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Climate Models and the Sun-Climate Connection: Why the Two are Currently Incompatible
Volume 8, Number 35: 31 August 2005

In a review of the temporal variability of various solar phenomena, Judith Lean of the Naval Research Laboratory's E.O. Hulburt Center for Space Research in Washington, DC, USA makes the following important but disturbing point about climate models and the sun-climate connection: "a major enigma is that general circulation climate models predict an immutable climate in response to decadal solar variability, whereas surface temperatures, cloud cover, drought, rainfall, tropical cyclones, and forest fires show a definite correlation with solar activity (Haigh, 2001, Rind, 2002)."  So what's going on here?

What's going on is that a vast repository of empirical findings from an array of scientific disciplines is being ignored by a small coterie of climate scientists that is focused almost exclusively on developing computer models of how they believe earth's climate system operates.  Any observation that fails to harmonize with that belief system is generally ignored by its practitioners, while those who champion their approach to the subject often question the judgment and/or motives of scientists who place greater confidence in real-world observations, be they based on instrumental or proxy data.

So just how real is the sun-climate connection that ranks so low on the climate modelers' scale of climatic significance?

Lean begins her foray into this aspect of her review by noting that the beginning of the Little Ice Age "coincided with anomalously low solar activity (the so-called Sporer and Maunder minima)," and that "the latter part coincided with both low solar activity (the Dalton minimum) and volcanic eruptions."  Then, after discussing the complexities of this potential relationship, she muses about another alternative: "Or might the Little Ice Age be simply the most recent cool episode of millennial climate-oscillation cycles?" ... which, we hasten to add, may be driven by a similar (millennial-scale) cycle of solar activity.

Lean also cites evidence that reveals the sensitivity of drought and rainfall to solar variability, stating that climate models are unable to reproduce the plethora (her word) of sun-climate connections.  In addition, she notes that simulations with climate models yield decadal and centennial variability even in the absence of external forcing, stating that "arguably, this very sensitivity of the climate system to unforced oscillation and stochastic noise predisposes it to nonlinear responses to small forcings such as by the sun," which argument pretty much invalidates the climate modellers' claim that solar forcing is too weak to produce the degree of warming and cooling that is often ascribed to it by scientists who are not fettered by the constraints of the climate modelling enterprise.

In further buttressing her position on the issue, Lean accurately reports that "various high-resolution paleoclimate records in ice cores, tree rings, lake and ocean sediment cores, and corals suggest that changes in the energy output of the sun itself may have contributed to sun-earth system variability," citing the work of Verschuren et al. (2000), Hodell et al. (2001), and Bond et al. (2001).  Indeed, she notes that "many geographically diverse records of past climate are coherent over time, with periods near 2400, 208 and 90 years that are also present in the 14C and 10Be archives," which isotopes (produced at the end of a complex chain of interactions that are initiated by galactic cosmic rays) contain information about various aspects of solar activity (Bard et al., 1997).

In her concluding paragraph, Lean rhetorically wonders "How much of earth's recent surface warming is induced by solar rather than anthropogenic forcings?"  We likewise wonder, suspecting that solar forcing may well be the dominant driver of 20th-century global warming, with anthropogenic CO2 emissions squeezed in somewhere in the far back seat of the climate-forcings bus.

Sherwood, Keith and Craig Idso

References
Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J.  1997.  Solar modulation of cosmogenic nuclide production over the last millennium: comparison between 14C and 10Be records.  Earth and Planetary Science Letters 150: 453-462.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G.  2001.  Persistent solar influence on North Atlantic climate during the Holocene.  Science 294: 2130-2136.

Haigh, J.D.  2001.  Climate variability and the influence of the sun.  Science 294: 2109-2111.

Hodell, D.A., Brenner, M., Curtis, J.H. and Guilderson, T.  Solar forcing of drought frequency in the Maya lowlands.  Science 292: 1367-1370.

Lean, J.  2005.  Living with a variable sun.  Physics Today 58 (6): 32-38.

Rind, D.  2002.  The sun's role in climate variations.  Science 296: 673-677.

Verschuren, D., Laird, K.R. and Cumming, B.F.  2000.  Rainfall and drought in equatorial east Africa during the past 1,100 years.  Nature 403: 410-414.