Climate Science ex Negativo?

I know fuck all about geophysics, but something in the recent BBC interview with Phil Jones, of "climategate" fame, struck me.

Most people quoted this bit (questions in bold, answers in normal type):
So, in answer to the question, the warming rates for all 4 periods [1860-1880, 1910-1940, 1975-1998 and 1975 to 2009] are similar and not statistically significantly different from each other.
But I found this more interesting:
H - If you agree that there were similar periods of warming since 1850 to the current period, and that the MWP is under debate, what factors convince you that recent warming has been largely man-made?

The fact that we can't explain the warming from the 1950s by solar and volcanic forcing - see my answer to your question D.

Here's question D and the full answer:
D - Do you agree that natural influences could have contributed significantly to the global warming observed from 1975-1998, and, if so, please could you specify each natural influence and express its radiative forcing over the period in Watts per square metre.

This area is slightly outside my area of expertise. When considering changes over this period we need to consider all possible factors (so human and natural influences as well as natural internal variability of the climate system). Natural influences (from volcanoes and the Sun) over this period could have contributed to the change over this period. Volcanic influences from the two large eruptions (El Chichon in 1982 and Pinatubo in 1991) would exert a negative influence. Solar influence was about flat over this period. Combining only these two natural influences, therefore, we might have expected some cooling over this period.

To reiterate, I have no understanding of the actual physics of this. But the argument that we know warming is man-made because we can't explain it with other factors we think are influences seems terribly weak. You might as well take that observation as an argument for divine intervention. And indeed this is just the kind of argument used by evolution skeptics: Inability to explain = Evidence for my favourite hypothesis. That, of course, is a fallacy.

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