12.4.3.4 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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