Wigley and Santer Find the IPCC is too Conservative on AGW [View all]

[joshcryer]

Wigley and Santer Find the IPCC is too Conservative on AGW
Tom Wigley and Ben Santer have published a new paper in Climate Dynamics entitled A probabilistic quantification of the anthropogenic component of twentieth century global warming (hereinafter WS12). The paper seeks to clarify this statement about human-caused global warming in the 2007 IPCC report:

"Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"

As WS12 notes, this statement has been criticized by various individuals, including Pat Michaels in testimony to US Congress and Judith Curry. Some of these criticisms stem from a failure to understand that the term "very likely" has a specific numerical definition, meaning greater than 90% probability. Some stem from the fact that the other terms used in the IPCC attribution statement like "most" and "mid-20th century" are somewhat vague.

...

"Here, the probability that the model-estimated GHG component of warming is greater than the entire observed trend (i.e., not just greater than ‘‘most’’ of the observed warming) is about 93%. Using IPCC terminology, therefore, it is very likely that GHG-induced warming is greater than the observed warming. Our conclusion is considerably stronger than the original IPCC statement."