Project Major Findings

Richard Alley

Much of the Penn State funding for Siple Dome was for salaries, and when the ice was slow to arrive, we used the money to do things related to the goals of Siple Dome if not always to the actual ice. Probably our main accomplishment is formulation of a rather complete hypothesis for causation of abrupt climate change and its relation to ice-age cycles, based on our data and ideas and those of many others. We argue that a weak 1500-year periodic oscillation, probably of solar origin, combined with outburst floods and ice-sheet surges has produced the large, abrupt, global climate changes that punctuated the ice age and into the current warm time. Our work contributes to the understanding that while the current warm climate is more resistant to such large perturbations than the ice-age climate, instability is possible. We have highlighted the importance of widespread feedbacks in transmitting the abrupt climate changes, and demonstrated the continued ability of northern-insolation forcing amplified by carbon-dioxide changes to explain the ice-age cycles on which the abrupt changes rode. This work has involved about a dozen refereed publications, was highlighted in a book I wrote (The Two-Mile Time Machine) that was the Phi Beta Kappa Science Award winner in 2001, and was instrumental in my selection as chair of the National Research Council committee that has just produced the report Abrupt Climate Change: Inevitable Surprises. Understanding of the importance of abrupt climate changes is a triumph not of one group but of the entire community, owes more to ice-core science than to any other effort, and certainly has benefited from Siple Dome funding including that directed to Penn State.

More closely tied to Siple Dome, we conducted a number of efforts that are just coming to fruition because they are necessarily collaborative in nature. These include providing data (from visible stratigraphy by several Penn Staters) and counting effort (primarily by Matt Spencer) to aid in the dating of the core, identifying visible features to help guide other sampling, and making a complete catalog of every melt layer in the core that will be used by graduate student Sarah Das in seasonal paleothermometry now that the dating and annual-average thermometry data are becoming available. Graduate student Matt Spencer developed an improved time-dependent firn-densification model (2001), and we have used it plus data from many others in the collaboration to produce improved estimates of changes in snow accumulation at Siple Dome, including the surprising result that warming caused a drop in accumulation across the abrupt climate step about 19 ka whereas further warming caused little permanent accumulation-rate change after the hiatus near 15 ka. We demonstrated that occurrence of elongated bubbles in the Siple Dome and Taylor Dome cores does not necessarily indicate flow disturbance, and we are near completion of a novel application of bubble-sizes to provide independent estimates of paleotemperatures. We have collaborated in the effort to characterize c-axis fabrics and grain sizes, and anticipate completion of data sets and interpretations. In short, we have learned some about Siple Dome ice and its climate history but hope to learn much more, and we have learned a great deal about abrupt climate changes and how Siple Dome might fit into this larger picture.

Appended is a selection of key refereed publications for which the Penn State contribution was funded largely by the Siple Dome project and that have been published or accepted for publication; other papers are in review or in advanced stages of preparation. Nine additional published refereed papers partially funded by Siple Dome, and assorted unrefereed contributions, abstracts, and theses, were omitted to save space.

Alley, R.B. 2000. The Younger Dryas cold interval as viewed from central Greenland. Quaternary Science Reviews 19, 213-226.

Alley, R.B. and P.U. Clark. 1999. The deglaciation of the northern hemisphere: a global perspective. Annual Reviews of Earth and Planetary Sciences 27, 149-182.

Alley, R.B. and K.M. Cuffey. 2001. Oxygen- and hydrogen-isotopic ratios of water in precipitation: Beyond paleothermometry. In J.W. Valley and D. Cole, eds., Stable Isotope Geochemistry, Reviews in Mineralogy and Geochemistry, v. 43, Mineralogical Society of America.

Alley, R.B. and J.J. Fitzpatrick. 1999. Conditions for bubble elongation in cold ice-sheet ice. Journal of Glaciology 45, 147-154.

Alley, R.B., A.M. Agustsdottir and P.J. Fawcett. 1999. Ice-core evidence of Late-Holocene reduction in north Atlantic ocean heat transport, in Clark, P.U., R.S. Webb and L.D. Keigwin, eds., Mechanisms of Global Climate Change at Millennial Time Scales, Geophysical Monograph 112, American Geophysical Union, Washington, DC, 301-312.

Alley, R.B., S. Anandakrishnan and P. Jung. 2001. Stochastic resonance in the North Atlantic. Paleoceanography 16, 190-198.

Alley, R.B., S. Anandakrishnan, P. Jung and A. Clough. 2001 Stochastic resonance in the north Atlantic: Further Insights. In D. Seidov, B.J. Haupt and M. Maslin, eds., The Oceans and Rapid Climate Change: Past, Present and Future, Geophysical Monograph 126, p. 57-68.

Alley, R.B., E.J. Brook and S. Anandakrishnan. 2002. A northern lead in the orbital band: North-south phasing of ice-age events. Quaternary Science Reviews, 21, 431-441.

Alley, R.B., P.U. Clark, L.D. Keigwin and R.S. Webb. 1999. Making sense of millennial-scale climate change, in Clark, P.U., R.S. Webb and L.D. Keigwin, eds., Mechanisms of Global Climate Change at Millennial Time Scales, Geophysical Monograph 112, American Geophysical Union, Washington, DC, 385-394.

Clark, P.U., R.B. Alley and D. Pollard. 1999. Northern hemisphere ice-sheet influences on global climate change. Science 286, 1103-1111.

Spencer, M., R.B. Alley, and T.T. Creyts. 2001. Firn densification: an empirical model. Journal of Glaciology 47, no. 159: 671-676.

Steig, E.J. and R.B. Alley. Lag, lead, in phase, or anti-phase: what does the comparison of ice cores from Greenland and Antarctica really tell us about millennial-scale climate change? In press. Annals of Glaciology.