I’ve been thinking further about this knotty problem, which has vexed climatologists and glaciologists for many years. Finally the light has come on. A little while ago the Hockey Schtick site reported a new paper which had found that the Arctic ocean between 50,000 and 11,000 years ago was warmer than it is now by 1-2C to a considerable depth. The authors think it is due to a reduced fresh water flux.
I think it’s due to the glacial period frozen-year-round Arctic ocean surface providing a skull cap of insulation over the bulk of the Arctic ocean preventing it from losing heat to space.
This would mean that the warmer water circulating up in to the Arctic from the equator and sub-tropical ocean would not be able to lose heat as quickly as it acquires it, leading to a gradual buildup of energy. If the heat can’t escape upwards it will build up downwards. The authors note that this warmer water was extending down from around 1000 to around 2500m depth. That’s a lot of excess energy.
So when the Milankovitch cycles bring the configuration of Earth’s orbital parameters to the point where increased summertime insolation at high latitudes in the Northern Hemisphere ablates the Arctic ocean’s surface ice to the point where the warmer ocean below melts through and starts radiating to the cold Arctic night-time atmosphere, there is going to be a lot of energy released. This will generally warm the atmophere, leading to further melting, increased exposure of the ocean, decreased albedo on the land surface, and a rapid positive feedback. This might explain the familiar non-linear rise in temperature from glacial to interglacial.
The standard explanation – increased co2, is a non starter. There is not enough energy from the forcing to explain the speed at which the shift from glacial to interglacial conditions proceeds. Only Earth’s oceans are capable of buffering the energy required. Sea ice melts predominantly from beneath not from above, as rolling icebergs neatly demonstrate.
On one of our previous ice age discussions, I posted a plot comparing the 100,000 year timescale temperature estimates with the decdal scale temperature measurements. Here’s the plot:
At the time, people pointed out that the change in forcing provided by the Milankovitch cycles weren’t enough to account for the glacial-interglacial rapidity of change. I think this finding on the extra heat stored in the Arctic ocean during the glacial period solves the problem. Just like an El NIno is fuelled by warm water from the subsurface Pacific Warm Pool which was previously hidden from the surface temperature record spreading out over the surface, so the Glacial-Interglacial transition is fuelled by the warm intermediate layer releasing energy into the newly uncovered Arctic ocean surface.
Given that co2 levels don’t start to rise at the end of glacial periods until 800-2800 years after the temperature starts to increase, and the unconvincing argument that the co2 increase then amplifies the temperature increase to give us the 6-9C lift estimated to occur, I think the warm ocean hypothesis is the best explanation.
I find the co2 argument unconvincing because there just isn’t enough energy to do the job. Mile thick continent wide ice sheets don’t melt from above in a couple of thousand years due to slighty elevated air temperature caused by any co2 driven effect. An ocean releasing a massive amount of energy into the arctic atmosphere to equilibriate with it after the insulative barrier is removed is a far more plausible explanation in my opinion.