Heads of the Hydra - The Hockey Stick and the Climate Wars - page 131

than fifty of which extended back to A.D. 1000, and more than thirty back to A.D. 500. 45 Meanwhile,
we had been conducting extensive tests of statistical reconstruction methods using synthetic
(pseudoproxy) networks derived from climate model simulations built to have properties similar to
actual proxy data networks. 46 These tests demonstrated that, given the available proxy data, our
methods were likely to capture past temperature variations faithfully.
We were now in a position to address explicitly the main recommendations offered in the 2006
NAS report for furthering progress in paleoclimate reconstructions of the last two thousand years. We
had made use of a greatly expanded dataset, adequate for reconstructing past temperatures even
without the much-maligned tree ring data, and employed complementary methods that had been
thoroughly tested and validated with model simulation experiments. We used two different statistical
approaches—the regularized-expectation maximization method we had been working with in recent
years, 47 and the simpler composite approach discussed in chapter four —neither of which employed
the PCA centering at the heart of McIntyre's attack on the original hockey stick. We published our
hemispheric temperature reconstructions in the Proceedings of the National Academy of Sciences
( PNAS ) in early fall 2008. 48 A year later, we published a follow-up article in Science describing the
spatial patterns of past temperature changes. 49
With far more ice core and sediment records now available, we were able to obtain a
meaningful reconstruction of the Northern Hemisphere average temperature for the past thirteen
hundred years without using tree ring data at all. If tree ring data were used, the reconstruction could
be extended, with some reservations, back over the past seventeen hundred years. 50 The amplitude of
century-to-century variations in the new reconstruction were somewhat larger than in other previously
published reconstructions (including the original hockey stick), with somewhat greater peak medieval
warmth. 51 That warmth, however, still did not approach the warmth of the most recent decades.
Recent warmth, it now at least tentatively appeared, was unprecedented for nearly the past two
millennia, and perhaps longer.
Stephen McIntyre wasted little time in launching a series of attacks on the PNAS paper,
employing—it would seem—the strategy of throwing as much mud against the wall as possible and
hoping that some would stick. Teaming up with his former coauthor Ross McKitrick, he submitted a
short letter to the editor of PNAS claiming that our reconstruction used “upside down proxy data.” 52
That was nonsensical; as we pointed out in our response, 53 one of our methods didn't assume any
orientation, while the other used an objective procedure for determining it. 54
McIntyre also appealed to the conclusions of the 2006 NAS report to claim that our continued
use of the very long bristlecone pine tree ring records was inappropriate. Yet this was a
misrepresentation of what the NAS had concluded. The NAS panel expressed some concerns about
so-called strip bark tree ring records, which include many of the long-lived bristlecone pines. These
trees grow at very high CO 2 -limited elevations, and there is the possibility that increases in growth
over the past two centuries may not be driven entirely by climate, but also by the phenomenon of CO 2
fertilization—something that had been called attention to and dealt with in MBH99 (see chapter 4 ).
The NAS report simply recommended efforts to better understand any potential biases by “performing
experimental studies on biophysical relationships between temperature and tree ring parameters.”
Such would be the focus of a paper published in PNAS by Mathew Salzer and coauthors the following
year, 55 demonstrating that the much-maligned bristlecone pines were good temperature proxy records