Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmgp62a..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #GP62A-02
Astronomy and Astrophysics
Astronomy
1035 Geochronology
Scientific paper
After over 20 years of "success", the very concept of astronomical tuning is now under attack. New data and new dates are available that enable us to review the past practice, and evaluate where it succeeded and where it failed. The issue is not just in the procedures used, but whether we know the climate target model well enough to use it as a tool rather than as a theory to be tested. The ultimate goal is to find a better time scale, and use that to uncover the mysteries of climate change. In the early days of tuning, the potential dangers of circular reasoning in orbital tuning were well appreciated. It was considered important to keep the number of tuneable parameters low, and then to test the approach by its success in matching the variations. But as tuning became more demanding, the number of parameters used increased. Imbrie has recommended that one should use one parameter for each data point. This would be a valid procedure if, in fact, the climate model was solidly established. But most analysts who accepted the Milankovitch tuning model recognized that, at best, it was only an approximation that still needed major work. If the wrong climate target is used, the timescale is invalid. Some of the early procedures to test the success of the theory proved to be flawed. It was once believed that the amplitude and phase variations in the climate signal were independent, that tuning could be tested by seeing if the amplitude variations were automatically matched. However, analysis by Neeman, Muller and MacDonald, and Huybers show that the process of tuning can also modulate the amplitude of the narrow-banded signal, thus reproducing the modulation that had been once considered unimpeachable proof of success. I will examine historical attempts at tuning in light of modern knowledge, including the Pacemaker, Specmap, and Low-Latitude stacks, and the "minimally tuned" Benthic stack. The outstanding success of tuning was the correct prediction of the age of the Brunhes-Matuyama reversal by Shackleton and colleagues. Modern day analysis of that success shows that it resulted from the coherence of the 100 kyr cycle -- not on the correctness of the Croll-Milankovitch insolation theory, as has often been assumed. A further problem comes from the fact that climate has proven to be multidimensional, with climate records in different proxies showing distinctly different behaviors. When the identical tuning target is used for all proxies, then the possibility of uncovering and clarifying such structure is rendered virtually impossible. New radiometric dates also provide an independent test of the effectiveness of past tuning. Ages of sea-level high stands have been determined for six glacial terminatons in the Pleistocene by Karner. The Devil's Hole radiometric dates, combined with new determinations of the ages of sea level high stands from corals in the Atlantic and Pacific, conflict with dates tuned to the Milankovitch model, leading to a "causality" crisis for the original tuning approach.
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