I had a brief email exchange with Professor Severinghaus about Steve McIntyre’s recent post on his discussion with Mann and others about the divergence problem. I post it without comment, with permission and without emphasizing any of his words:

Dear James,

This is fascinating. I had no idea these emails were in the public domain.

In general Steve has gotten most of this right. There really is a problem

with the trees not being sensitive to temperature after about 1950. My

current best guess is that the higher CO2 since then has caused greater

warming at night (which is corroborated by minimum temperature trends,

since minimum temperatures usually occur at night). Trees respire more

at higher temperature, so they lose carbon when nights are warmer

than average. So their ring width has not increased as much as it would

have if the warming had been uniformly distributed over the diurnal cycle.

I think this is all published now so it should be possible to set the whole

record straight. But I did indeed feel at the time that Mike Mann had not

given me a straight answer. So if there is a response written, it won’t be

one defending Mike.

Jeff

Cheers,

James Padgett

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