Milutin Milankovic built on James Croll’s notion that perturbation of Earth’s astronomical behaviour is likely to cause variations in solar heating that might lie behind repeated glacial epochs. One of the most fertile discoveries in Earth science since World War 2 is that the periodicities that Milankovic calculated do seem to dominate the time-record of climate change over the last 2.5 million years, when 50 glacial-interglacial cycles forced changes in the oxygen isotope content of fossils from deep-ocean cores. These data record the variations in long-term storage of water in continental ice sheets, and are a near-incontrovertible ‘proxy’ for both varying extents of glaciation and sea levels Much the same kinds of signals also appear to turn up in time series of other kinds from sediments of a much wider range of ages. Milankovic theory now has as much popular support as Alfred Wegener’s idea of drifting continents. But problematic aspects refuse to go away. Not the least of these is the conversion of depth to time in oceanic sediments from which the longest and most detailed records have emerged. An analysis of the frequencies involved in past climate change rests or falls on the accurate conversion of depths in sediment cores to time. In oceanic sediments this is by no means an easy job, because of a lack of material that can be dated precisely.
The first attempt to unscramble the complex variation in ocean cores used a few calibration points in climate time series onshore, whose shapes seemed to match those of ocean records. The most crucial of these was that for the rise in sea level at the end of the ice age before last (called Termination II) recorded in coral reefs in the Caribbean. The most widely used date for Termination II in the Caribbean is 127+6 thousand years (ka). It was from using this date as a global time correlation that the Milankovic signals of 100, 41, 23 and 19 ka periodicities popped out of the mathematical analysis. One surprise was that the match with the prediction of varying solar heating referred to the Northern rather than the Southern Hemisphere, or the planet as a whole. A great deal of later work hangs on that, and much of it has simply assumed the Northern-Hemisphere pacemaker, such as the widely used SPECMAP time scale.
Daniel Karner and Richard Muller of the University of California in Berkeley summarise the most contrary pieces of evidence for the timing of climatic change in a recent issue of Science (Karner, D.B. and Muller, R.A., 2000. A causality problem for Milankovitch. Science, 288, p.2143-2144). Using a different dating approach the Caribbean timing of Termination II comes out at 132 ka, while for a series of coral reefs in Papua New Guinea it appears as early as 142 ka. Detailed climate changes recorded in stalactitic material from a cave in Nevada (Devils Hole) also show an ‘early’ Termination II. All these ages are as precise or better than the accepted 127ka date for Termination II, so Karner and Muller see a big problem. Whereas the end of the last glaciation (Termination I) is pretty well tied down to about 12 ka, and corresponds to increased solar heating in the Northern Hemisphere from the Milankovic predictions, Termination II bucks that by 5 to 10 thousand years. A theory that is only 50% believable needs a serious seeing to!
For the last four terminations, the most varied and informative data come from cores through the Antarctic ice sheet. Though that too has its problems regarding calibration of depth to time, a recent evaluation of the climate variations over the last 420 ka (Petit, J.R. et al., 2000. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature, 399, p. 429-436) shows significant differences between the last four terminations. Karner and Muller suggest that each glacial-interglacial cycle may have different controls, and encourage a new look at the wealth of data, unbiased by earlier ideas centring on pacing by a single, astronomical pacemaker and accepting that climate controls are multidimensional.