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Instrument meteorological records rarely extend back more than 200 years; therefore, a long-term
perspective on solar forcing must rely on the records provided by paleoclimate archives—principally, ice
cores, lake and marine sediments, stalagmites, corals, and tree rings. Within each of these natural
archives, a number of parameters can be measured and their relationship to climate assessed through
calibrations with overlapping instrument data. In this way, paleoclimate proxies extend the record of past
climate over past millennia.
Workshop presentations on this topic included discussions of detection of solar signals from
paleorecords and temperature proxies as well as the role of cyclic and secular forcing at Earth's surface
and the corresponding climate response.
Detection of the Solar Signal in Climate from Paleorecords
Raymond S. Bradley, University of Massachusetts
Paleoclimate archives also provide an index of past solar activity, through the record of changes
in cosmogenic isotopes recorded in tree rings and ice cores. In particular, variations in the cosmogenic
isotopes 10 Be and 14 C indicate changes in the production rate of these isotopes. Raymond Bradley
described how, over the past 12,000 years, these variations have been controlled mainly by changes in
Earth's magnetic field, and the field was weaker than today's for much of that period (only ~40% of the
present day value at 7000 years before present). Thus, isolating solar magnetic effects on the production
rate of 10 Be and 14 C requires that changes in the geomagnetic field strength be removed, leaving the
heliomagnetic signal as a residual. Unfortunately, past changes in Earth's magnetic field are not well
constrained, and Bradley indicated that further research on this topic is needed to refine the signal of
whatever residual solar signal may be present. Furthermore, although attempts have been made to
calibrate changes in 10 Be in terms of variations in total solar irradiance (TSI), 19 it is still debatable how
variations in cosmogenic isotope production relate to changes in total or spectrally distributed irradiance.
Bradley noted that, despite these limitations, paleoclimatologists have generally accepted that the
record of 10 Be or 14 C anomalies provides an index of changes in TSI, and have often sought to correlate
paleoclimatic records with these data. The results have been mixed. In the case of the high-resolution
Belukha glacier ice core data (from the Siberian Altai), a well-defined correlation between δ 18 O (a proxy
for March-November mean temperature) and 10 Be was found, with the strongest correlation associated
with a 20-year lag in the temperature response. 20 Furthermore, spectral analysis of the δ 18 O record
revealed statistically significant variance at 10.8, 86, and 205 years, frequencies known to be prominent in
the 14 C anomaly record. However, when other nearby proxy records were examined, there was no
evidence for a similar relationship to solar forcing, leaving open the question of whether the Belukha
record is superior to the others, or whether the observed relationship is only of local significance.
Bradley discussed how a number of high-resolution ice core records have noted a strong
relationship between 14 C anomalies and changes in oxygen isotopes in the stalagmite carbonate. For
example, Neff et al. (2001) noted a high correlation between δ 18 O in a stalagmite from Oman, and 14 C,
which they suggest is related to solar influences on monsoon-derived rainfall. 21 Similarly, Wang et al.
19 See, for example, F. Steinhilber, J. Beer, and C. Fröhlich, Total solar irradiance during the Holocene,
Geophysical Research Letters 36:L19704, 2009.
20 A. Eichler, S. Olivier, K. Henderson, A. Laube, J. Beer, T. Papina, H.W. Gäggeler, and M. Schwikowski,
Temperature response in the Altai region lags solar forcing, Geophysical, Research, Letters 36: L01808, 2009.
21 U. Neff, S.J. Burns, A. Mangini, M. Mudelsee, D. Fleitmann, and A. Matter, Strong coherence between solar
variability and the monsoon in Oman between 0 and 6 kyr ago, Nature 411:290-293, 2001.
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