A new paper led by Potsdam authors has received much publicity because it claims that human emissions will delay the next ice age for 100,000y. This is based on model simulations which predict another glacial period starting naturally only in 50,000y time.
Using an ensemble of simulations generated by an Earth system model of intermediate complexity constrained by palaeoclimatic data, we suggest that glacial inception was narrowly missed before the beginning of the Industrial Revolution. The missed inception can be accounted for by the combined effect of relatively high late-Holocene CO2 concentrations and the low orbital eccentricity of the Earth7. Additionally, our analysis suggests that even in the absence of human perturbations no substantial build-up of ice sheets would occur within the next several thousand years and that the current interglacial would probably last for another 50,000 years. However, moderate anthropogenic cumulative CO2 emissions of 1,000 to 1,500 gigatonnes of carbon will postpone the next glacial inception by at least 100,000 years. Our simulations demonstrate that under natural conditions alone the Earth system would be expected to remain in the present delicately balanced interglacial climate state, steering clear of both large-scale glaciation of the Northern Hemisphere and its complete deglaciation, for an unusually long time.
Currently the earth’s orbit is in a low eccentricity cycle which is similar to the one 400,000 years ago. We can therefore make an analogy with that interglacial to determine when this one will end. The growth of northern ice sheets begins when summer insolation is insufficient to completely melt back winter expansion, starting within the arctic circle. An expanding ice surface reduces albedo thereby leading to less absorption of solar energy. This ice albedo feedback effect accelerates the growth of ice sheets, reducing CO2 levels until another ice age begins. Something else is needed to end deep glaciations. A likely mechanism is that described in the previous post. Low CO2 levels lead to the death of boreal forests, soil erosion and dust storms which deposited on the ice sheets break the ice-albedo feedback loop.
We are now 15,000 years into the current interglacial (12,000 if you exclude Younger Dryas). What will cause it to end ? The answer is a large reduction in summer insolation inside the arctic circle. Currently the precession of the equinox is unfavorable for northern summers since it coincides with aphelion (largest distance from the sun). We are lucky that the earth’s orbit is close to circular and obliquity low so that the effect is rather small, although the earth has indeed cooled a little since 8000 years ago. We narrowly avoided another glaciation a couple of hundred years ago. Now polar summer insolation is beginning to increase again.
The northern hemisphere will naturally warm slightly for about another 4000 years before another dip in summer insolation will slide us back into another glaciation. We can see when this will happen by comparing the current interglacial to that 400,000 years ago. The earth eccentricity follows a 420,000 year ‘amplification’ cycle superimposed on the 100,000 year cycle. Ice ages tend to follow the 100,000 year cycle but their strength depends on the super-cycle. About 400,000 years ago we had a similar pattern of eccentricity to the current one leading to weaker precession peaks. Changes in obliquity are constant and simply increase/decrease net radiation at both poles and extend the arctic/antarctic circles.
The arrow in figure 1 shows the interglacial 400Ky ago compared to average June/July insolation at the north pole (blue curve). Note how similar it is to the Holocene. It ended when summer insolation fell below 520 W/m2. Currently summer insolation is ~540 W/m2. Another precession dip will occur in about 15,000 years time when summer insolation will again fall below 520 W/m2 reaching 515 W/m2. This is when the next ice age is most likely to begin. The Potsdam authors of the new paper seem to want to ignore this dip preferring instead to wait another 50,000 years for a slightly bigger drop of ~511 W/m2. The aim of their paper though is to claim this too will be delayed (in their model) because we have pumped too much CO2 into the atmosphere !