Monday, July 28, 2008

Tree rings

Mann et al. "hockey-stick" temperature graph.

You might remember that much of the hysteria surrounding the IPCC's assessment of global warming was based on Mann et al.'s "hockey stick" graph. This seemed to show reasonably steady temperatures followed by very rapid warming in recent years (as well as mysteriously removing the Mediaeval Warm Period).

The IPCC's initial reports, most notably that produced in 2001, were all based around this graph, and much of the IPCC's credibility was centred around it. Which was a pity. For the IPCC, I mean.

Because two interested men, Steve McIntyre and Ross McKitrick, demonstrated that the hockey-stick graph was utterly unreliable. A large part of this unreliability affects almost all AGW predictions, because all of these models require substantial amounts of data.

The trouble is that we simply don't have enough data: we have only been measuring and recording temperature since the late 1800s (and even then, mostly in the USA) and, obviously, accuracy and method have improved since that time. Unfortunately, so has urban sprawl, as surfacestations.org has been demonstrating.

As such, we are forced to rely on proxies: these include ice cores (most notably the Vostock cores) and, in the case of the hockey-stick graph, a heavy reliance on tree rings. And the trouble is that tree rings are, as Climate Skeptic explains, more than a little unreliable.
One of the issues scientists are facing with tree ring analyses is called "divergence." Basically, when tree rings are measured, they have "data" in the form of rings and ring widths going back as much as 1000 years (if you pick the right tree!) This data must be scaled -- a ring width variation of .02mm must be scaled in some way so that it translates to a temperature variation. What scientists do is take the last few decades of tree rings, for which we have simultaneous surface temperature recordings, and scale the two data sets against each other. Then they can use this scale when going backwards to convert ring widths to temperatures.

But a funny thing happened on the way to the Nobel Prize ceremony. It turns out that if you go back to the same trees 10 years later and gather updated samples, the ring widths, based on the scaling factors derived previously, do not match well with what we know current temperatures to be.

The initial reaction from Mann and his peers was to try to save their analysis by arguing that there was some other modern anthropogenic effect that was throwing off the scaling for current temperatures (though no one could name what such an effect might be). Upon further reflection, though, scientists are starting to wonder whether tree rings have much predictive power at all. Even Keith Briffa, the man brought into the fourth IPCC to try to save the hockey stick after Mann was discredited, has recently expressed concerns:
There exists very large potential for over-calibration in multiple regressions and in spatial reconstructions, due to numerous chronology predictors (lag variables or networks of chronologies – even when using PC regression techniques). Frequently, the much vaunted ‘verification’ of tree-ring regression equations is of limited rigour, and tells us virtually nothing about the validity of long-timescale climate estimates or those that represent extrapolations beyond the range of calibrated variability.

Using smoothed data from multiple source regions, it is all too easy to calibrate large scale (NH) temperature trends, perhaps by chance alone.

Every single climate projection model is based on proxies—sure, most will use a mix of different proxies, but many of the original proxy studies used other proxies to attempt to verify their data.

As such, as each scientist builds on the inaccuracies of the previous ones, the proxy temperature graphs require more... er... "adjustment". And when people build climate models based on such data, then you are really in trouble. As we have seen.
But this is what really got me the other day. Steve McIntyre (who else) has a post that analyzes each of the tree ring series in the latest Mann hockey stick. Apparently, each series has a calibration period, where the scaling is set, and a verification period, an additional period for which we have measured temperature data to verify the scaling. A couple of points were obvious as he stepped through each series:
  1. Each series individually has terrible predictive ability. Each were able to be scaled, but each has so much noise in them that in many cases, standard T-tests can't even be run and when they are, confidence intervals are huge. For example, the series NOAMER PC1 (the series McIntyre showed years ago dominates the hockey stick) predicts that the mean temperature value in the verification period should be between -1C and -16C. For a mean temperature, this is an unbelievably wide range. To give one a sense of scale, that is a 27F range, which is roughly equivalent to the difference in average annual temperatures between Phoenix and Minneapolis! A temperature forecast with error bars that could encompass both Phoenix and Minneapolis is not very useful.

  2. Even with the huge confidence intervals above, the series above does not verify! (the verification value is -.19). In fact, only one out of numerous data series individually verifies, and even this one was manually fudged to make it work.

And this is why those who base their faith in catastrophic anthropogenic climate change are making a severe mistake: many of those who do so, have an absolute faith in the IPCC and take their word as Gospel.

But, if the IPCC's conclusions and models are built on false—and demonstrably falsifiable—data, then the IPCC's models and conclusions are simply not reliable. Apart from anything else, if even the IPCC thought that they were absolutely correct, they wouldn't have to keep updating their conclusions, would they?

11 comments:

extraink said...

Can I suggest that you read New Scientist's take and, as an equally partisan alternative to the idiots at Climate Skeptic, the actual scientists at RealClimate. The first article is probably the more interesting, particularly from "Flaws in Methodology" onwards. Cheers.

TheFatBigot said...

