Workshop Presentations - The Effects of Solar Variability on Earth's Climate - page 17

(2005) examine δ 18 O in a well-dated monsoon-derived stalagmite from southern China and found a highly
statistically significant relationship with 14 C anomalies over the past ~9,000 years. 22
Bradley asserted that it is noteworthy that all these studies focus on changes in the hydrological
cycle of each region, rather than changes in temperature. This points to the possibility that if there is a
solar effect on climate, it is manifested in terms of changes in the general circulation, rather than in a
direct temperature signal. Bradley noted that in fact, despite the serious limitations in terms of statistical
significance of most of the published paleoclimate studies that claim to find a solar signal in the records,
there is nevertheless a clear geographical pattern in the overall signal that emerges when all records are
mapped out. Specifically, periods of high cosmogenic isotope production (which might be related to
reduced irradiance) appear to be associated with weaker monsoon rainfall in Oman, India, and southern
China. There is also evidence for colder conditions at high latitudes, more extensive sea-ice in the North
Atlantic, and wetter and colder conditions in western Europe, suggesting a general expansion of the polar
vortex and a southward displacement of the westerlies when solar activity is low. There is a
corresponding displacement, or seasonal shift in the intertropical convergence zone, affecting rainfall
distribution in Central and South America and equatorial Africa. 23,24,25 Meehl et al. (2004) suggested that
this pattern results from regional differences in radiation receipts, with cloud-free zones differentially
warming more than cloudy regions during periods of higher TSI, leading to changes in circulation
patterns. 26 Others have related solar-driven changes in stratospheric heating to changes in tropospheric
circulation. 27 Bradley concluded that either top-down or bottom-up effects (or both) may be relevant in
explaining the pattern of hydrological changes that appear to be present in the paleoclimatic records.
However, he asserted that it is clear that the current evidence for solar forcing from paleoclimate is very
limited, and most records do not provide the necessary resolution or signal strength to detect a solar signal
if it is present. Bradley suggested that further studies could be designed to address this question in a more
rigorous and systematic manner.
Detecting the Solar Cycle Via Temperature Proxies Back to the Maunder Minimum
Gerald R. North, Texas A&M University
Gerald North described an approach to detection of a solar signal in 18 O climate records thought
to record air temperature at the time of deposition on snow/ice fields. Some of these records are claimed
to resolve timescales fine enough to spectrally resolve signals at the 11-year-cycle period. North
described efforts to do this in oxygen isotope data from the Dye-3 core from Greenland and the Taylor
Dome cores from Antarctica, both of which revealed a weak 11-year signal. He described how further
research, involving additional well-dated records and band pass filtering, may further elucidate the
temporal evolution of such signals in relation to the long-term record of solar forcing.
22 Y. Wang, H. Cheng, R.L. Edwards, Y. He, X. Kong, Z. An, J. Wu, M.J. Kelly, C. A. Dykoski, and X. Li, The
Holocene Asian Monsoon: Links to solar changes and North Atlantic climate, Science 308:854-857, 2005.
23 D.E. Black, L.C. Peterson, J.T. Overpeck, A. Kaplan, M.N. Evans, and M. Kashgarian, Eight centuries of
North Atlantic ocean atmosphere variability, Science 286:1709-1713, 1999.
24 D.E. Black, R.C. Thunell, L.C. Peterson, A. Kaplan, and E.J. Tappa, A 2000-year record of tropical North
Atlantic hydrographic variability, Paleoceanography 19:PA2022, 2004.
25 J.C. Stager, D. Ryves, B.F. Cumming, L.D. Meeker, and J. Beer, Solar variability and the levels of Lake
Victoria, East Africa, during the last millennium, Journal of Palelimnology 33:243-251, 2005.
26 G.A. Meehl, and C. Tebaldi, More intense, more frequent, and longer lasting heat waves in the 21st century,
Science 305:994-997, 2004.
27 See, for example, D.T. Shindell, G. Faluvegi, and N. Bell, Preindustrial-to-present-day radiative forcing by
tropospheric ozone from improved simulations with the GISS chemistry-climate GCM, Atmospheric Chemistry and
Physics 3:1675-1702, 2003.