New Scientist 22nd May 2010 had a story on the Hulu cave system near Nanjing in China which contains some very useful stalagmites that have been used to date the Ice Ages – or at least the last four. James Croll, a Scottish self taught physicist, proposed that periodic change in the earth’s orbit caused the amount of sunshine reaching the surface of the earth to vary over time. Low sunshine in winter led to snow accumulating. As ice sheets grew the earth reflected more heat back into space amplifying the effects of orbital changes. While he was wrong about the timing of the Ice Ages, the article says, his orbital theory was revived in the 20th century by Milutin Milankovitch, something of a genius. In spite of that Milankovitch was also ignored until the 1960s and 1970s when his theory was amalgamated into the consensus.
The tilt of earth’s axis is thought to increase and decrease every 41,000 years, making summers hotter and winters colder. The discovery of oxygen isotopes in bottom dwelling oceanic plankton was responsible for ending the fallow years as the isotopes showed there had been dozens of Ice Ages and they could be seen quite clearly in ocean sediments. It was thought the waxing and waning of ice sheets coincided with orbital change. However, shortly a problem emerged. Early Ice Ages appeared to conform to the 41,000 year cycle – up to two million years ago. At that point the pattern changed. A series of more severe Ice Ages occurred – but at 100,000 year intervals. It has been broached that this was really 120,00 year intervals, or three times 41,000 years, although every graph appears to show a 100,000 year cycle. This is a big mystery because although the shape of the earth’s orbit alters slightly over periods of 95,000 and 125,000 years, this is a much weaker effect on the seasons than the 41,000 year cycle.
Other theories began to be explored – such as the earth passing through interplanetary dust clouds. Or perhaps the Sun was sometimes brighter or less so. Then global warming got into the act and it was claimed that ice cores in Antarctica seemed to show a correlation between global temperatures and the level of greenhouse gases in the atmosphere – specifically C02. Small changes in sunlight intensity at 100,000 year intervals might be amplified greatly by rises in C02 levels. Ice cores and marine sediments, it was said, were not exact – which is an interesting observation. Palaeo coral reefs were consulted and it was found there were surges in sea level at the end of the last two Ice Ages – but there was a lack of coral to see what happened at earlier events. Stalagmites were then explored – but caves of the right age were necessary and this is where the Chinese caves come back into the story. They are able to reveal the strength of summer monsoons, it is alleged. Monsoon rains seem to have failed in concert with the end of the last four Ice Ages (over 400,000 years) which indicates disruption to the ocean circulation system – which impacts on monsoon intensity. In turn this is thought to release C02 from the atmosphere. This conclusion, strained as it seems, then followed with a spiffing rejoinder on AGW – apparently, the journalist at New Scientist thinks human emissions of C02 are capable of thwarting the arrival of the next Ice Age. Very strange.
Note … it is therefore an assumption that Ice Ages occur every 100,000 years. Oxygen isotopes may change for other reasons – such as the orbit of the Sun around the barycentre of the universe which occurs roughly at 95,000 year intervals, or we may even view such cycles as products of computers.