2011-03-04 10:36:28Milankovitch cycles initiated by southern winter insolation?
John Cook


The prevailing theory on what initiated deglaciations over the past million years was Northern summers - the timing of increasing summer insolation in the north matched the deglaciation cycle. This has always done my head in - deglaciation starts in the south so how can northern insolation be the initiator? A new paper might explain this:

Synchronicity of Antarctic temperatures and local solar insolation on orbital timescales

The Milankovitch theory states that global climate variability on orbital timescales from tens to hundreds of thousands of years is dominated by the summer insolation at high northern latitudes1, 2. The supporting evidence includes reconstructed air temperatures in Antarctica that are nearly in phase with boreal summer insolation and out of phase with local summer insolation3, 4, 5. Antarctic climate is therefore thought to be driven by northern summer insolation5. A clear mechanism that links the two hemispheres on orbital timescales is, however, missing. We propose that key Antarctic temperature records derived from ice cores are biased towards austral winter because of a seasonal cycle in snow accumulation. Using present-day estimates of this bias in the ‘recorder’ system, here we show that the local insolation can explain the orbital component of the temperature record without having to invoke a link to the Northern Hemisphere. Therefore, the Antarctic ice-core-derived temperature record, one of the best-dated records of the late Pleistocene temperature evolution, cannot be used to support or contradict the Milankovitch hypothesis that global climate changes are driven by Northern Hemisphere summer insolation variations.

I don't know if this is worth blogging about - it's a very technical, obscure subject - but it's been something I've been scratching my head about for a while so if it's true, I feel a bit more comfortable about my understanding of deglaciation.

2011-03-04 12:27:07Sounds fascinating
Daniel Bailey
Daniel Bailey

I'd offer, but I'm certain the maths are completely beyond me.

2011-06-24 04:44:39Been examining this too
John Garrett

Hi John,

I read this article with great interest only recently (I have  1-2 month lag on my reading of Nature). A couple things come to mind: 1) the reviewer pointed out that the effect from local antarctic conditions was small, less than the orbital forcing assumed and 2) milankovitch theory is also reflected in ocean sediments, which are thought to be less affected by local conditions. In other words, I didn't see it as a major threat to the milankovitch theory, but it did make me less secure about understanding the trigger to deglaciation. If I were to illustrate the trigger, I'd include the Atlantic MOC and show three connection points to the MOC: 1. northern deepwater and glacial melt, 2. the Algulhas leakage, and 3 the south polar front and ocean circulation. Another recent article described a poleward shift of the southern westerlies as allowing greater leakage of salt water into the MOC and this may be strengthening the MOC while freshwater into the north atlantic weekens. Just explaining the system and the areas of study in play may be an interesting post, though it would have no conclusion at the moment.


2011-06-24 05:17:38
Steve Brown


I'll be covering the role of the MOC during the Last Interglacial in part 4.  I'm not familiar with this Nature article, but I might be interested in following it up at a later date.

Two factors come to mind about Antarctica.  The Antarctic Circumpolar Current effectively cuts off Antarctica from the global climate system; it's why it has such a slow response to warming in interglacials.  Also, you need to be careful which species of foraminifer in the ocean sediment cores you are doing the proxy temperature analysis on.  Critters that live near the surface will show up localised surface temperature delta-180 readings, rather than a hemispheric or global temperature signal that you would expect from benthic forams.  I'm sure that anything that gets into Nature will have considered these.