Working Group I: The Scientific Basis

Other reports in this collection Summary of optimal fingerprinting studies

Results from optimal fingerprint methods indicate a discernible human influence on climate in temperature observations at the surface and aloft and over a range of applications. These methods can also provide a quantitative estimate of the magnitude of this influence. The use of a number of forced climate signals, and the extensive treatment of various (but not all) sources of uncertainty increases our confidence that a considerable part of the recent warming can be attributed to anthropogenic influences. The estimated signals and scaling factors remain subject to the considerable uncertainty in our knowledge of historic climate forcing from sources other than greenhouse gases. While estimates of the amplitude of a single anthropogenic signal are quite consistent between different model signals (see Figures 12.10, 12.12) and different approaches, joint estimates of the amplitude of several signals vary between models and approaches. Thus quantitative separation of the observed warming into anthropogenic and naturally forced components requires considerable caution. Nonetheless, all recent studies reject natural forcing and internal variability alone as a possible explanation of recent climate change. Analyses based on a single anthropogenic signal focusing on continental and global scales indicate that:

  • Changes over the past 30 to 50 years are very unlikely to be due to internal variability as simulated by current models.
  • The combined response to greenhouse and sulphate forcing is more consistent with the observed record than the response to greenhouse gases alone.
  • Inclusion of the simulated response to stratospheric ozone depletion improves the simulation of the vertical structure of the response.

Analyses based on multiple anthropogenic and natural signals indicate that:

  • The combination of natural external forcing (solar and volcanic) and internal variability is unlikely to account for the spatio-temporal pattern of change over the past 30 to 50 years, even allowing for possible amplification of the amplitude of natural responses by unknown feedback processes.
  • Anthropogenic greenhouse gases are likely to have made a significant and substantial contribution to the warming observed over the second half of the 20th century, possibly larger than the total observed warming.
  • The contribution from anthropogenic sulphate aerosols is less clear, but appears to lie in a range broadly consistent with the spread of current model simulations. A high sulphate aerosol forcing is consistently associated with a stronger response to greenhouse forcing.
  • Natural external forcing may have contributed to the warming that occurred in the early 20th century.

Results based on variables other than continental and global scale temperature are more ambiguous.

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