Working Group I: The Scientific Basis

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12.1.2 Summary of the First and Second Assessment Reports

The first IPCC Scientific Assessment in 1990 (IPCC, 1990) concluded that the global mean surface temperature had increased by 0.3 to 0.6°C over the previous 100 years and that the magnitude of this warming was broadly consistent with the predictions of climate models forced by increasing concentrations of greenhouse gases. However, it remained to be established that the observed warming (or part of it) could be attributed to the enhanced greenhouse effect. Some of the reasons for this were that there was only limited agreement between model predictions and observations, because climate models were still in the early stages of development; there was inadequate knowledge of natural variability and other possible anthropogenic effects on climate and there was a scarcity of suitable observational data, particularly long, reliable time-series.

By the time of the SAR in 1995, considerable progress had been made in attempts to identify an anthropogenic effect on climate. The first area of significant advance was that climate models were beginning to incorporate the possible climatic effects of human-induced changes in sulphate aerosols and stratospheric ozone. The second area of progress was in better defining the background variability of the climate system through multi-century model experiments that assumed no changes in forcing. These provided important information about the possible characteristics of the internal component of natural climate variability. The third area of progress was in the application of pattern-based methods that attempted to attribute some part of the observed changes in climate to human activities, although these studies were still in their infancy at that time.

The SAR judged that the observed trend in global climate over the previous 100 years was unlikely to be entirely natural in origin. This led to the following, now well-known, conclusion: “Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors. Nevertheless, the balance of evidence suggests that there is a discernible human influence on global climate”. It also noted that the magnitude of the influence was uncertain.

12.1.3 Developments since the Second Assessment Report

In the following sections, we assess research developments since the SAR in areas crucial to the detection of climate change and the attribution of its causes. First, in Section 12.2, we review advances in the different elements that are needed in any detection and attribution study, including observational data, estimates of internal climate variability, natural and anthropogenic climate forcings and their simulated responses, and statistical methods for comparing observed and modelled climate change. We draw heavily on the assessments in earlier chapters of this report, particularly Chapter 2 - Observed Climate Variability and Change, Chapter 6 - Radiative Forcing of Climate Change, Chapter 8 - Model Evaluation, and Chapter 9 - Projections of Future Climate Change.

In Section 12.3, a qualitative assessment is made of observed and modelled climate change, identifying general areas of agreement and difference. This is based on the observed climate changes identified with most confidence in Chapter 2 and the model projections of climate change from Chapter 9.

Next, in Section 12.4, advances obtained with quantitative methods for climate change detection and attribution are assessed. These include results obtained with time-series methods, pattern correlation methods, and optimal fingerprint methods. The interpretation of optimal fingerprinting as an estimation problem, finding the scaling factors required to bring the amplitude of model-simulated changes into agreement with observed changes, is discussed. Some remaining uncertainties are discussed in Section 12.5 and the key findings are drawn together in Section 12.6.

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