Monday, January 11, 2010

4. Insights from Ice Cores

The figure shows a famous record that was obtained from an ice core, 3.6 kilometers (2.2 miles) deep, that was drilled at the Russian Vostok station in Antarctica. Analysis of the air trapped in tiny bubbles gives the ambient concentration of CO2 and the ratio of oxygen isotopes in the ice permits an estimate of the temperature. The lower blue curve shows the CO2 concentration (left scale, in parts per million by volume, or ppmv)) and the upper red curve the variations in temperature (right scale) during the last 420,000 years. The graph shows the present interglacial warm period (the Holocene) and the previous four interglacials during the Pleistocene epoch. Note that the average temperatures during previous interglacials were higher than at present. If the climate is now warming, it is merely returning to the interglacial norm.

At first glance, the strong correlation of the temperature and the CO2 concentration in this record supports the theory of anthropogenic global warming (AGW), but this is an illusion. Detailed examination reveals that the transitions from frigid glacial to warm interglacial conditions were often abrupt, taking only a few decades, and that the increase in CO2 followed rather than preceded the increase in temperature, with a typical delay of 800 years. The probable explanation is that the oceans released the gas when they warmed, after a long delay because of their thermal inertia. The CO2 remained high for many centuries after glacial conditions returned, presumably due to a decreasing volume of biomass as well as to slow uptake by the oceans.

The Intergovernmental Panel on Climate Change (IPCC( acknowledges these facts, but still claims that "it is very likely that glacial-interglacial CO2 variations have strongly amplified climate variations." If this were true, the temperature records should show accelerated warming when the CO2 increased, about 800 years after the onset of a deglaciation. The data show no evidence of such a rise, but it is of course impossible to exclude a small effect.

The ice core records, like the finding that only a fraction of the recent growth in CO2 is anthropogenic,  suggests that CO2 was a consequence and not a cause of global warming, and that its concentration had little or no effect on the variations in temperature. The data from the Pleistocene strengthen but are insufficiently detailed to prove the conclusion that the AGW doctrine is mistaken.

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