According to Emily Shuckburgh and Prof. Erik Wolf in a letter to the Times , the next Ice Age is not expected for another 50,000 years. Their claim seems to be based on a 2016 paper in Nature by Andrey Ganopolski, R. Winkelmann & H. J. Schellnhuber, Nature 529, 200–203 (14 January 2016).

“Even without man-made climate change we would expect the beginning of a new ice age no earlier than in 50.000 years from now”

The paper also claims that the Earth narrowly avoided the inception of a new ice age just before the Industrial Revolution, because CO2 levels were 40ppm above some hypothetical threshold of 240ppm. Of course humans were responsible for this (luckily this time) due to deforestation and land use change.

The leading hypothesis currently is that Ice Ages initiate when summer insolation reduces sufficiently so that it fails to melt back the previous winter snow. Ice then slowly accumulates leading to an increase in albedo as the northern ice sheets slowly grow. There are two causes for this effect, both of which interplay one with the other. Changes in the obliquity of the earth modulate summer insolation at both poles. Precession of the seasons, due to the precession of the earth’s axis, change the timing of the summer equinox. Combined together they then modulates the distance ‘R’ to the sun during summer months, simply because the earth has an elliptical orbit . The strength of this precession ‘forcing’ is amplified by at high eccentricity.

Until 800,000 year ago ice ages followed the 41,000 year obliquity cycle. Low obliquity reduced summer insolation at the poles and glaciers expanded. High obliquity reversed this initiating an interglacial. No-one knows for sure why this change happened, but it is usually assumed that the ice sheets became too large for obliquity alone to melt them back. They then also needed the help of the precession term acting on the expanded northern ice sheets. However, this does not explain why these insolation minima only work once ice sheets have reached some critical size. One attractive explanation to explain this is CO2 starvation . CO2 levels in the atmosphere naturally increase with the onset of an interglacial as a result of the warmer temperatures and an enhanced life cycle. These increases in CO2 act as a small positive feedback on temperatures.

Glaciation begin at high values of CO2, which in general then fall with reducing temperatures and reductions in biosphere activity. However during the Eemian CO2 levels remained above 260ppm for some 35,000 years into the last ice age. If CO2 plays any role in Ice Ages, it is just a supporting role as a feedback.

So when would the next ice age naturally begin had humans not burned any fossil fuels ? The Anglian interglacial some 400,000 years had similar orbital eccentricity to that during the Holocene. The preceding glaciation was also very severe like the that preceeding the Holocene.

The Anglian interglacial lasted about 25,000 years which is roughly twice as long as average. Cooling initiated on a reducing obliquity coinciding with a northern summer minimum. The Holocene interglacial has northern and summer hemispheres inverted but obliquity still follows almost the same pattern. The minimum to which Ganopolski refers to as a close call pre-industrial inception is really nothing of the sort, since obliquity was still too high. I believe cooling would naturally begin another glaciation before 10 thousand years from now as we approach minimum obliquity. At the latest it starts 15,000 years from now. So will anthropogenic global warming delay the onset of the next ice age for 100,000 years as the authors argue ?

Let’s assume that in the worst case we manage to double atmospheric CO2 levels before curbing carbon emissions (perhaps we have magic fusion reactors by then). Then quoting an acknowledged expert in Ocean Climate Chemistry – David Archer

“Dissolution into ocean water sequesters 70–80% of the CO2 release on a time scale of several hundred years. Chemical neutralization of CO2 by reaction with CaCO3 on the sea floor accounts for another 9–15% decrease in the atmospheric concentration on a time scale of 5.5–6.8 kyr. Reaction with CaCO3 on land accounts for another 3–8%, with a time scale of 8.2 kyr. The final equilibrium with CaCO3 leaves 7.5–8% of the CO2 release remaining in the atmosphere. The carbonate chemistry of the oceans in contact with CaCO3 will act to buffer atmospheric CO2 at this higher concentration until the entire fossil fuel CO2 release is consumed by weathering of basic igneous rocks on a time scale of 200 kyr.”

So after 15,000 years we end up with CO2 levels = 280+ (0.08)*280 = 302 ppm. The remaining anthropogenic CO2 forcing works out at only 0.4 W/m2, whereas the drop in summer insolation over the Arctic between now and the next obliquity minimum is 22W/m2 . That is 50 times larger!

Following both the Eemian and the Anglian consequent glaciations all began with CO2 levels well over 280ppm. There is no reason to suppose that the Holocene will be any different, assuming that CO2 levels peak below 600 ppm this century.

Perhaps in 10,000 years time we will have learned to control the earth’s climate to the advantage of all life through managed CO2 emissions.

There again perhaps not.

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