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6.8.3. When Will Attribution Occur?

The fact that the cause (the enhanced greenhouse effect) and effect (global warming) have not unambiguously been linked leads to the question: when is this [attribution] likely to occur? Detection is not a simple yes/no issue. Rather, it involves the gradual accumulation of observational evidence in support of model predictions. The scientist's task is to reduce the uncertainties associated with the understanding of the climate system. Thus, while the ultimate objective of climate research is detection, it is really the climate sensitivity (section 2.8) one is interested in. A better understanding of this key parameter will not only allow one to argue with increased confidence that anthropogenic global warming is (or is not) occurring, but it will support prediction of future climate change, both anthropogenic and natural.

Figure 6.14 illustrates how the uncertainty in the climate sensitivity puts constraints on the timing of the detection issue. GCMs indicate that the climate sensitivity (the CO2 doubling temperature) lies in the region 1.5 to 4.5C. A high climate sensitivity will allow detectionArbitrarily, detection may be agreed to mean at least a 1oC warming since the late nineteenth century (i.e. another 0.5 degrees Celsius) (Wigley & Barnett, 1990). before 2010. Conversely, if it is low, detection may not occur for another 50 years.