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The "Unprecedented" Surface Air Temperature of the Past Decade
Volume 3, Number 17: 9 August 2000

It has been claimed that the last decade of the 20th century was the warmest of the past hundred years and possibly the warmest of the entire past millennium (Mann et al., 1998, 1999). "Unprecedented" is the word that is typically used to convey this message. It has also been claimed that this observation - which we sincerely question (see our editorials of 15 June, 1 July, 15 July, and 2 August 2000) - is a cause for much concern, because the temperatures in question are supposedly so unprecedented. In fact, those who would have us believe that these putative high air temperatures are the result of anthropogenic CO2 emissions resulting from the burning of fossil fuels - which is essentially the same group of concerned individuals - contend that the surface air temperature of the planet is currently so uncommonly high that we must radically reformulate the energetic basis of the entire industrialized world to avoid a host of unwanted climatic consequences.

Forget for the moment that current mean global air temperature may not, in reality, be as high as is typically claimed. Forget also that this unique, or not-so-unique, air temperature level may not really be due to elevated levels of atmospheric CO2. Forget both of these very crucial points of controversy and concentrate solely on the word "unprecedented." Then ask yourself: Is this characterization correct?

As with most questions of this type, the answer depends upon one's vantage point and powers of observation. Are you near-sighted or far-sighted? Narrow-minded or broad-minded? Able to see the forest or blinded by the trees? In the case in point, it is our contention that those who are calling the current temperature regime of the planet unprecedented and, therefore, a sign of dangerous human interference, portending even worse things to come, are not acknowledging that from a broader perspective this characterization is simply not true.

Consider the findings of Petit et al. (1999), who retrieved the deepest ice core ever recovered from the Russian Vostok drilling station in East Antarctica and extracted a 420,000-year history of near-surface air temperature and atmospheric CO2 concentration from it. This record covers the current interglacial period in which we live, i.e., the Holocene, as well as the preceding four such climatic intervals. Why is this fact so important? Simply for what it tells us about the uniqueness or non-uniqueness of the Holocene; and that is that the current interglacial is by far the coolest of the five most recent such periods. In fact, the four interglacials that preceded the Holocene were, on average, more than 2C warmer than the one in which we currently live.

This observation raises some interesting questions. First, is the temperature of the last decade of the 20th century unprecedented in terms of typical interglacial temperatures? The answer, clearly, is that it is, but not in the way suggested by those who claim it is unusually warm. It is unprecedented in that it is unusually cool. Second, is the current "high" planetary temperature a sign of dangerous human interference? Clearly, it is not, for the attribution of a human influence is based on the supposition that the current temperature is uncommonly high (Crowley, 2000), when it has clearly been significantly warmer than it is now in all preceding interglacials for which we have good temperature data; and there is absolutely no one who would attribute those high temperatures to human influences. But can the higher temperatures of the preceding interglacials be attributed to higher CO2 concentrations caused by some non-human influence? Absolutely not, for atmospheric CO2 concentrations during all four prior interglacials never rose above approximately 290 ppm, whereas the air's CO2 concentration today stands at nearly 370 ppm.

Combining these two observations, we have a situation where, compared with the mean conditions of the preceding four interglacial periods, there is currently 80 ppm more CO2 in the air than there was then, and it is currently 2C colder than it was then. Now 80 ppm is a big change in CO2 concentration, just as 2C is a big change in mean global air temperature, which all adds up to one big discrepancy in what we are being told about the nature of the "unprecedented" situation in which we currently find ourselves: a situation where the large relative drop in atmospheric CO2 concentration of the previous four interglacials compared to our current situation is associated with those interglacials' large relative rise in air temperature compared to current conditions, which represents a climate change that is just the opposite of what the conventional CO2 greenhouse effect is supposed to produce.

Yes, things are unprecedented, all right, but not in the way the public is being led to believe.

Dr. Craig D. Idso
Dr. Keith E. Idso
Vice President

Crowley, T.J. 2000. Causes of climate change over the past 1000 years. Science 289: 270-276.

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.