"The 1990s have been warmer than any other comparable time period of the last 600 years." The origin of this oft-repeated statement is the highly-publicized paper of Mann et al. (1998) in which the authors use a few long historical and instrumental weather records together with various proxy climate indicators (such as tree-ring data, ice core data, corals, and varved sediment cores) to reconstruct the likely temperature history of the globe over the past six centuries. This data set, however, is not comprehensive enough to support this statement, nor are some of the direct and proxy climate indicators it uses free of biases that suggest warming where none, or much less, is actually present (see our Editorials of 15 June 2000, 1 July 2000, 15 July 2000, 2 August 2000, 9 August 2000). But even if it were true - which we fervently believe it is not - that observation would by no means validate climate model predictions of CO2-induced global warming, a purpose to which many people have applied it.
The warming of the last decade (or century for that matter) in conjunction with rising atmospheric CO2 concentrations is often offered as "evidence" that future increases in CO2 will result in catastrophic global warming. However, as we pointed out in our Volume 2 Number 7 Editorial CO2 and Temperature: The Great Geophysical Waltz and in our Journal Review CO2 and Temperature: Ice Core Correlations, CO2-temperature trends over a much longer period of time (250,000 years) indicate that, contrary to current climate model implications, this simplistic notion has little real-world data to support it. Following the penultimate deglaciation, for example, atmospheric CO2 concentrations exhibited no net change for approximately 15,000 years, during which period air temperatures dropped all the way back to values characteristic of glacial times (Fischer et al., 1999). Then, when CO2 finally began to decline, air temperatures remained constant for a few thousand years, after which they actually rose for about 6,000 years. And even when the two parameters increased in unison, as they did during the three most recent glacial terminations, temperature always rose first, followed by CO2 concentrations some 400 to 1,000 years later.
Clearly, the concomitant increase in atmospheric CO2 and air temperature over the last century or so proves nothing of a cause-and-effect nature. When all available CO2 and temperature records are analyzed, one can find much longer periods of absolutely no correlation and even opposing trends. Thus, the balance of evidence would tend to suggest that the recent century-long rise in temperatures is not driven by the ongoing rise in the air's CO2 content.
References
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.
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.