations were much lower during all of those much warmer periods, providing further evidence that it is the sun that rules the world's climate, not CO2.

Sherwood, Keith and Craig Idso

Reference
Polissar, P.J., Abbott, M.B., Wolfe, A.P., Bezada, M., Rull, V. and Bradley, R.S. 2006. Solar modulation of Little Ice Age climate in the tropical Andes. Proceedings of the National Academy of Sciences USA 103: 8937-8942.