IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis

9.1.3 The Basis from which We Begin

Evidence of a human influence on the recent evolution of the climate has accumulated steadily during the past two decades. The first IPCC Assessment Report (IPCC, 1990) contained little observational evidence of a detectable anthropogenic influence on climate. However, six years later the IPCC Working Group I SAR (IPCC, 1996) concluded that ‘the balance of evidence’ suggested there had been a ‘discernible’ human influence on the climate of the 20th century. Considerably more evidence accumulated during the subsequent five years, such that the TAR (IPCC, 2001) was able to draw a much stronger conclusion, not just on the detectability of a human influence, but on its contribution to climate change during the 20th century.

The evidence that was available at the time of the TAR was considerable. Using results from a range of detection studies of the instrumental record, which was assessed using fingerprints and estimates of internal climate variability from several climate models, it was found that the warming over the 20th century was ‘very unlikely to be due to internal variability alone as estimated by current models’.

Simulations of global mean 20th-century temperature change that accounted for anthropogenic greenhouse gases and sulphate aerosols as well as solar and volcanic forcing were found to be generally consistent with observations. In contrast, a limited number of simulations of the response to known natural forcings alone indicated that these may have contributed to the observed warming in the first half of the 20th century, but could not provide an adequate explanation of the warming in the second half of the 20th century, nor the observed changes in the vertical structure of the atmosphere.

Attribution studies had begun to use techniques to determine whether there was evidence that the responses to several different forcing agents were simultaneously present in observations, mainly of surface temperature and of temperature in the free atmosphere. A distinct greenhouse gas signal was found to be detectable whether or not other external influences were explicitly considered, and the amplitude of the simulated greenhouse gas response was generally found to be consistent with observationally based estimates on the scales that were considered. Also, in most studies, the estimated rate and magnitude of warming over the second half of the 20th century due to increasing greenhouse gas concentrations alone was comparable with, or larger than, the observed warming. This result was found to be robust to attempts to account for uncertainties, such as observational uncertainty and sampling error in estimates of the response to external forcing, as well as differences in assumptions and analysis techniques.

The TAR also reported on a range of evidence of qualitative consistencies between observed climate changes and model responses to anthropogenic forcing, including global temperature rise, increasing land-ocean temperature contrast, diminishing arctic sea ice extent, glacial retreat and increases in precipitation at high northern latitudes.

A number of uncertainties remained at the time of the TAR. For example, large uncertainties remained in estimates of internal climate variability. However, even substantially inflated (doubled or more) estimates of model-simulated internal variance were found unlikely to be large enough to nullify the detection of an anthropogenic influence on climate. Uncertainties in external forcing were also reported, particularly in anthropogenic aerosol, solar and volcanic forcing, and in the magnitude of the corresponding climate responses. These uncertainties contributed to uncertainties in detection and attribution studies. Particularly, estimates of the contribution to the 20th-century warming by natural forcings and anthropogenic forcings other than greenhouse gases showed some discrepancies with climate simulations and were model dependent. These results made it difficult to attribute the observed climate change to one specific combination of external influences.

Based on the available studies and understanding of the uncertainties, the TAR concluded that ‘in the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations’. Since the TAR, a larger number of model simulations using more complete forcings have become available, evidence on a wider range of variables has been analysed and many important uncertainties have been further explored and in many cases reduced. These advances are assessed in this chapter.