IT is terribly unfashionable to admit it, but I’ve just never been able to believe that the late 20th Century was particularly warm. This admission despite ‘the hockey stick’ graph that featured so prominently in the United Nation’s third IPCC assessment report, and despite that amazing looking chart in Al Gore’s movie that also showed a recent spike in global temperatures relative to the last many thousand years.
My key problem with the ‘the hockey stick’ has always been that the upward spike representing runaway global warming in the 20th Century was never of the same stuff as the rest of the chart. That is the spike is largely based on the instrumental temperature record i.e. the thermometer record, while the downward trending line that it was grafted on to, is based on proxies, in particular estimates of temperature derived from studies of tree rings.
It has always, for me, been a case of Michael Mann comparing apples and oranges, or to put it another way sticking an apple on the end of a banana.
Worst the grafting was necessary because the proxy record, i.e. the tree ring record, shows that global temperatures have declined since about 1960.
Of course we know that global temperature haven’t declined since 1960, or thereabout, so there must be something wrong with the proxy record. This is known as “the divergence problem” and it is a problem, because if tree rings are not a good indicator of global temperature after 1960, how can they be a good indicator of global temperature prior to 1960?
Indeed there doesn’t appear to be a reliable method for reconstructing the last 100 or so years based on the standard techniques used to reconstruct the last 2,000, 4,000 and even 11,000 years of global temperature.
So when someone claims the past 10 years have been hotter than the past 11,300 years, as the Australian Broadcasting Commission did recently , there is good reason to cringe.
Of course the ABC didn’t make it up. They were reporting on the work of climate scientists recently published in a reputable journal. In particular a paper by Shaun Marcott and colleagues published in Science .
Sceptic, mathematician and blogger, Steve McIntyre, broke the original hockey stick into bits to do a thorough analysis, showing that the entire shaft, not just post 1960, was a fancy construct to create the impression of runaway global warming , and he’s done the same with this new Marcott fabrication .
While some who read this blog may cringe at my use of the word fabrication, it is more than justified because as Dr Marcott now admits in his own words:
“[T]he 20th 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. Our primary conclusions are based on a comparison of the longer term paleotemperature changes from our reconstruction with the well-documented temperature changes that have occurred over the last century, as documented by the instrumental record.”
In summary, Dr Marcott created the perception of a spike in temperatures the same way Michael Mann did in that first hockey stick paper that featured so prominently in the third IPCC report, by comparing apples and oranges… or perhaps best described as grafting an apple onto the end of a banana.
1. Earth on track to be hottest in human history: study . March 8, 2013
2. A reconstruction of regional and global temperature for the past 11,300 years. Marcott et al.. Science, Volume 339, No. 6124, pages 1198-1201.
Abstract: Surface temperature reconstructions of the past 1500 years suggest that recent warming is unprecedented in that time. Here we provide a broader perspective by reconstructing regional and global temperature anomalies for the past 11,300 years from 73 globally distributed records. Early Holocene (10,000 to 5000 years ago) warmth is followed by ~0.7°C cooling through the middle to late Holocene (
4. Response by Marcott et al. http://www.realclimate.org/index.php/archives/2013/03/response-by-marcott-et-al/