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What triggers ice ages?

RealClimate – February 16, 2007

This is a moderately technical blog entry discussing an article in Climatic Change.  It is not too in depth though and the average interested reader should be able to understand it.  The premise of the article is a hypothesis that there needs to be more than just changes in the Earth’s orbit to trigger the entry into or out of an ice age.

Martinson & Pitman III’s hypothesis states that the fresh water input works in concert with the Milankovitch cycle and the albedo feedback. They conclude that ‘major’ terminations can only follow from glacial build-up of sufficient magnitude to isolate the Arctic, inhibiting the inflow of fresh water to the point that salinity buildup in the surface layer from slow but continuous growth of sea-ice, causes overturn of the Arctic (through the effect on the atmospheric circulation and the ocean currents). The vertical overturning brings warmer water up from below, setting conditions that are more favorable for ice melting.

…the hypothesis does not mention variations in the greenhouse gases (GHGs).

so far, the hypothesis proposed by Martinson and Pitman III is merely a speculation, and we are waiting to see if the hypothesis can be tested through numerical model experiments (which would require higher resolution sea-ice and ocean models than used in todays global climate models).

Greenhouse gases only contribute a couple of W/m2, vs. the seasonal Milankovich forcing of >40. For this new idea to have merit, it had better have heat fluxes at least on par with the radiative forcing from CO2.

It is tricky to completely isolate the individual causes because changes in GHG may produce altered cloud and sea ice distribution. But roughly speaking, if you do an LGM run and only reduce sea level, put in the ice sheets, change the vegetation, add some dust (though that one is still rough), then you get about 50% the way you want to go.

You can read the blog entry here.

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