Making sense of glacial-interglacial cycles?

The competing periodicities of  the three astronomical “drivers” of climate – orbital eccentricity (~100 ka), axial obliquity (~40 ka) and axial precession (~20 ka) – lie behind several models for the climate changes of the last 0.7 Ma.  Taking in the theories that sway towards the influence of variables in the Earth system itself, around 30 models have some currency at present.  Since climate forecasters have to take account of which factors drive climate in the absence of human emissions, as well as piece together their own particular models, it is easy to see how critics of global warming get a wide hearing: compared with creationists, they have it easy!  Is there any way of resolving what is quite bluntly a theoretical mess?  It is a mess simply because the available data are so complex, and in the case of both main sources, ocean-floor sediments and ice cores, not only are their devils in the detail, but there are whopping contradictions, such as the mismatches in timing between the Greenland and Antarctic ice cores.  Add all the other sources, such as stalactites, tree rings etcetera, together with caveats like the difficulty in time calibration using 14C dating,  and the volume of diverse records become bewildering.  It is tempting that a reversion to a statistical approach, that includes more bells and whistles than hitherto (see Evolutionary rhythms below), can resolve matters.  Peter Huybers and Carl Wunch, of Woods Hole Oceanographic Institution and MIT, have tried that for pacing of the last 0.7 Ma of climate cycles (Huybers, P. * Wunsche, C. 2005.  Obliquity pacing of the late Pleistocene glacial terminations.  Nature, v. 434, p. 491-494).  Generally accepted “wisdom” holds that the last 7 glacial-interglacial cycles are paced by ~100 ka eccentricity forcing, even though it has the weakest effect on solar heating, by a very long way.  But there are smidgens of evidence for some interaction between that and the much stronger influence of changes in the Earth’s axial tilt or obliquity.  Huybers and Wunsch go for the Popperian rigor of first defining a null hypothesis, that obliquity has no effect, and then designing a test.   It isn’t easy to decide how the contrary hypothesis that it does can be evaluated though.  The clearest features in all climate records are the ends of glacial epochs or termination: they are sudden, sharp and generally look the same.  Most other features have some kind of pattern, but little consistent comparability.  Using the most advanced statistical techniques, which employ many iterations to test for stability in statistical models, they can show that the null hypothesis fails.  The positive result is that the time between terminations that are repeatedly modelled  falls into two envelopes, around 120 and 80 ka, which simple arithmetic shows are divisible by 40 ka.  But how can axial obliquity only have an effect every two of three of its cycles, while a single cycle does not appear in the time-series; is it nature skipping beats somehow.  One means that the authors suggest is that the underlying pace of eccentricity can effect the temperature at the base of ice sheets, depending on their thickness.  If they are thin, then the heating is insufficient to trigger ice-sheet collapse because the base is very cold, whereas if ice is thick the effects of thermal conductivity and heat flow makes the ice base warmer and more subject to perturbation beyond its failure limit.  It was at this point that I gave up, but wish the authors good luck in promoting their possibly unifying hypothesis for what finishes off glacial epochs…..


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