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:
Analyses based on multiple anthropogenic and natural signals indicate that:
Results based on variables other than continental and global scale temperature are more ambiguous.
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