Natural versus anthropogenic forcing

Since the IPCC (1990, 1992, 1995) scientific assessments, progress has been made towards identifying possible anthropogenic effects on climate. Firstly, model experiments are now incorporating the possible climatic effects of human-induced sulphate aerosols and stratospheric ozone. As a result, the potential climate change signal due to human activities is better defined, although several uncertainties still remain. Secondly, the background climatic variability has been better defined. This is crucial for distinguishing human effects on climate from natural climate variability on decadal to centennial time-scales, including both internal and external components (e.g. changes in solar variability or volcanic dust loading). Thirdly, some progress has been made in the application of pattern-based methods in an attempt to attribute some part of the observed climate changes to human activities (cause-effect relationship). The majority of the studies that have attempted to detect an anthropogenic effect on climate have used mean annual global temperature. Most of these studies indicate that the observed global temperature change during the last century is unlikely only to result from natural temperature fluctuations. However, the records cannot be considered clear evidence of anthropogenic forcing and changes in the Earth's surface temperature.

Recent studies have compared observations with the patterns of temperature change predicted by models as a response to anthropogenic forcing. The background for pattern-based approaches is that different forcing mechanisms show different response patterns. Some studies have compared patterns of temperature variations with model patterns from simulations using changes in CO2 and anthropogenic sulphate aerosols. The results presented in the IPCC (1995) report indicate that the record of mean global temperature over the past century is probably not entirely of natural origin and that there is a climate response to forcing by greenhouse gases and sulphate aerosols. The ability to quantify the magnitude of this effect is, however, limited as a result of uncertainties in longer-term natural climate variability and the forcing and response patterns to changes in greenhouse gases, aerosols and other human influences (IPCC, 1995 and references therein).

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