Things I took for granted: tree rings are accurate temperature proxies

treeringthermometer

The biggest hurdle I took in my quest to understand the global warming story was a graph called the Hockey Stick. It represents the temperatures over the last 1,000 years. I believed what I was seeing and saw this as a proof of the current anthropogenic global warming. I found it everywhere and was done by scientists, so naively I thought it had to be correct. If temperatures stayed stable for about 1,000 years and the last hundred years took of like a rocket, how much proof do one need to have, considering that carbon dioxide was a greenhouse gas and we emit loads of it into the atmosphere.

Tree-rings showed how warm or cold the climate was, I had no real problem with that, I learned that in primary school. The higher the temperature, the wider the tree-ring. The lower the temperature, the smaller the tree-ring. Just count and measure them and you are done. This was confirmed by scientists who declared that Bristlecones pines were good proxies for temperatures of the past because they live long.

There was the word: proxies. Thousand years ago there were no thermometers. The temperatures back then were measured not by instruments that measure temperature, but by something that is influenced by temperatures. That is a proxy. Temperatures have an influence on the width of tree-rings. True, but are they good indicators of past temperatures?

Looking at the background. Trees are complex organisms. They react on temperature, sure, but also on a bunch of other things. Beside temperature they react on:

  • precipitation
  • nutrients
  • disease
  • wind
  • sunlight
  • competition with other trees
  • competition with animals
  • local variations
  • concentration of carbon dioxide in the air
  • events like storms, lighting,…
  • and probably many, many more…

That makes it different from thermometer. Thermometers measure temperature via the expansion/contraction of a substance, which is representative for the current temperature. There is a direct relation between the expansion/contraction and the temperature. Hence the ability to measure temperature.
On the other hand, temperature has an influence on the width of tree rings, but this is not direct. The tree rings translate the temperature signal, but also those other signals. The temperature signal is diluted in the other signals, there will be a lot of noise in the tree rings. Getting rid of the noise and distilling only the temperature signal will not be possible if nothing is known about the other signals.

There are other things: datasets from trees are sparse. There aren’t that many very old trees. How to compare with modern datasets with real thermometers? Temperature readings of thermometers are read every day at least two times, sometimes a reading every hour. Tree rings one a year. Tree-rings depend on the tree and the period it lived.
A lot of fuss about sparse data, where did I hear that before?

Back to my own story: the Hockey Stick was the difficult hurdle to take. It was difficult because I began to realize that the media brought one sided information about the climate, but wasn’t that far to realize that it was necessary to be critical, not just assume that something was true because the majority thinks so.

Why did I took it for granted? It is a combination of several things:

  • I just looked at the graph and what it meant seemed obvious. I even saw it as proof that humans were causing global warming.
  • I trusted science. I had no reason to believe the scientists were biased in any way or that the information that reached me was one-sided.
  • The graph was found everywhere I searched for historical temperature data. I had no reason to think this could be one-sided information, it seemed straight forward.
  • It was presented as something evident: there was no doubt about this. For example scientists state that “tree rings are a good proxy” because they live long. I didn’t realize that he probably meant a proxy in time, not necessarily a reliable proxy for temperature. Big difference.
  • The basis looks simple and straight forward: warmer: bigger rings. colder: smaller rings. It didn’t seem rocket science. What I forgot to take into account was that a tree is a living thing and reacting on the many influences in its environment. It was brought too simple, but a little bit of thinking would have discovered flaws in the reasoning.

When I think back about this period, I ask myself the big question: how could I ever believed tree rings are thermometers in disguise? How could I ever have believed this stuff?

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