Towards the detection and attribution of an anthropogenic effect on climate.

Article Link

Santer, B. D., K. E. Taylor, T. M. L. Wigley, J. E. Penner, P. D. Jones and U. Cubasch, 1995. Towards the detection and attribution of an anthropogenic effect on climate. Climate Dynamics 12, 77-100.

Essay about this article

Santer, et al. test the hypothesis that regional-scale cooling due to anthropogenic sulfate aerosols may be partially obscuring a warming signal associated with increasing atmospheric CO2 concentration. They are attempting to identify climate change “fingerprints,” identify actual climate signals over and above natural variability and model “noise”, rule out alternative explanations, and identify future modeling and observational challenges. Going beyond the conclusion of the IPCC First Assessment Report (1990), that “The unequivocal detection of the enhanced greenhouse effect is not likely for a decade or more,” this study provides scientific support for the statement in the IPCC Second Assessment Report (1995), that ”The balance of evidence suggests a discernible human influence on global climate.”

Using statistics derived from two coupled atmosphere-ocean models, the authors show that model runs with changes in both CO2 and sulfate aerosol concentrations best emulate the actual climate over the past 50 years. In seeking to identify an anthropogenic signal emerging from both actual climate variability and model background “noise,” the authors acknowledge that the state of the climate modeling art does not make this possible. Although the cooling effects of sulfates are known in theory, this paper presents the first model-derived evidence that both global CO2 levels and regional sulfate aerosol concentrations are significant factors in determining the distribution and time evolution of Earth’s surface temperatures. Unsurprisingly, more factors than these two need to be taken into account.

Given that the “real world” has cooled in certain eras and in certain areas, the authors recommend that the significant and large effect of sulfate aerosols they identified in this study of model statistics be included in future climate simulations. Seasonal and regional cooling and warming patterns, however, still require further investigation. The authors find that Taylor and Penner (1994), the first model experiments to incorporate the basic atmospheric chemistry and climatic effects of sulfate aerosols, produced more varied temperature patterns than those typical of atmospheric GCMs coupled to mixed-layer oceans.

Model runs with increasing CO2 as the dominant influence on climate, that is with no other (potentially negative) forcings and negligible internal natural variability, generate strong multi-decadal positive trends. Since this is not in accord with observations, other factors must be in play. On the other hand, sulfate only model runs err in the opposite direction, giving an overall cooling pattern, which is also not realistic. Combining both CO2 and anthropogenic sulfate effects in the model brings the results closer in line with the observed temperatures of past and may provide some guidance on the evolution of temperature patterns in the future. The combined CO2 / sulfate runs demonstrate spatially coherent warming and cooling, with suppressed temperatures in the northern hemisphere likely due to the increased sulfur emissions and subsequent sulfate forcing that occurs there. These results are statistically significant when compared to model runs with no forcing, but only background noise.

Although CO2 radiative forcing is “relatively” well-known, the spatial and temporal aspects of sulfate aerosol forcing are not. Current models do not adequately handle atmospheric chemistry, ocean dynamics, and other trace gases and aerosols. Santer et al. express high confidence in an anthropogenic effect on climate but have not shown conclusively that it is due uniquely to the combination of anthropogenic CO2 and sulfate aerosols. Ongoing modeling efforts and observations are needed to examine (the authors said “eliminate”) other forcing factors, including solar variability and volcanic eruptions, but also potentially many more influences on the Earth’s radiation budget.

Additional readings:

Kiehl, J.T. and B.P. Bregleb, 1993. The relative role of sulfate aerosols and greenhose gases in climate forcing. Science 260, 311-314.

MacCracken, M. C., and H. Moses, 1982. The first detection of carbon dioxide effects: Workshop summary, 8-10 June 1981. Bull. Amer. Meteorol. Soc. 63, 1164-1178.

Taylor, K.E. and J.E. Penner, 1994. Response of the climate system to atmospheric aerosols and greenhouse gases. Nature 369, 734-737.

Discussion Questions

a. What is a climate change “fingerprint” and what is “noise?”

b. In what ways is climate change science a forensic undertaking?

c. Other factors, for example black carbon or soot, have been recently discovered to influence Earth’s heat budget. How are such new discoveries factored into climate change forensics?

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