Climate Science

Understanding Past CO2

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Science  10 Aug 2012:
Vol. 337, Issue 6095, pp. 625
DOI: 10.1126/science.337.6095.625-b

Overall, atmospheric CO2 levels have decreased during the Cenozoic (the geological era that started 65 million years ago), coinciding with a broad cooling of Earth. Several prominent warm excursions, however, provide opportunities to calibrate the climate of CO2 and offer comparisons for understanding the long-term effect of human CO2 emissions. A requirement for such an analysis is a good understanding of past CO2 levels. One prominent warmer excursion occurred during the Miocene, about 15 million years ago, when temperatures are thought to have warmed by about 3°C and sea level rose as, it is thought, the growth of the Antarctic Ice Sheet abated. Foster et al. analyzed two deepsea boron isotopic ratio of seawater varies with pH, which in turn is infl uenced by atmospheric CO2 levels. Their data, broadly consistent with other measurements, imply that CO2 levels during the warmest part of the Miocene excursion were about 400 ppm, comparable to the levels that we have reached recently. Broad excursions in the global ice volume during the Miocene coupled with these modest atmospheric CO2 levels suggest that the Antarctic Ice Sheet was rather sensitive to this modest forcing at that time.


Earth Planet. Sci. Lett. 341—344, 243 (2012).

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