Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E., and Stievenard, M. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399: 429-436.
What was done
The authors, partners in a long-term collaboration among Russia, the United States and France, retrieved the deepest ice core ever recovered - reaching a depth of 3,623 meters - from the Russian Vostok station in East Antarctica. By careful analysis of this historic ice core, they reconstructed trends of many climatic and environmental parameters, including temperature and CO2 concentration, over a period of 420,000 years.
What was learned
Over four glacial-interglacial cycles, the succession of changes through each cycle of glacial growth and termination was similar, with atmospheric and climatic properties oscillating between fairly stable lower and upper bounds. Surface temperature, for example, varied over a range of approximately 12°C, while atmospheric CO2 concentration ranged from a low of 180 ppm to a high of 290 ppm.
The authors note that "the new data confirm that the warmest temperature at stage 7.5 [238,000 years ago] was slightly warmer than the Holocene [the current interglacial]." They also note that the interglacials preceding and following the one at 238,000 years ago were warmer still. In fact, from the graphs they present, it can be seen that all of the four interglacials that preceded the Holocene were warmer than the current one, and by an average temperature in excess of 2°C.
The authors additionally found that (1) "the Holocene, which has already lasted 11,000 years, is, by far, the longest stable warm period recorded in Antarctica during the past 420,000 years," (2) "the climate record makes it unlikely that the West Antarctic ice sheet collapsed during the past 420,000 years," (3) "during glacial inception ... the CO2 decrease lags the temperature decrease by several thousand years," and (4) "the same sequence of climate forcing operated during each termination: orbital forcing followed by two strong amplifiers, greenhouse gases acting first, then deglaciation and ice-albedo feedback."
What it means
Noting that "there is a close correlation between Antarctic temperature and atmospheric concentrations of CO2 and CH4 [methane]," the authors conclude that "this discovery suggests that greenhouse gases are important as amplifiers of the initial orbital forcing and may have significantly contributed to the glacial-interglacial changes." They also note that "this correlation, together with the uniquely elevated concentrations of these gases today" -- which they correctly claim are "unprecedented during the past 420,000 years" -- is thus "of relevance with respect to the continuing debate on the future of earth's climate," leaving us with the impression that the rising CO2 content of the air may presage dire climatic consequences for the planet. Their actual findings, however, suggest just the opposite.
First of all, correlation implies nothing about causation. For purposes of attribution, it must be clearly demonstrated what is cause and what is effect; and in the real world of nature, a criterion that must always be met in this regard is that cause always comes first and effect always follows. And in checking the Vostok ice core record, the authors have rightly noted that "during glacial inception ... the CO2 decrease lags the temperature decrease by several thousand years." Furthermore, they also correctly note that "the same sequence of climate forcing operated during each termination: orbital forcing followed [our italics] by two strong amplifiers, greenhouse gases acting first, then deglaciation and ice-albedo feedback." Simply put, changes in orbital forcing start the temperature moving either up or down, which then makes the air's CO2 content either rise or fall. To say that the correlation of temperature and CO2 "suggests that greenhouse gases are important as amplifiers of the initial orbital forcing" is just not correct. In fact, the authors are forced to admit that the real reason for their making this claim is that "results from various climate simulations [our italics again] make it reasonable to assume [our italics yet again] that greenhouse gases have, at a global scale, contributed significantly to the globally averaged glacial-interglacial temperature change."
Some other interesting points: First, the preceding four interglacials were all warmer than the current one, even though the atmospheric CO2 concentration at those times was no higher, in the mean, than it was over the bulk of the Holocene. Hence, it is not illogical to expect that any global warming we may yet experience in the near future may be totally unrelated to the ongoing rise in the air's CO2 content. Second, even though the planet was considerably warmer during the preceding four interglacials, there is no evidence that the West Antarctic ice sheet ever collapsed during any of these warm periods. Shouldn't that make us feel secure instead of scared? Third, since the current interglacial is by far the longest stable warm period of the past 420,000 years, we are probably significantly overdue for the next ice age. Ought we not thus welcome the ongoing rise in the air's CO2 content, if we truly believe that it enhances global warming?
This paper will someday be realized by all concerned to be a major step forward in our quest to understand the dynamics of climatic and atmospheric change on earth, and this in spite of the authors' half-hearted attempts to tow the currently politically correct line with regard to this issue. The truth cannot be hid forever and will ultimately shine forth, as we believe they actually realize is occurring in this signal contribution to our understanding of things climatic.
Reviewed 15 June 1999