“.But there are real questions to be asked of the paleo
reconstruction. First, I should point out that we calibrated versus
1902-1980, then “verified” the approach using an independent data set for
1854-1901. The results were good, giving me confidence that if we had a
comparable proxy data set for post-1980 (we don’t!) our proxy-based
reconstruction would capture that period well. Unfortunately, the proxy
network we used has not been updated, and furthermore there are many/some/
tree ring sites where there has been a “decoupling” between the long-term
relationship between climate and tree growth, so that things fall apart in
recent decades.this makes it very difficult to demonstrate what I just
claimed. We can only call on evidence from many other proxies for
“unprecedented” states in recent years (e.g. glaciers, isotopes in tropical
ice etc..). But there are (at least) two other problems — Keith Briffa
points out that the very strong trend in the 20th century calibration
period accounts for much of the success of our calibration and makes it
unlikely that we would be able be able to reconstruct such an extraordinary
period as the 1990s with much success (I may be mis-quoting him somewhat,
but that is the general thrust of his criticism). Indeed, in the
verification period, the biggest “miss” was an apparently very warm year in
the late 19th century that we did not get right at all. This makes
criticisms of the “antis” difficult to respond to (they have not yet risen
to this level of sophistication, but they are “on the scent”).
Furthermore, it may be that Mann et al simply don’t have the
long-term trend right, due to underestimation of low frequency info. in the
(very few) proxies that we used. We tried to demonstrate that this was not
a problem of the tree ring data we used by re-running the reconstruction
with & without tree rings, and indeed the two efforts were very similar –
but we could only do this back to about 1700. Whether we have the 1000
year trend right is far less certain (& one reason why I hedge my bets on
whether there were any periods in Medieval times that might have been
“warm”, to the irritation of my co-authors!). So, possibly if you crank up
the trend over 1000 years, you find that the envelope of uncertainty is
comparable with at least some of the future scenarios, which of course begs
the question as to what the likely forcing was 1000 years ago.”
