Attempts to explain the last million years of glacial cycles have concentrated mainly on the growth of Northern Ice sheets. Traditionally the summer insolation at 65N is thought to control the growth and extent of ice sheets, with interglacials coinciding with summer melt-back. A new paper ‘Hemispheric sea ice distribution sets the glacial tempo‘ argues instead that Antarctic sea-ice sets the timing of glaciations.
Variations in the earth’s orbit (Milankovitz cycles) change the intensity of polar summer insolation and consequent summer melt back. Obliquity affects both poles equally, whereas precession of the equinoxes is modulated by the earth’s eccentricity. The extent of sea ice in the Arctic is limited by the closeness of northern continents, whereas Antarctica is not. Lee et al. argue that at minimum summer insolation over Antarctica, amplified by high eccentricity, leads to a rapid growth in annual sea ice coverage in the southern hemisphere. This reduces the earth’s net albedo enough to initiate a new ice age leading consequently also to the growth in northern ice sheets. This asymmetry in the response of sea ice to precession between North and South poles is enough, they argue, to kick start a new ice age.
Currently perihelion coincides with the Antarctic summer insolation at a maximum. Despite this an asymmetry in sea ice cover averaged between 1980-2015 is evident.
|Sea ice extent (million km2)||NH||SH|
The authors use a climate model to calculate that June perihelion leads to sufficient increase in summer antarctic sea ice to reduce the net annual insolation for the earth, triggering a new ice age thus reducing albedo.
The paper does not discuss how ice ages end, but proposes that glaciations can only start when the precession term is amplified by maximum eccentricity leading to the 100,000y cycles. Until 1 million years ago obliquity alone drove the glacial cycle until the earth cooled below a threshold. This is a nice idea to explain the 100k problem but does it work? Here are the calculated summer maxima of insolation calculated from the work of Laskar et al.
One problem is that eccentricity itself follows a 400,000 years super cycle so that some summer minima in the Antarctic (dashed line) nearer minimum eccentricity are actually larger than others at maxima eccentricity. Obliquity affects both poles equally which is why it alone was sufficient to drive glacial cycles fro the previous 3 million years.
Now let’s see if minima in Antarctic insolation match up to the onset of glaciations.
There is indeed some correlation but it is not fully convincing. Basically the problem is that there is only a 12000y gap between maximum northern and maximum southern polar summer insolation. So why should only 1 in 6 southern minima initiate a new glacial cycle and likewise 1 in 6 northern minima herald a new interglacial? Perhaps only those minima coincident with minimum obliquity count. This leads to a slightly better correlation with cooling. If we now just look at the last glacial cycle and compare the Greenland (NP) ice data with that from Antarctica, there is further insight.
It is remarkable how similar the overall Arctic/Antarctic temperature dependence is. Whatever initiates a glaciation and terminates it is a global phenomena without significant time delay. This favours a global change in albedo and probably also connected heat flow from Antarctica to the Arctic and vice versa through the AMOC. Is the fall in temperature 12000 years ago in the Antarctic caused by by the fall in summer insolation? If so then the next glacial cycle will surely begin within 5000y as Antarctic summer insolation is currently in decline.
The new proposal in this paper is that sea ice growth is unhindered in the Antarctic Ocean but limited in extent in the Arctic. This produces an imbalance in global albedo whenever Antarctic summer insolation reaches an (eccentricity enhanced) minimum. However despite this nice idea, I am dubious that this paper is a breakthrough in understanding glacial cycles. It still seems to need the Arctic to end ice ages even if the Antarctic starts them. Yet something else is needed as well to short circuit those intermediate insolation minima which apparently have little effect until the every 100 ky eccentricity peaks. This is the same problem as relying on 65N insolation to pace glaciations.