Global Warming: The "Evidence" Begins to Unravel

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The 'Hockey Stick' graph - compared with a version with more data

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uploaded by Colonel Boyle

One of the most iconic images in the climate debate over the last 10 years has been the so-called "hockey stick" graph. This image, published countless times by organisations from the Intergovernmental Panel on Climate Change (IPCC) on down has been one of the backbones of the case that the 20th Century saw a surge in temperature that mirrored the increase in atmospheric CO2 caused by increasing industrialisation.

However, there has been a long standing controversy over the methodology used to compile the data behind the hockey stick. Essentially, there are no real records of global temperature for most of the world prior to the satellite era. Countries such as the USA and Britain have a relatively long record of keeping historical records of temperature measured by local weather stations, but the area they cover in terms of both geography and time is very limited in the context of the whole planet.

Consequently, when reconstructing past temperatures, scientists have had to rely on 'proxies'. A proxy is something that stands in for an actual measurements. The basis of the hockey stick graph were measurements taken from tree ring growth in ancient timbers (in many cases fossilised). By observing the growth rates of trees against average temperatures, samples of trees that can be many thousands of years old were used to create a picture of climate on a year-by-year graph - which resulted in the effect we now call the hockey stick.

The use of tree rings to measure past temperature - properly known as dendrochronology - has long been problematical. Because any individual tree or batch of trees can be affected by local conditions such as changes in soil quality, rainfall or overall tree cover the use of tree rings as a proxy for temperature needs a big sample of trees over a wide area in order to properly representative.

Today's most prevalent version of the hockey stick graph originates from the work of a UK scientist - Keith Briffa. Briffa's work was based on a sample of tree ring data from the Yamal region of Siberia, which dates back well over a thousand years, and should therefore be an excellent source of data.

However, following the publication of his iconic graph in 2000, some scientists wanted to see the actual source data. In science, publication of raw data behind a result is normal process, as it gives other scientists the chance and means to validate, replicate or falsify the result. For various reasons given by Briffa and the people and organisations for which he worked, this data was not published until the Philosophical Transactions of the Royal Society published updates of Briffa's work this year. This publication makes all published authors submit their raw data alongside their research as part of policy and so, for the first time, other climate scientists and statisticians were able to look at the source data.

What they found was not a large, robust sample size of the kind that would make for an adequate proxy. Statistician Steve McIntyre identified that the sample dwindled to as few as 10 trees for the temperature reconstruction of the 1990s - and even reached as low as just 5 trees for 1995. 

To put that in the simplest possible terms, that meant that the temperature for the entire globe was being estimated from the growth rate of just 5 trees in places. As mentioned earlier, even if tree ring growth can be used as a proxy of temperature, it can only be considered to be representative if the sample size is as large and as diverse as possible.

McIntyre showed how the addition of tree data from another site - also in Siberia - resulted in a wildly different looking graph, with a far more stable temperature record.

Clearly this makes the science behind the hockey stick graph look alarmingly thin. With even the land temperature records of the USA also coming in for unfavourable scrutiny, those clamouring for extra taxes and legislation are going to have to do rather better than claiming that the science is 'settled'.