6.8. Detection of Anthropogenic Global Warming
Sections 6.4. to 6.6. reviewed the anthropogenic increase in atmospheric concentrations of greenhouse gases, and the associated increase in greenhouse radiative forcing. Section 6.7 examined the observed climate changes, principally in surface temperature, that have taken place during the last 100 to 140 years. In this section, the evidence for a causal link between the anthropogenic increase in greenhouse radiative forcing and the observed global warming will be reviewed.
The word "detection" by climate scientists has been used to refer to the identification of the significant change in climate during the twentieth century and its association with the anthropogenically enhanced greenhouse effect. Until 1995 most reviews (MacCracken & Luther, 1985; Bolin et al., 1986; Wigley & Barnett, 1990) had concluded that the enhanced greenhouse effect has not yet been detected unequivocally in the observational record. However, they have also noted that the global-mean temperature change over the past 100 years is consistent with the greenhouse hypothesis. Since then, the most recent scientific report from the Intergovernmental Panel on Climate Change (IPCC, 1995) has proposed that the balance of [modelling] evidence suggests a discernible human influence on the global climate does exist.
Traditionally, the difficulty in greenhouse detection has arisen because there are numerous other causes of climatic variability (see chapter 2), and some of these [forcing mechanisms] may be operating on time scales (101 to 102 years) comparable to that of the anthropogenic greenhouse forcing. In addition, Wigley & Raper (1990) have shown that the inherent variability (random fluctuations) of the climate system can produce warming or cooling trends of up to 0.3°C per century. The detection problem can be conveniently described in terms of "signal" and "noise" (Maddan & Ramanathan, 1980). The signal here is the time-dependent climatic response to greenhouse forcing, whilst the noise is any non-greenhouse climate variation, either periodic, quasi-periodic or random.