I received the following note from Paul Williams. It is an interesting critique of the use of bristlecone pines as an indication of past temperatures. In the note, Williams explains the pines may be a better proxy for carbon dioxide (CO2) than temperature. So, Williams concludes, the famous hockey stick graph may not be a ‘temperature hockey stick’, but rather a ‘CO2 Hockey Stick’:
“The “Hockey Stick” is the famous graph showing the results of studies done by Michael Mann, Raymond Bradley and Malcolm Hughes.
Mann, ME; Bradley, RS; Hughes, MK: ‘Global-scale temperature patterns
and climate forcing over the past six centuries’, NATURE |VOL 392 | 23 APRIL 1998
Mann, ME; Bradley, RS; Hughes, MK:‘Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations’, Geophysical Research Letters, Vol. 26, No. 6, 1999
These papers are often called MBH98/99.
The Hockey Stick depicts relatively constant temperatures from 1000AD up to 1900, (the shaft of the “hockey stick”), followed by a sharp rise in temperature, (the “blade”). This graph is used extensively to support the argument that humans are causing global warming by emitting large quantities of CO2 and other “greenhouse gasses” into the atmosphere.
Steve McIntyre and Ross McKitrick, (MM), challenged the statistical basis of MBH98/99, claiming that the conclusion, (that the 1990s were likely to have been the warmest years in the last millennium), was not supported by the data and statistical workings described in the papers.
One of the aspects of MBH98/99 that the Wegman Report touched on, was the use of Bristlecone pines as indicators of temperature, or “temperature proxies”.
I have no expertise or qualifications in this field, but the use of bristlecones can be understood without knowing maths and statistics, and it is one of McIntyre and McKittrick’s main objections to the hockey stick, yet it gets little discussion compared to the obscure statistical arguments.
Bristlecones are pine trees living at altitudes up to about 4,000 metres in the dry mountains of California and Utah. Some of them are very old, over 4,000 years. They live in soils that are very low in nutrients, in areas with low precipitation. They have developed a survival system known as “dieback”. When a tree is stressed due to lack of nutrients or moisture, part of the tree will die, thus lowering the requirements for the nutrients or water for the tree as a whole. The tree survives by maintaining a strip of viable bark that carries nutrients to the surviving branches and canopy. Thus a large, old tree 18 metres tall, may be sustained by a 40cm strip of bark up one side.
Because of their great age, Bristlecones tend to dominate dendrochronologies, or climate records based on tree rings, that extend back in time for long periods. So they are very important in the data that MBH98/99 used to draw their conclusions. They are also important in some of the other studies that support the Hockey Stick. As the “Hockey Team” said in their post at Real Climate, “The Missing Piece at the Wegman Hearing”, in which they show that doing the statistics differently still leads to a Hockey Stick shape,
“Why doesn’t it make any difference? It’s because the PC analysis was used to encapsulate all of the statistically relevant information in the N. American tree ring network and so whatever patterns are in there they will always influence the final reconstruction.”
But is the pattern that’s in the Bristlecones a true reflection of temperature? As Wegman mentions, it is known that Bristlecones have reacted to increased atmospheric CO2 since about 1850. This CO2 fertilisation was allowed for in MBH99, but only by using the 19th century CO2 figures, as though the increase in CO2 that happened in the 20th century had no additional effect on the Bristlecones. This may in fact be correct, as they react more to CO2 increases at lower levels than at higher, but it is a point that needs to be verified.
It’s worth noting that MBH98 was the first “multi proxy” temperature reconstruction to include Bristlecones. Several of the following studies that support the Hockey Stick also use Bristlecones. Before MBH98, Bristlecones were not considered useful for temperature reconstructions.
Thus Bristlecones react strongly to atmospheric CO2 levels. They may be a better indicator of CO2 than of temperature. So the underlying pattern is a Hockey Stick, but not a temperature Hockey Stick, instead it is a CO2 Hockey Stick. And this pattern shows up in other studies that use them.
Ross McKitrick’s presentation to the Australian APEC Study Group, 2005. A non-technical summary of the main issues that McIntyre and McKitrick have raised about the Hockey Stick.
Climate Audit website. (Steve McIntyre)
RealClimate website. (Michael Mann and others)