The past million years

To go back before recorded human history, scientists have to rely on indirect methods to unravel much of the story of the past climate. A particularly valuable information source is the record stored in the ice that caps Greenland and the Antarctic continent. These ice caps are several thousands of metres thick. Snow deposited on their surface gradually becomes compacted as further snow falls, becoming solid ice. The ice moves steadily downwards, eventually flowing outwards at the bottom of the ice-sheet. Ice near the top of the layer will have been deposited fairly recently; ice near the bottom will have fallen on the surface many tens or hundreds of thousands of years ago. Analysis of the ice at different levels can, therefore, provide information about the conditions prevailing at different times in the past.

Deep cores have been drilled out of the ice at several locations in both Greenland and Antarctica. At Russia's Vostok station in east Antarctica, for instance, drilling has been carried out for over 25 years. The longest and most recent core reached a depth of over 3.5 km; the ice at the bottom of the hole fell as snow on the surface of the Antarctic continent well over half a million years ago (Figure 4.6b).

Small bubbles of air are trapped within the ice. Analysis of the composition of that air shows what was present in the atmosphere for the time at which the ice

Figure 4.6 (a) Variations over the last 160 000 years of polar temperature and atmospheric carbon dioxide concentrations derived from the Vostok ice core from Antarctica. It is estimated that the variation of global average temperature is about half that in the polar regions. Also shown is the current carbon dioxide concentration of about 380 ppm and the likely rise during the twenty-first century under various projections of its growth. (b) Variations of deuterium (SD), a proxy for local temperature; 918O, a proxy for global ice volume fluctuations; and the atmospheric concentrations of CO2 and CH4 derived from air trapped within ice cores from Antarctica. Shading indicates interglacial periods.

10"

"S

CO2 in 2100 (with business as usual)

Double pre-industrial CO2

Lowest possible CO2 stabilisation level by 2100

CO2 now

T"

Time (thousands of years)

T"

-700

-400

-300

Time (thousands of years)

1—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—r

600 500 400 300 200 100 0

Time (before 2005)

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  • Stanley
    What caused global warming in the past 1000000 years?
    3 months ago

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