Tuesday, November 24, 2009

Tree Ring Circus™

For those of you laymen following the CRU document dump I'd like to make a few points about one of the central issues: Using tree ring width as a thermometer in order to have a record of temperature from the era before thermometers were invented. Tree rings were the genesis of the famous hockey stick "proving" that temperatures had been flat for a thousand years or more and suddenly with CO2 rising Earth's temperature system was rising out of control. Doom was coming if things continued this way and electricity production and automobiles powered by carbon and hydrocarbon fuels were the cause of the coming catastrophe.

But the boys reading the rings found something interesting. The ring "temperature" in the modern era was going down while the "real" temperature was going up. This is called the "great divergence" (really it is just called "divergence" but I like my phrase better). So what did they do to make the rings agree with the measured temperatures? They grafted on "real" temperatures. This is not exactly kosher if you don't know what caused the great divergence.

What the divergence tells us is that tree rings are not a good way to measure temperature because we lack all the relevant confounding variables. Like rainfall. Air quality (volcanic eruption?). CO2 content of the atmosphere. Nearby forest fires. Cloud cover. etc.

To say something caused the divergence is true. But if you discard the later data you also have to discard the prior data until you know the cause.

And it is dishonest in the extreme to append “real” values on a chart of calculated values without making that explicit each an every time the chart is presented and the data used. Especially in a public policy context. Like the hockey stick.

The idea of deriving temperatures to an accuracy of .1 deg C or even .5 deg C from tree rings is a triumph of self delusion over error bars.

If you want to dig into this some good search terms are: dendrochronology, Yamal, hockey stick, tree ring, Real Climate, Watts, McIntyre, Climate Audit, temperature.

Her is a link rich site. A search for "Watts" on the page will get you started on a deeper look.

4 comments:

simentt said...

The fact that there have been series that have been used 'inverted' - ie that the underlying rings are given the opposite 'meaning' that they have in other series - by mistake should give some food for thought.

It would seem that trees have been harvested en masse, and that those who have the 'clearest expected signal' are then used for the series, so as not to drown the present signal in noise.

The divergence-problem then occurs when new trees do not show this signal clearly.

The alternate explanation, is that only trees with the wished-for pattern have been used to draw the graphs, and that the same amount of diligence has not been applied when finding new trees to extend the series.

-S

Neil said...

I suspect that the factually damning stuff is still a week or so from coming out. Forget about the e-mails; there was a lot of raw data in those documents.

Once that data is re-analyzed in a straightforward manner, I suspect there's not going to be much actual data left to support AGW.

The emails sure sexed it up for the new media though. And sex sells.

Tom said...

Another problem with the tree ring data is that climate changes cause various effects, not just temperature changes. So a cooler climate may also be drier, for example, in a particular place.

This would amplify the signal, make it look as if the temperature had changed even more than it really had.

This would have to be accounted for in each individual location. We have to KNOW how the various factors interact or the data is simply mush.

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