Heads of the Hydra - The Hockey Stick and the Climate Wars - page 123

Third Assessment Report (TAR) of 2001, and, with temperatures trending upward and other climate
variables following suit, every additional year of data added to the overall significance of the
observed climate changes. This was particularly relevant for certain measurements, such as the extent
of summer sea ice in the Arctic, for which direct satellite observations only went back a few decades,
and for which every additional year of measurement highlighted the striking rate of decline.
Though encouraged by IPCC colleagues to take on the role of lead author for two different
chapters, I had chosen to sit out the AR4. Since my own scientific findings had come under attack by
climate change deniers, I felt it was important that the IPCC be able to provide a thoroughly
independent assessment of the underlying scientific issues. Moreover, I certainly didn't want to serve
as a lightning rod. I was satisfied to have served as an expert reviewer, providing constructive
criticism of the report. Unlike the previous report, which had combined all observations (whether
instrumental or paleoclimate) into a single observations chapter, AR4 devoted, for the first time, an
entire chapter to paleoclimate, including a substantial section devoted to the “The Last 2000 Years.”
It was a wise choice, as it allowed controversial matters to be aired more fully.
With an assessment of the existing literature that was both more thorough and more up-to-date
than the 2006 NAS assessment, the IPCC was in a position to come to more definitive conclusions
regarding the evidence from paleoclimate reconstructions. There were now more than a dozen
published reconstructions of Northern Hemisphere average temperature. They all came to a common
conclusion: The most recent decades were warmer than any comparable period in the past as far back
in time as the reconstructions went. Several of the reconstructions now extended further back than the
original hockey stick, as far back as two thousand years in some cases. 4 Moreover, a number of
reconstructions were based on fundamentally different sources of proxy information than those used in
the hockey stick, such as boreholes and, even more recently, information from mountain glaciers
around the world, yet still came to essentially the same conclusion. Such additional evidence led the
authors of the AR4 report to reach even stronger conclusions regarding the unprecedented nature of
recent warmth than either their earlier 2001 assessment or the 2006 NAS report:
The TAR pointed to the “exceptional warmth of the late 20th century, relative to the past
1,000 years.” Subsequent evidence has strengthened this conclusion. It is very likely that
average Northern Hemisphere temperatures during the second half of the 20th century were
higher than for any other 50-year period in the last 500 years. It is also likely that this 50-
year period was the warmest Northern Hemisphere period in the last 1.3 kyr [1300 years],
and that this warmth was more widespread than during any other 50-year period in the last
1.3 kyr. 5
Despite this strengthening of our original conclusions in the new IPCC report, in the distorted
up-is-down, black-is-white world of climate change denial, the IPCC had supposedly “dropped the
hockey stick”! 6 The claim was entirely false. The hockey stick graph simply was not featured in the
Summary for Policy Makers as it had been in the TAR. Why would it be? The purpose of the policy
maker's summary was to highlight key new findings, not old ones. The original MBH99 hockey stick
was indeed one of the dozen reconstructions shown in the IPCC paleoclimate chapter (see figure
12.2), which collectively formed the basis for the strengthened conclusions of AR4 regarding the