In an effort to understand present climate and how increases in anthropogenic CO2 emissions may impact future climate, scientists often look for clues in climates of epochs past. One such clue is the persistence of millennial-scale temperature oscillations throughout the Pleistocene (see Raymo et al., 1998 and Climate Oscillations in our Subject index), where variations of 3 to 4.5°C are observed during glacial periods and variations of 0.5 to 1°C are observed during interglacials (Oppo et al., 1998).
Other clues have been found in proxy temperature and CO2 records of the Miocene and Eocene epochs. Working with sediment cores from three deep sea drilling sites,
Pagani et al. (1999) reconstructed a history of atmospheric CO2 concentration over the early to late Miocene (25 to 9 million years ago), finding that atmospheric CO2 concentrations in the Miocene were similar to concentrations observed during the Pleistocene, i.e., 180 to 290 ppm, but that at the height of the Miocene climatic optimum (approximately 17 million years ago), deep water and high-latitude surface water temperatures were as much as 6°C warmer than they are today. Thus, the authors state that the "uniformly low" concentration of atmospheric CO2 during the Miocene "appears in conflict with greenhouse theories of climate change." They also report "there is no evidence for a sharp decline in [atmospheric] CO2 associated with EAIS [East Antarctic Ice Sheet] expansion" during the Miocene and that "atmospheric carbon dioxide rises following the expansion of EAIS," which findings are also in conflict with the greenhouse theory of climate change.
With regard to the climate of the Eocene (55 to 35 million years ago), a similar decoupling of temperature and atmospheric CO2 is reported (Pearson and Palmer, 2000). Once again, at a time when temperatures have been estimated to have been as much as 5°C warmer than today, atmospheric CO2 concentrations were determined to lie between 180 and 550 ppm, with a best estimate of 385 ppm (Pearson and Palmer, 1999). For those searching for clues as to how future climate may be impacted by increases in atmospheric carbon dioxide from the great climate epochs of the past, it would thus appear, in the words of paleoclimatologist Thomas Crowley, as quoted in Science (Vol. 284, p. 1745), that "it could be the whole carbon dioxide paradigm is crumbling," at least, as news writer Richard Kerr adds, "when it comes to explaining very long-term climate change."
References
Oppo, D.W., McManus, J.F. and Cullen, J.L. 1998. Abrupt climate events 500,000 to 340,000 years ago: Evidence from subpolar North Atlantic sediments. Science 279: 1335-1338.
Pagani, M., Authur, M.A. and Freeman, K.H. 1999. Miocene evolution of atmospheric carbon dioxide. Paleoceanography 14: 273-292.
Pearson, P.N. and Palmer, M.R. 1999. Middle Eocene seawater pH and atmospheric carbon dioxide concentrations. Science 284: 1824-1826.
Raymo, M.E., Ganley, K., Carter, S., Oppo, D.W. and McManus, J. 1998. Millennial-scale climate instability during the early Pleistocene epoch. Nature 392: 699-702.