Posted by Curt on 3 April, 2013 at 11:42 am. 2 comments already!


Rob Lyons @ Spiked:

The exposure of yet another dodgy piece of climate-change alarmism shows the need for serious scepticism.

‘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 inScience 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.

The real inconvenient truth – for Mann, Gore and the IPCC – is that the ‘hockey stick’ was a mirage. A Canadian geologist, Stephen McIntyre, received one of those government leaflets. Sceptical, he decided to investigate the methods and data behind the paper. He found that the data used relied heavily on tree-ring measurements from one particular kind of tree – American bristlecone pines – which showed a spurt in growth in the twentieth century that had nothing to do with climate change. Furthermore, there were errors in Mann’s methods such that even random data processed in this way could produce hockey-stick graphs. This was hardly the damning evidence of manmade influence on the climate it had been made out to be.

So no wonder alarm bells rang out when last month’s new hockey-stick graph emerged. Not only did it seem to confirm Mann’s original, but it was used in exactly the same way – as a STFU to climate sceptics who suggest that current, comparatively mild temperatures could be partly or wholly explained by natural trends that have nothing to do with human greenhouse gas emissions.

A press release from the National Science Foundation, which funded the research, claims the paper finds that ‘during the last 5,000 years, the Earth on average cooled about 1.3 degrees Fahrenheit [0.7 degrees Celsius] – until the last 100 years, when it warmed about 1.3 degrees Fahrenheit’. That’s an astonishing ‘bounceback’ in temperatures. In an article for the environmentalist website Grist, Marcott is quoted as saying: ‘What we found is that temperatures increased in the last 100 years as much as they had cooled in the last 6,000 or 7,000. In other words, the rate of change is much greater than anything we’ve seen in the whole Holocene.’

The Science paper was soon getting torn apart. One academic at the University of Nottingham, Paul Matthews, noted in acomment on Montford’s blog that the data relied upon ‘show no dramatic increase in the twentieth century’. Oddly, he pointed out, Marcott’s own PhD thesis on the same subject ‘uses the same data sets and plots similar graphs, but with no trace of any sharp increase’. Of course, the change in Marcott’s results could easily be the product of further work, but this disparity seems strange nonetheless.

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