Throughout earth's geologic history, major ice ages have occurred over and over again, with ten of them affecting the planet in the past one million years and another ten in the million or so years before that. Each one persists for about 90,000 years, after which it is followed by an approximate 10,000-year interglacial. [See Earth's Climatic History: The Last 2,000,000 Years for references and additional information regarding this topic.]
Our understanding of climatic conditions during glacial-interglacial cycles is limited; but a constant flow of new studies is providing ever more information about the topic. This growing body of knowledge is helpful, for it gives us a long historical baseline against which the uniqueness of the current state of earth's climate may be compared, thereby enabling us to better understand the significance of current climate trends and determine their causes.
It has been claimed by certain scientists, for example, that the ongoing rise in the air's CO2 content actually made the decade of the 1990s the warmest period of the entire past millennium (Mann et al., 1998, 1999). "Unprecedented" is the word proponents of this idea routinely use to describe the current temperature of the globe, which appellation we "hotly" contest (see our Editorials of 15 June, 1 July, 15 July and 2 August 2000); and, in fact, when the mean temperature of the 1990s is compared with the warmest temperatures of the four prior interglacials (for which we have high-quality reconstructed temperature records), the 1990s are found to have been much cooler than all of these other periods.
Petit et al. (1999), for example, reconstructed a temperature history from the Vostok ice core in East Antarctica that spans the past 420,000 years, determining 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 in their paper, it can be seen that all four of the interglacials that preceded the Holocene were warmer than the current one, and by an average temperature in excess of 2°C.
Similar findings have been obtained from the Dome Fuji ice core, which was extracted from a site in an entirely different sector of East Antarctica that is separated from the Vostok ice-core site by 1500 km (Watanabe et al., 2003). Although of somewhat shorter duration and, therefore, covering only the last three glacial-interglacial periods (marine stages 5.5, 7.5 and 9.3), this independent proxy temperature record also reveals that the last three interglacials, in the words of Watanabe et al., "were much warmer than the most recent 1,000 years (~4.5°C for stage 5.5 and up to 6°C for stage 9.3)."
This prior interglacial warmth is also evident in a 550,000-year sea surface temperature (SST) data set derived by Herbert et al. (2001) from marine sediment cores obtained along the western coast of North America, from around 22°N latitude at the southern tip of the Baja Peninsula to around 42°N latitude off the coast of Oregon. According to this reconstructed SST data set, in the words of the scientists who developed it, "the previous interglacial (isotope stage 5e) produced surface waters several degrees warmer than today," such that "waters as warm as those now at Santa Barbara occurred along the Oregon margin." Furthermore, as can readily be seen from the SST data in Figure 2 of their paper, SSTs for this region in the current interglacial have not reached the warm peaks witnessed in all four of the preceding interglacial periods, falling short by a margin of 1 to 4°C.
Thus, over the past half-million years, and within the context of the most recent five full interglacials, it is clear that the average near-surface air temperature of the earth during the 1990s was not unusually warm, but unusually cool. And not even the much greater atmospheric CO2 content of the latter period was able to reverse this incongruity, which only exists, of course, in the minds of those who believe we understand earth's climate system well enough to have absolute confidence in the output of today's climate models.
This observation raises an interesting question. Is earth's current temperature indicative of dangerous human interference with the planet's thermoregulatory system? Clearly, it is not; for the climate alarmists' claim of a human influence on today's climate is based solely on their contention that earth's current temperature is uncommonly high (Crowley, 2000), when it clearly is not. The earth was significantly warmer than it is today in all of the preceding interglacials for which we have good temperature data; and there is absolutely no one who would attribute those high temperatures of the past to human influence or even natural increases in the air's CO2 content, for the atmospheric CO2 concentration during all four prior interglacials never rose above approximately 290 ppm.
The supposed causal relationship between earth's atmospheric CO2 concentration and global temperature has been further weakened by the studies of Fischer et al. (1999), Indermuhle et al. (1999), and Stephens and Keeling (2000). In the Fischer et al. study, records of atmospheric CO2 and air temperature derived from Antarctic ice cores were examined across a quarter of a million years. Over this immense time span, the three most dramatic warming events were those associated with the terminations of the last three ice ages; and for each of these tremendous global warmings, earth's air temperature rose well before there was any increase in atmospheric CO2. In fact, the air's CO2 content did not begin to rise until 400 to 1,000 years after the planet began to warm.
Clearly, increases in the air's CO2 content did not trigger these massive changes in climate. In addition, there was a 15,000-year period following one of the glacial terminations when the air's CO2 content was essentially constant but air temperatures dropped all the way down to values characteristic of glacial times. Also, just as increases in atmospheric CO2 did not trigger any of the major global warmings that led to the demise of the last three ice ages, neither was the induction of the most recent ice age driven by a decrease in CO2. And when the air's CO2 content finally did begin to drop after the last ice age was fully established, air temperatures either remained fairly constant or actually rose, doing just the opposite of what climate models suggest should have happened if changes in atmospheric CO2 drive changes in climate.
In much the same vein, Indermuhle et al. (1999) determined that after the termination of the last great ice age, the CO2 content of the air gradually rose by approximately 25 ppm in almost linear fashion between 8,200 and 1,200 years ago, over a period of time that saw a slow but steady decline in global air temperature, which is once again just the opposite of what would be expected if changes in atmospheric CO2 affect climate in the way affirmed by the popular CO2-greenhouse effect theory.
