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Detecting Climate Change

Introduction

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 "attribution" to the enhanced greenhouse effect due to man-made greenhouse gas emissions. Most reviews have noted that the global average temperature change over the past 100 years is consistent with the enhanced greenhouse effect theory. This fact sheet reviews the evidence for a causal link between the man-made enhancement of the greenhouse effect and the observed global warming.

The difficulty in "greenhouse detection and attribution" arises because there are numerous other causes of climatic variability, and some of these may be operating on time scales (10 to 100 years) comparable to that of the enhanced greenhouse effect. In addition, the inherent variability (random fluctuations) of the climate system can produce warming or cooling trends of up to 0.3C per century.

Greenhouse Modelling versus Observation

Global average temperature has increased by around 0.4 to 0.8C over the past 100 years. At the same time, greenhouse gas concentrations, and atmospheric aerosols emissions, have increased substantially. To assess whether the two are associated requires the use of computer model simulations of the likely climatic effects of the changing atmospheric composition, and the comparison of the results with observations.

The diagram below compares the observational global temperature record (solid line) with different model scenarios (dashed lines). The different model scenarios represent different possibilities of the sensitivity of the global climate. Results from this more simple (energy balance) model reveal similarities with the observations on the century time scale. On shorter time scales, these models have failed to reproduce the inter-decadal variability of the observational temperature record.

Modelling 20th Century Climate Change


More complex simulations of the observational temperature record use general circulation models (GCMs). Being more computationally complex than energy-balance models, GCMs can simulate both global average temperature changes and inter-regional differences. In addition, the climatic influence of both greenhouse gases and aerosols can be incorporated, and even the influence of some natural factors like changes in the amount of radiation from the sun. Finally, such GCMs can take account of the time delay between changes in the atmosphere and changes in the climate as a result of the climate system's background inertia. By taking into account the global cooling potential of man-made sulphate aerosols, other natural influences on the global climate, and the climate's inertia, GCMs simulate a rise in temperature close to the observed 0.6C. (If greenhouse gases alone were influencing climate, one would expect global temperatures to have risen by some 0.6 to 1.3C over the last 100 years.)

Such models have significantly increased the confidence in scientists' assertion that current global warming is due to an enhanced greenhouse effect. Indeed, in light of the most recent evidence, and taking into account the remaining uncertainties, the Intergovernmental Panel on Climate Change (IPCC) now argues that most of the observed warming in the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.

When Will Detection Occur?

Although the IPCC believe that recent climatic change is mostly likely the result of man-made activities, there are still some who maintain that an unambigous link between the two has yet to be fully proven. More critically, 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.

Conclusion

Today, it is now generally accepted amongst the scientific community that global warming has been linked to human activities. Whilst a few scientists still argue over the detection issue, recent improvements in climate modelling, in particular, the ability to simulate time-dependent climate changes and the cooling potential of atmospheric aerosols, have allowed the Intergovernmental Panel on Climate Change to argue that most of the observed warming in the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.