Ice cores and deep sea bed cores provide the best available record of changes in global temperature and CO2 content of the atmosphere going back 800,000 years. The data shows a clear periodicity in global temperatures which is thought to be linked to the Milankovitch cycle.
Back in 1920, Milutin Milankovitch, a Serbian mathematician, proposed that fine changes in Earth’s orbit around the Sun could explain an approximately 100,000 year cycle in glaciation seen from geological evidence. The tilt of the Earth’s axis swings slightly over a 41,000 year cycle – the eccentricity of Earth’s orbit moves from almost circular to more elliptical and back again over a 413,000 year cycle – and overlaying that you have not only the precession of the equinoxes, which is an inherent wobble in the Earth’s axial spin over a 26,000 year cycle, but also a precession of the whole of Earth’s orbit over a 23,000 year cycle.
Ice core data does show a rough concordance between glaciation and the synchronicity of these orbital cycles. Even though there’s no significant change in the mean amount of solar radiation reaching the Earth over the period of its annual orbit – the orbital changes can lead to increased polar shadowing and cooling.
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Once ice does start advancing from the poles, a positive feedback loop can develop – since more ice increases the albedo of Earth’s surface and reflects more of the Sun’s heat back into space, thus reducing mean global temperatures.
It’s thought that what limits the ice advancing is increasing CO2 in the atmosphere – which can be measured from trapped bubbles of air in the ice cores. More ice formation leads to less exposed land area for photosynthesis and silicate rock weathering to remove CO2 from the atmosphere. So the more ice that’s formed, the more CO2 accumulates in the atmosphere – which causes mean global temperatures to rise, which limits ongoing ice formation.
Of course the opposite is true in an ice-melting phase. Ice melting also follows a positive feedback loop since less ice means less albedo, meaning less solar radiation is reflected back into space and mean global temperatures rise. But again, CO2 becomes the limiting factor. With more exposed land, more CO2 is drawn from the atmosphere by photosynthesizing forests and rock weathering. A consequent drop in atmospheric CO2 cools the planet and hence limits ongoing ice melting.
But there lies the rub. We are in an ice-melting phase of the Milankovitch cycle now, where the Earth’s orbit is closer to circular and the Earth’s tilt is closer to perpendicular. But CO2 levels aren’t declining – partly because we’ve chopped a lot of trees and forests down, but mostly because of anthropogenic CO2 production. Without the limiting factor of declining CO2 we’ve seen in previous Milankovitch cycles, presumably the ice is just going to keep on melting as the albedo of the Earth surface declines.
So you might want to rethink that next coastal real estate purchase – or hope for the best from Copenhagen.