The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 136

(a)

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0 25 50 75 100

Age (thousands of years ago)

(b)

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Age (thousands of years ago)

(a) 18 OIsotopeand(b)methanerecordsfromthenorthernandsouthernhemispheres(Greenlandand
Antarctica),smoothed(witha5000-yearilter).

Fig. 4.6

in generating this lag? The marine cores from the northern and southern Atlantic and
East Pacific indicate that the global thermohaline circulation was reduced (presumably
due to fresh meltwater in the North Atlantic from the melting northern ice sheets, as
we shall shortly see). The southern ocean warmed. About 300 years later atmospheric
carbon dioxide levels started to rise as the deep ocean warmed, releasing the gas. This
amplified the Milankovitch warming of the Earth. The thermohaline circulation then
switched back to its 'warm north' mode 14 700 years ago (or perhaps nearer 15 000
years ago). At this point with the thermohaline circulation switched on, increased
Milankovitch June solar energy and finally increased atmospheric carbon dioxide, the
fate of the northern hemisphere ice sheets was sealed. One other interesting aspect