Climate Variability and Global Warming
Although uncertainties in global warming are many and varied, they are not as great as stated in the recent U.S. National Research Council (NRC) report ([1][1]). As Richard A. Kerr notes in his News of the Week article, President George W. Bush seized upon these uncertainties to justify the administration's limited response (“Bush backs spending for a ‘global problem,’” 15 Jun., p. [1978][2]). Specifically, Bush “emphasized that the contribution of natural climate variability to the past century's warming is uncertain,” to quote Kerr.
Unfortunately, in the NRC report, two aspects of natural climate variability are conflated. First, there is natural variability that is tied to external forcings, such as variations in the Sun, volcanoes, and the orbital variations of Earth around the Sun. The latter is the driving force for the major ice ages and interglacial periods. Second, there is natural variability that is internal to the climate system, arising, for instance, from interactions between the atmosphere and ocean, such as El Nino. This internal variability occurs even in an unchanging climate.
In the NRC report and in its summary, natural variability is said to be “quite large,” but both kinds of variability are treated as if they are internal. Glacial to interglacial swings are discussed without mention of the known causes. Several lines of evidence, from the instrumental and paleoclimate records ([2][3]) and from climate models ([3][4]), strongly suggest that the recent increase in global mean temperature is beyond that possible from internal processes and thus must be caused by an increase in heating. This reasoning also puts limits on how large aerosol cooling could be. Further, known causes such as changes in the Sun and volcanic activity in the past 50 years have, if anything, led to cooling in this interval, leaving only the human-caused increase in greenhouse gases as the culprit. This reasoning has also been quantitatively confirmed with climate models ([3][4], [4][5]).
A consequence of mistreatment of natural climate variability in the NRC report is that the caveats are overstated. Natural climate variability is dealt with much more thoroughly in the recent Intergovernmental Panel on Climate Change (IPCC) assessment ([4][5]), which was developed over about 3 years (versus 1 month for the NRC report). The summary from the IPCC is that “[t]here is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.”
1. [↵][6]Committee on the Science of Climate Change, Division of Earth and Life Studies, National Research Council, Climate Change Science: An Analysis of Some Key Questions (National Academy Press, Washington, DC, 2001). Available at .
2. [↵][7]1. M. E. Mann, 2. R. S. Bradley, 3. M. K. Hughes
, Geophys. Res. Lett. 26, 759 (1999).
[OpenUrl][8][CrossRef][9][Web of Science][10]
3. [↵][11]1. P. A. Stott 2. et al.
, Science 281, 2133 (2000).
[OpenUrl][12]
4. [↵][13]IPCC, Climate Change 2001: The Scientific Basis, J. T. Houghton et al., Eds. (Cambridge Univ. Press, Cambridge, in press).
[1]: #ref-1
[2]: /lookup/doi/10.1126/science.292.5524.1978
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[9]: /lookup/external-ref?access_num=10.1029/1999GL900070&link_type=DOI
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