Solar Forcing of Climate. 2: Evidence from the Past |
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Authors: | Gerard J M Versteegh |
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Institution: | (1) Hanse Wissenschaftskolleg, Lehmkuhlenbusch 4, D-27753 Delmenhorst, Germany;(2) Royal Netherlands Institute for Sea Research, P.O. Box 59, NL-1797 AB Den Burg, The Netherlands;(3) Institut für Biogeochemie und Meereschemie, Fachbereich Geowissenschaften, University of Hamburg, Bundesstr, 55, D-20146 Hamburg, Germany |
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Abstract: | The nature of the climatic response to solar forcing and its geographical coherence is reviewed. This information is of direct
relevance for evaluating solar forcing mechanisms and validating climate models.
Interpretation of Sun-climate relationships is hampered by difficulties in (1) translating proxy records into quantitative
climate parameters (2) obtaining accurate age assessments (3) elucidating spatial patterns and relationships (4) separating
solar forcing from other forcing mechanisms (5) lacking physical understanding of the solar forcing mechanisms. This often
limits assessment of past solar forcing of climate to identification of correlations between environmental change and solar
variability.
The noisy character and often insufficient temporal resolution of proxy records often exclude the detection of high frequency
decadal and bi-decadal cycles. However, on multi-decadal and longer time scales, notably the ∼90 years Gleisberg, and ∼200
years Suess cycles in the 10Be and 14C proxy records of solar activity are also well presented in the environmental proxy records. The additional ∼1500 years Bond
cycle may result from interference between centennial-band solar cycles.
Proxy evidence for Sun-climate relations is hardly present for Africa, South America and the marine realm; probably more due
to a lack of information than a lack of response to solar forcing. At low latitudes, equatorward movement of the ITCZ (upward
component of the Hadley cell) occurs upon a decrease in solar activity, explaining humidity changes for (1) Mesoamerica and
adjacent North and South American regions and (2) East Africa and the Indian and Chinese Monsoon systems. At middle latitudes
equatorward movement of the zonal circulation during solar minima probably (co-)induces wet and cool episodes in Western Europe,
and Terra del Fuego as well as humidity changes in Southern Africa, Australia, New Zealand and the Mediterranean. The polar
regions seem to expand during solar minima which, at least for the northern hemisphere is evident in southward extension of
the Atlantic ice cover. The forcing-induced migration of climate regimes implies that solar forcing induces a non linear response
at a given location. This complicates the assessment of Sun-climate relations and calls for nonlinear analysis of multiple
long and high resolution records at regional scale. Unfortunately nonlinear Sun-climate analysis is still a largely barren
field, despite the fact that major global climate configurations (e.g. the ENSO and AO) follow nonlinear dynamics.
The strength of solar forcing relative to other forcings (e.g. volcanism, ocean circulation patterns, tides, and geomagnetism)
is another source of dynamic responses. Notably the climatic effects of tides and geomagnetism are hitherto largely enigmatic.
Few but well-dated studies suggest almost instantaneous, climatic deteriorations in response to rapid decreases in solar activity.
Such early responses put severe limits to the solar forcing mechanisms and the extent of this phenomenon should be a key issue
for future Sun-climate studies. |
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Keywords: | proxy climate solar forcing Holocene |
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