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Solar Forcing of Climate: A Means of Amplification?
Tobias, S.M. and Weiss, N.O.  2000.  Resonant interactions between solar activity and climate.  Journal of Climate 13: 3745-3759.

Numerous studies have yielded high correlations between time courses of a number of different climatic phenomena and historical trends of various measures of solar activity (see, for example, the many entries under Solar Climatic Effects in our Subject Index).  It is difficult, however, for certain scientists to believe that earth's thermal and hydrologic cycles could be so strongly influenced by so weak an influence as solar forcing appears to be in terms of its degree of variability.  Hence, the quest to discover how weak solar signals could be sufficiently amplified to produce the many intriguing climatic histories that appear to be the offspring of solar variability has become somewhat of a Holy Grail for a major branch of climatology.

What was done
Noting that "solar magnetic activity exhibits chaotically modulated cycles ... which are responsible for slight variations in solar luminosity and modulation of the solar wind," the authors attack the solar forcing of climate problem by means of a model in which the solar dynamo and earth's climate are represented by low-order systems, each of which in isolation supports chaotic oscillations but which when run together sometimes resonate.

What was learned
The authors determined that "solutions oscillate about either of two fixed points, representing warm and cold states, flipping sporadically between them."  They also discovered that a weak nonlinear input from the solar dynamo "has a significant effect when the 'typical frequencies' of each system are in resonance."

What it means
The authors' findings are best stated in their own words: "It is clear that the resonance provides a powerful mechanism for amplifying climate forcing by solar activity."  Hence, there need no longer be any reluctance to accept as fact the observation that the many correlations that have been documented between solar variability and the time histories of various climatic phenomena do indeed have a cause that is of extraterrestrial origin.

Reviewed 6 December 2000