Another problem for this theory arises from the work of Stephens and Keeling (2000), who in a study of the influence of Antarctic sea ice on glacial-interglacial CO2 variations, proposed a mechanism to explain the observed synchrony between Antarctic temperature and atmospheric CO2 concentration during glacial-interglacial transitions. Their mechanism presumes that temperature is the independent variable that alters sea ice extent, which then alters the sea-to-air CO2 flux in the high-latitude region of the Southern Ocean and consequently changes the CO2 content of the atmosphere. Hence, in their explanation for the gross correlation of CO2 and air temperature over glacial-interglacial cycles, atmospheric CO2 variations are clearly the result of temperature variations, and not vice versa.
Other studies have revealed yet other things related to the current concern over potential global warming. Ding et al. (1999), for example, developed a high-resolution record of climate changes over the last two glacial-interglacial cycles based on a study of grain sizes in soil cores removed from sections of the northwestern part of the Chinese Loess Plateau. These scientists detected the presence of large-amplitude millennial-scale climatic oscillations over both of the previous glacial periods, but very little such variation during the prior interglacial. Greater climatic stability during warmer interglacial periods has also been reported by Shemesh et al. (2001) and Alley (2000). Hence, because periods such as that in which we now live, i.e., interglacials, appear to be much more climatically stable than cooler glacial periods, we will likely not experience a significant increase in extreme weather events of the type routinely predicted by climate alarmists to accompany global warming. And especially should we not experience rapid climate changes of the type suggested to become more likely by Alley et al. (2002, 2003), as described in our Editorials of 2 April 2003 and 23 April 2003.
Finally, the isotopic and palynological study of a lacustrine sequence derived from a core obtained on the flank of the Pindus Mountain Range in northwest Greece has revealed that the development of the last great glacial period occurred about 10,800 years after the beginning of the last interglacial period (Frogley et al., 1999). As this is approximately the age of the current interglacial, we would appear to be scheduled to experience the start of a new ice age any day now, geologically speaking. Yet we obsess about global warming. Is this healthy? Is it wise? Is it even rational? Surely, the study of glacial-interglacial cycles can provide us much food for thought about current climate concerns.
Alley, R.B. 2000. Ice-core evidence of abrupt climate changes. Proceedings of the National Academy of Sciences USA 97: 1331-1334.
Alley, R.B., Marotzke, J., Nordhaus, W.D., Overpeck, J.T., Peteet, D.M., Pielke Jr., R.A., Pierrehumbert, R.T., Rhines, P.B., Stocker, T.F., Talley, L.D. and Wallace, J.M. 2002. Abrupt Climate Change: Inevitable Surprises. National Research Council, National Academy Press, Washington, DC.
Alley, R.B., Marotzke, J., Nordhaus, W.D., Overpeck, J.T., Peteet, D.M., Pielke Jr., R.A., Pierrehumbert, R.T., Rhines, P.B., Stocker, T.F., Talley, L.D. and Wallace, J.M. 2003. Abrupt climate change. Science 299: 2005-2010.
Crowley, T.J. 2000. Causes of climate change over the past 1000 years. Science 289: 270-276.
Ding, Z.L., Ren, J.Z., Yang, S.L. and Liu, T.S. 1999. Climate instability during the penultimate glaciation: Evidence from two high-resolution loess records, China. Journal of Geophysical Research 104: 20,123-20,132.
Fischer, H., Wahlen, M., Smith, J., Mastroianni, D. and Deck B. 1999. Ice core records of atmospheric CO2 around the last three glacial terminations. Science 283: 1712-1714.
Frogley, M.R., Tzedakis, P.C. and Heaton, T.H.E. 1999. Climate variability in Northwest Greece during the last interglacial. Science 285: 1886-1889.
Herbert, T.D., Schuffert, J.D., Andreasen, D., Heusser, L., Lyle, M., Mix, A., Ravelo, A.C., Stott, L.D. and Herguera, J.C. 2001. Collapse of the California Current during glacial maxima linked to climate change on land. Science 293: 71-76.
Indermuhle, A., Stocker, T.F., Joos, F., Fischer, H., Smith, H.J., Wahlen, M., Deck, B., Mastroianni, D., Tschumi, J., Blunier, T., Meyer, R. and Stauffer, B. 1999. Holocene carbon-cycle dynamics based on CO2 trapped in ice at Taylor Dome, Antarctica. Nature 398: 121-126.
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties and limitations. Geophysical Research Letters 26: 759-762.
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.
Shemesh, A., Rietti-Shati, M., Rioual, P., Battarbee, R., de Beaulieu, J.-L., Reille, M., Andrieu, V. and Svobodova, H. 2001. An oxygen isotope record of lacustrine opal from a European maar indicates climatic stability during the last interglacial. Geophysical Research Letters 28: 2305-2308.
Stephens, B.B. and Keeling, R.F. 2000. The influence of Antarctic sea ice on glacial-interglacial CO2 variations. Nature 404: 171-174.
Watanabe, O., Jouzel, J., Johnsen, S., Parrenin, F., Shoji, H. and Yoshida, N. 2003. Homogeneous climate variability across East Antarctica over the past three glacial cycles. Nature 422: 509-512.