of these various uncertainties, estimates of the value of the climate sensitivity cannot be improved by using observational data alone; the range of possible values for ∆T deduced in this way is substantially larger than the standard model-based range for ∆T, 1.5–4.5°C. It is, however, much more difficult to derive sensitivities below 1.5°C from the observational data than it is to obtain values above 4.5°C.

Where do we go from here? In the most recent IPCC report (31) and in an earlier report (34), it was noted that studies of global-mean temperature alone are insufficient to show a compelling cause–effect relationship between anthropogenic forcing and climate change. Such studies, as shown above, can demonstrate that the observed warming is consistent with a substantial anthropogenic effect on climate but cannot accurately quantify this effect. To show a cause–effect linkage, more sophisticated techniques are required that make use of the patterns of observed climate change, either in the near-surface horizontal (latitude/longitude) plane (39, 40) or in the vertical (zonal mean/height) plane ( 41 , 42 ). Such pattern-based studies have shown increasing and statistically significant similarities between model predictions and observed temperature changes. These results, combined with the evidence from global-mean analyses, provide convincing evidence for a discernible human influence on global climate; but further work is required to better quantify the magnitude of the human influence and reduce uncertainties in the climate sensitivity.

This work was supported by the U.S. Department of Energy (Grant DE-FG02-86ER60397) and the National Oceanic and Atmospheric Administration (Grant NA96AANAG0347). The National Center for Atmospheric Research is sponsored by the National Science Foundation. IPCC temperature data were provided by David Parker, U.K. Meteorological Office.

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