‘Global
temperatures are warmer than at any time in at least 4,000 years… and over the
coming decades are likely to surpass levels not seen on the planet since before
the last ice age.’ That was the pithy message offered by New York Times eco-columnist Justin Gillis, reporting
on a new reconstruction of past global temperatures published in Science last month. The Atlantic was blunter: ‘We’re Screwed: 11,000
Years’ Worth of Climate Data Prove It.’
The Science paper is an
attempt to chart changes in global temperatures for the past 11,000 years. In
the absence of actual thermometer records any earlier than the late seventeenth
century, paleoclimatologists use ‘proxy’ data - things like tree rings - to
estimate changing temperatures. The researchers, led by Shaun Marcott of Oregon
State University, found ‘Early Holocene (10,000 to 5,000 years ago) warmth is
followed by ~0.7 degree Celsius cooling through the middle to late Holocene
(less than 5,000 years ago), culminating in the coolest temperatures of the
Holocene during the Little Ice Age, about 200 years ago’.
However, then things changed dramatically:
‘Current global temperatures of the past decade have not yet exceeded peak
interglacial values but are warmer than during ~75 per cent of the Holocene
temperature history.’ The accompanying graph of temperature changes, as shown in the Atlantic article, is startling. Temperatures
are more or less stable until just over 1,000 years ago, when a marked cooling
started. Then, after a recovery since the Little Ice Age, the line takes off
like a rocket in the twentieth century. What clearer evidence could there be
for manmade global warming?
The shape of the graph is very much like a
hockey stick on its side - long and straight with a sharp bend at the end - and
there has been plenty of past trouble caused by ‘hockey stick’ graphs. In 1999,
a paper published in Nature by Michael Mann and colleagues
suggested that the current period was the warmest in at least 1,000 years.
Mann’s graph also showed no Medieval Warm Period, previously assumed to have
been a spell of warmer weather in the years roughly from 900 to 1300. Such was
the impact of this paper that it become the centrepiece of the IPCC’s Third
Assessment Report in 2001 and got a major plug in Al Gore’s film, An Inconvenient Truth. As
Andrew Montford has noted previously
on spiked, the Canadian
government even sent out a leaflet to every household featuring the ‘hockey
stick’ graph.



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