I seem to have returned to an old obsession. Tying to understand the underlying causes and timings of ice ages is as frustrating as it is fascinating.
By far the largest glaciation that occurred over Britain was 480,000 years ago in what geologists call the ‘Anglian’ ice age. Two thirds of Britain and Ireland were then covered by a thick ice sheet extending from the west coast of Ireland across the North Seas all the way to Norway. The British Ice sheet reached as far south as Oxfordshire and Essex diverting the original course of the River Thames, which flowed north to the Wash, to its current position through what is now London. Geological evidence for this can be seen in old destroyed river beds and layers of boulder clay laid down by glaciers.
The second largest glaciation was the most recent one which reached its maximum extent at LGM 25,000 years ago. It is called by geologists the ‘Devensian’ . It turns out that these two glacial cycles were very similar, despite being spaced by 400,000 years, because of a regular cycle in the earth’s eccentricity. When eccentricity is low glaciations are always longer and deeper. To see how this happens, we can start by comparing the last 800,000 years of ice-core data as shown below.
The ‘Anglian’ ice volume, as measurement by Benthic Fora dO18, exceeded that of the LGM, and proves that this was also a global phenomena. The Anglian interglacial also lasted far longer than normal, at least 25,000 years, and this is most likely because of the weak value of eccentricity. A very similar situation is re-occurring now, luckily for us, because an even lower eccentricity peak will occur in 30,000 years time. Low eccentricity mainly acts to suppress the effects of precession, leaving just the 41000 year obliquity cycle to cause seasonal changes at the poles. For reasons that are not yet fully clear, this alone has not been sufficient to end ice ages for a million years. From 5 million to 1 million years ago glaciations followed the 41,000 year obliquity cycle, but as the earth cooled and glaciations became more extensive this then switched to a 100k year cycle. No-one has yet convincingly explained quite why this occurred.
The reference calculations of Laskar et al. which solve the solar system multi-body gravitational orbital equations also show a remarkable 2.8 million year cycle in eccentricity. I have never seen this mentioned before. We are entering a periodicity very similar to that which occurred just under 3 million years ago.
So what happened 2.8 million years ago? The climate was much warmer then and the glacial cycle was driven by the 41ky obliquity cycle. Yet at that exact time two successive glaciations missed a beat doubling the glacial cycle to 82,000 y.
Further evidence that eccentricity over the last million years subtly modulates all glaciations can be seen by comparing the net annual average insolation the earth receives from the sun. Despite Keppler’s law there is still a small variation in the net solar insolation that the earth receives, dependent on eccentricity and slightly modulated by obliquity. The values shown below are all calculated using the Laskar’s 2010 orbital solution model.
The change in net solar forcing change is very small ( <0.5 W/m2) and unlikely to be as important as changes in how the radiation is distribution, but it is rarely mentioned. When eccentricity is low glaciations become more severe, more sawtoothed and yet somehow still end with a bang. How do they end? Is this perhaps when Gaia intervenes to restore a climate that is more conducive to life on earth?