Friday, April 5th 2013, 7:12 AM EDT
‘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.
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 a comment 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.
Another critic, Willis Eschenbach, argued that 10 per cent of the proxies used by Marcott et al did not qualify for inclusion even according to their own criteria: ‘A fine example of their choice of proxies can be seen in the fact that they’ve included a proxy which claims a cooling of about nine degrees in the last 10,000 years … although to be fair, they’ve also included some proxies that show seven degrees of warming over the same period.’ Could any sensible results be obtained from data that is clearly all over the place?
On Easter Sunday, Marcott and his colleagues published a response on the Real Climate blog. Most notable was this comment: ‘Our global paleotemperature reconstruction includes a so-called “uptick” in temperatures during the twentieth century. However, in the paper we make the point that this particular feature is of shorter duration than the inherent smoothing in our statistical averaging procedure, and that it is based on only a few available paleo-reconstructions of the type we used. Thus, the twentieth-century portion of our paleotemperature stack is not statistically robust, cannot be considered representative of global temperature changes, and therefore is not the basis of any of our conclusions.’
In other words, all that stuff about having the highest temperatures for millennia and about eye-popping warming over the past 100 years appears to have no basis in the paper’s actual temperature reconstruction. As climate policy expert Roger Pielke Jr points out, that correction/clarification needs to be trumpeted as loudly as the original claims were. Remove the twentieth-century portion of the reconstruction and it merely shows that temperatures have been falling overall for the past few thousand years. That doesn’t exactly support the case for urgent action on climate change, does it?
The lesson from this particular episode is the need for scepticism about any claim being made right now about the world’s climate. The issue is highly politicised, with each side in the argument leaping on every new study in an attempt to prove, once and for all, whether humans are influencing the climate and to what extent. The best antidote to such partisan thinking is the fullest and most open debate possible. Current temperatures, for example, might be unusual by historical standards, but that is something that needs to be demonstrated, not merely asserted.
Sadly, however, those who believe we must urgently clamp down on greenhouse-gas emissions have often demanded an end to debate. For example, back in 2006, green author Mark Lynas speculated about opponents of the climate-change consensus being put on trial: ‘I wonder what sentences judges might hand down at future international criminal tribunals on those who will be partially but directly responsible for millions of deaths from starvation, famine and disease in decades ahead. I put this in a similar moral category to Holocaust denial – except that this time the Holocaust is yet to come, and we still have time to avoid it. Those who try to ensure we don’t will one day have to answer for their crimes.’
David Roberts, a columnist for Grist, declared in 2006: ‘When we’ve finally gotten serious about global warming, when the impacts are really hitting us and we’re in a full worldwide scramble to minimise the damage, we should have war crimes trials for these bastards - some sort of climate Nuremberg.’ James Hansen, the godfather of the climate scare, declared that CEOs of companies that ‘dispute what is understood scientifically’ and ‘fund contrarians’ are guilty of ‘crimes against humanity’.
If we ever got to the kind of world suggested by these and numerous other environmentalists, we would just have to take as gospel the skewed reporting of research like the recent Science paper. Given that the kind of policies advocated by greens would be very costly, we need all the debate we can get. It is plausible that humans might be having an effect on climate, but plausible is not the same as true. Whether human influence is significant and demands urgent action is a question demanding unflinching and open-minded analysis and discussion. To turn round Lynas’s phrase, what if there is no Holocaust to come? Without such discussion, we’ll be left with bad science and bad policy - and the real damage will have been done by those clamouring for action.
Rob Lyons is deputy editor of spiked.
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