Saturday, 27 October 2012

Where's the hockey stick gone? New study (by hockey stick contributor) says 'tree ring data was biased'

I feel like a storyteller here. Once upon a time a bunch of scientists got together, and between them made a potion of figures and measurements which when put together made a hockey stick. The trouble was when anyone tried to use it it changed shape and wouldn't work, but no one else could replace it and everyone was forced to try and use it even though it never seemed to be stable enough to work.

Many years later, although one of his tricks was required to create it, by casting a spell over hundreds of tree rings to make all but a few disappear and use them to both raise the tip of the hockey stick, and then vanish in a tangle of lines when it started heading the wrong way, Keith Briffa saw the angry clouds of fate heading his way, threatening to rain on his chips and make his hockey stick so soggy it could melt away. To quickly get out of the path he and two colleagues went back to the spell and took out all the funny incantations, such as 'abracadabra and alakazam, make these rings for Michael Mann' or 'by the powers of Greyskull make these threes into sevens' and the like, leaving only the boring stuff like 'one and one is two, two and two is four' etc, which is deadly dull but impossible to question.

And then they concluded the trick was only an illusion and should not be relied on. He is probably going to be awarded a halo for this redeeming action, and had they not been part of the same illusion a Nobel Prize. I may even send him a Christmas card myself now.

Where's the beef?

Here be the summary:

We describe the analysis of existing and new maximum-latewood-density (MXD) and tree-ring width (TRW) data from the Torneträsk region of northern Sweden and the construction of 1500 year chronologies. Some previous work found that MXD and TRW chronologies from Torneträsk were inconsistent over the most recent 200 years, even though they both reflect predominantly summer temperature influences on tree growth. We show that this was partly a result of systematic bias in MXD data measurements and partly a result of inhomogeneous sample selection from living trees (modern sample bias). We use refinements of the simple Regional Curve Standardisation (RCS) method of chronology construction to identify and mitigate these biases. The new MXD and TRW chronologies now present a largely consistent picture of long-timescale changes in past summer temperature in this region over their full length, indicating similar levels of summer warmth in the medieval period (MWP, c. CE 900–1100) and the latter half of the 20th century. Future work involving the updating of MXD chronologies using differently sourced measurements may require similar analysis and appropriate adjustment to that described here to make the data suitable for the production of un-biased RCS chronologies. The use of ‘growth-rate’ based multiple RCS curves is recommended to identify and mitigate the problem of ‘modern sample bias’.

Michael Mann, the official owner of the hockey stick, has yet to answer the revised decision, although it is only but hours old. My bet is he isn't going to though, and for the double no one's going to ask him. Not even Keith, he's already off the hook.

No comments:

Post a Comment