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DB2: An interesting e-mail from Tom Wigley to Keith Briffa dated June 30, 2000:

Paleo data cannot inform us *directly* about how the climate sensitivity (as climate sensitivity is defined). Note the stressed word. The whole point here is that the text cannot afford to make statements that are manifestly incorrect. This is *not* mere pedantry...

Quantifying climate sensitivity from real world data cannot even be done using present-day data, including satellite data. If you think that one could do better with paleo data, then you're fooling yourself.

I actually know Tom Wigley from NCAR in the 1990s. He is being very careful and precise with what he is saying here. I think he means is that it is impossible to recover the short-term response of the climate system (the standard definition) by direct (i.e., forward modeling) of paleoclimate data -- which is correct. Paleoclimate shows the long-term response of the climate system, which includes ice albedo and ocean circulation feedbacks that are not considered in the standard definition of climate sensitivity. Therefore, you can't extract the short-term response by direct means. Instead it is necessary to resort to modeling, such Hansen (2008, 2013) did by constructing simple ice sheet models and "working backwards" to infer that a climate sensivity of 3 +/- 1 C, with the ice albedo feedbacks folded in, reproduced the paleo data.

Keep in mind also that this e-mail was written 13 years ago, and a lot of work has been done since then. In particular, the really exquisite analysis of Parrenin et al (2013) is a game-changer in my opinion. I think we will see a lot more paleo data analyses in the near future.

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