The Making of the Hockey Stick - The Hockey Stick and the Climate Wars - page 28

IPCC had suddenly made proxy reconstructions of past temperature policy-relevant. The Lamb curve,
as we've seen, implicitly raised the issue of whether modern warming really was unusual; it seemed
at the time to resolve that proposition in the negative. Like any controversial and high-profile
scientific finding, the curve became a straw man for other scientists to either confirm, refine, or
reject. And indeed, other scientists would soon approach the issue using more up-to-date data and
more rigorous approaches.
The first truly quantitative reconstruction of past temperature changes at the hemispheric or
global scale was attempted by Ray Bradley and Phil Jones in 1993. 1 Their approach was perhaps
primitive by today's standards, but their contribution to our understanding was significant. The two
researchers assembled a set of about two dozen proxy records representing temperature variations in
distinct regions of the Northern Hemisphere (largely during the summer season), supplemented with
the few long historical temperature records available. They formed a composite of the proxy records
representing the average over all regions and then scaled the composite to match the scale of the
modern instrumental temperature record 2 to produce an estimate of Northern Hemisphere average
temperature back in time. The Bradley and Jones reconstruction stretched back to A.D. 1500 and was
adopted as the new standard in the 1995 IPCC Second Assessment Report, replacing—to the chagrin
of climate change contrarians—the considerably less quantitative or reliable Lamb curve of the 1990
report.
The Bradley and Jones reconstruction did not encompass the medieval period, but it did, for the
first time, characterize the extent of Northern Hemisphere average cooling during the period known as
the Little Ice Age. Despite the existence of greater cooling in some regions (e.g., Europe) at certain
times (e.g., the seventeenth century), the temperature changes recorded in the various proxy records
were not synchronous and, in some cases, were even of opposite sign. As a result, the average
cooling over the entire Northern Hemisphere at the height of the Little Ice Age was modest—less than
1°C cooler than the late twentieth century, nearly a factor of two smaller than what Lamb had
originally estimated for central England.
Numerous efforts were made in the ensuing years to advance the science further. In 1995,
Bradley collaborated with solar physicists Judith Lean and Juerg Beer to investigate the relative roles
that both natural and human factors might have played in the long-term temperature changes
documented by the Bradley and Jones reconstruction. 3 The group built on earlier studies by scientists
such as John Eddy, 4 who correlated estimated changes in solar output (derived from historical
sunspot measurements and radiocarbon data) 5 with Lamb's estimates of past temperature changes in
an attempt to deduce the impact on temperatures of an apparent lowering of solar output during the
peak of the European Little Ice Age in the latter half of the seventeenth century. During this period—
termed the “Maunder minimum,” after British astronomer Edward W. Maunder (1851-1928) who had
first studied this anomalous period—no sunspots were observed at all, implying a substantial
reduction in solar output at the time. Eddy concluded that there was a statistical connection between
the Maunder minimum and coincident changes in climate, including not only cooling temperatures in
Europe, but also shifting drought patterns in North America. With the data available at the time, he
was unable to provide a meaningful quantitative estimate of the effect.
Bradley and his collaborators revisited the problem with aid of the more extensive quantitative
estimates that were now available. They confirmed that the dip in solar output during the Maunder
minimum appeared responsible for a cooling of a bit less than 0.5 °C during the Little Ice Age. A