Thank you Mr Extraink, I needed a good laugh.

Mark Wadsworth said...

They missed off mediaeval warm period and also little Ice Age.

Roger Thornhill said...

I can see it now: all 1000 year old trees will have to be cut down so they can "prove" their AGW theory.

Guthrum said...

A little bit more unscientific than tree rings, but has I suggest has more credibility than the IPCC is the history of clothing in the last two thousand years, which we have evidence going back to the bog people in Denmark.

The fall of the Roman Empire was largely due to twenty years of bad harvests and bad wet summers linked to volcanic activity in the far east. Standard dress changed dramatically in this period.

The medieval period was warm hence looser lightweight dress, as the 17th century progressed furs, high collars, hats became de rigeur- also the Thames froze over regularly, it then warmed again through the eighteeth century, until Dickens time in the 1840's when repeated cold periods reintroduced heavy clothing again and we got white christmas'

Nature is in constant flux, and all of this happened without one 4x4 being involved.

Dodgy Geezer said...

Extraink,

Your reference is two years out of date, and does not disprove McIntyre at all, in spite of your implication.

It makes incorrect assertions about McIntyre's work, and then finishes with 'we shall have to wait and see...'

Well, thanks to your 28 month old reference, we can. And what we find is that the temperatures around the world have now been going down for two years as fast as they were going up in the 1980s. But the CO2 figure continues to rise. So whatever is driving temperatures (and we know very little about cyclic climate variations) it CAN'T be the simple 'more CO2, higher temperature' which the IPCC modellers claim.

What is far worse is the attitude you are displaying, which is now endemic in this branch of science. AGW supporters have politicised science and turned it into a slanging match, where the 'other side' are called "idiots" and smeared in the way the APC have recently displayed.

The original Mann/Hansen/IPCC hypothesis of primarily CO2-driven Global Warming is now comprehensively broken, and a last-ditch political fight is being used to prop up the science. This will not work, and science itself will be ruined in the process. Does this not cause you any concern?

V said...

The only thing that is saving humanity as we know it is the total inability of politicians to implement a process which actually works as intended!

By the time they get around to green taxing everything out of use, the weather is going to get as cold as it was in the early 1980's.

It has been known since the 1700's that sun spots and good crops go together. We currently have no sun spots - and it is getting colder - but it will get ignored by the eco mentalists and the IPCC because it doesn't prove what they are trying to prove - and that is not science - its politics!

John A said...

extraink - those were hilarious! But why not add links to Dr. James Hansen warning about "Ice Age in 10 years" in the Seventies? Or Mann et al saying that if the Medieval Warm existed at all it was limited to small parts of Northwestern Europe (ie, the UK and Denmark) to be immediately reminded of Chinese written records of the same phenomenon? Ditto the Little Ice Age?

Ever heard of Chaos Theory? Know how it came to be accepted? Seems a climate specialist named Lorenz showed [his] computer models of weather could not possibly predict (even in retrospect) weather...

TheFatBigot said...

How very kind of you to bring up the Little Ice Age, Mr A. I have long wondered whether any IPCC model has projected a LIA either in the UK/Denmark/Greenland or in China.

And do not forget, the IPCC models no longer predict, now they just project; we know this because not only has the wording started to change in AGWers' writings but it has changed by consensus.

PompeyTeuton said...

Just to draw another thread into the debate, I'm not especially convinced that we're headed for something that merits either the term Global Warming or Global Cooling, but I'm still massively in favour of what now get called 'green energy sources'. Why? Because even optimistic estimates say that non-renewable sources are going to run out in most people's lifetimes, so it makes sense to be prepared sooner rather than later for when they do. Also, I'm not exactly a fan of OPEC, so researching any way we can have of NOT using petrochemicals (and if not 'green' resources then nuclear ueber alles as far as I'm concerned) in our 'basic goods' certainly gets my support and hopefully state funding as well...

TheFatBigot said...

With you (almost) all the way there Mr Teuton provided a sensible cost-benefit balance exists.

Solar panels and little windmills on the roof are not economic yet but might become so over time, in which case let those who want to play with them now do so but don't force them on the rest until the price is reasonable.

They are no different from any other new technology, horrendously expensive to start with but the income from the mugs who pay full-whack supports the research to improve the product until it is affordable. Sadly, our government wants to be windmill mug-in-chief.

As for OPEC, we have no bargaining position (as poor Gordon found out when he made an arse of himself, yet again, on his recent begging mission). The sooner we cease to be dependent on a cartel the better.

It pains me to say it, but the Frogs got it right when they said two-fingers to Greenpeace and the other eco-fascists and got on and built the nuclear power stations they needed.

Johnny Froggo isn't a complete mug, he knows how to milk the system. Become self-sufficient through lots of nuclear then support EUSSR targets for reducing emissions from coal-fired power stations, hey-presto, you can sell your surplus at an enhanced price and it's still cheaper for your customer than paying the EUSSR fine by running its own coal station.

£100billion on windmills? Oh dear god.