Email 4133, April 2005, David Rind, NASA GISS: "what Mike Mann continually fails to understand, and no amount of references will solve, is that there is practically no reliable tropical data for most of the time period, and without knowing the tropical sensitivity, we have no way of knowing how cold (or warm) the globe actually got"

Email 4133

[David Rind, NASA GISS] 2) Concerning the hockey stick (which took up probably 3/4 of the review pages!): what Mike Mann continually fails to understand, and no amount of references will solve, is that there is practically no reliable tropical data for most of the time period, and without knowing the tropical sensitivity, we have no way of knowing how cold (or warm) the globe actually got. (And similarly, without knowing the tropical sensitivity for the LGM, we don't know what it's global cooling was, and without knowing it for 2xCO2, we don't know what the future sensitivity would be.) It cannot be reconstructed with any confidence from the extratropical response, even if we were to know that well, because the extratropical response is partly driven by in situ feedbacks, so can occur with a variety of tropical responses. [We have a paper in press (two papers, actually), discussing this aspect - I've actually sent them to IPCC and several of the chapter leads with respect to their discussions of AO/NAO variations with climate.] Therefore the detailed comments Mike provides concerning the extratropical issues - how much does snow cover alter the ground temperature versus the surface air temperature - are to some extent beside the point. I've made the comment to Mike several times, but it doesn't seem to get across - during the 20th century, according to Jim Hansen's temperature reconstruction, the tropical warming has been 60% of that in the extratropics (and that includes the amplifying AO/NAO extratropical change). I believe that in Mike's reconstruction, it averages about 30%. How well we know the numbers for the first part of this century is also somewhat uncertain, so I can't say Mike is wrong - but the point is, I don't know that he's right, nor do I think anybody else knows either.