The tree ring debate faded from the news as has CO2 concentrations in the atmosphere while everyone analyses and over analyses the ClimateGate e-mails and who knew what when. New CO2 evidence however suggests we might want to take a look at those tree rings again.
The tree ring kerfuffle, if you will, was over an e-mail string indicating that data had been manipulated. using a “trick”. A good detailed explanation of the “trick” can be found at http://www.theclimateconspiracy.com/?p=354
In brief, there were TWO “tricks”. The first was to simply omit some data from the tree rings. The second was to fill in the missing data with actual temperature readings. Here’s how the final temperature reconstruction from tree ring data looked:
But here is how the original graphs of the data looked:
At the time of submission to the IPCC and published as a precursor to Copenhagen where the world was supposed to agree on major new carbon taxes to help stop global warming, this “trick” was never mentioned. When it blew up due to the revelations in the ClimateGate e-mails, everyone asked the same question. Why would anyone substitute tree ring data with temperature data but only for a small portion of the graph? The point of the tree ring study was to show that it matched the temperature record and predictions of run away warming. So why discard the data in the very part of the graph that was most important on that question?
The scientists involved came up with a pretty slippery answer. There was a problem with tree ring data they said, it stopped responding to temperature. The problem was noted from about 1950 on, with some areas showing a bigger problem than others. Lots of theories as to why though no real answers, it’s still a mystery. Funny they felt no need to explain that before they got caught doing a “trick”. In any event, this has what to do with CO2 concentrations?
The “normal” amount of CO2 in the atmosphere has always been thought to be 280 ppm (parts per million). This was based on studies of ice cores, and then supplemented by actual readings showing that CO2 started to rise fast in the 1900’s and temperature with it. The result is a temperature versus CO2 graph that looks like this:
But recently, researchers have been questioning the 280 ppm flat line before 1900. The first reason is that a paper was published explaining why CO2 levels from ice cores were too low. In brief, they “max out” at about 280 ppm:
Then in addition, we have the revelation that there were scientists doing all sorts of research on atmospheric CO2 concentrations going back to the early 1800’s. A full accounting of how their data was scrubbed to match the ice core data, and what the data should have looked like can be found here:
Some have been critical of the manner in which Beck reconstructed global atmospheric CO2 levels from this data, complaining that the measurements were taken from a lot of different places, using different instruments, and that the data series have been spliced together and may not be valid. Odd, if that’s true then temperature readings from all over the world spliced together must also be invalid. And splicing temperature data into tree ring data…
But let’s run with it for now. Here is how Beck reconstructed CO2 concentrations, and graphed them compared to the IPCC temperature series as well as solar sun spot cycles. The bottom graph shows temperature versus CO2 as the IPCC would have us view it:
Not such a good correlation to CO2 anymore. But back to the tree rings. What makes trees grow? Well sunshine obviously, hence the theoretical linkage to temperature. Moisture is an issue, if there isn’t enough (or too much) it will influence tree growth. Soil nutrients. Oh yeah, and CO2. So here we have scientists all over the world trying to figure out why their tree rings stopped responding to temperature around 1950. Some areas worse than others, but variations in moisture and sunshine explained the difference, but not the root cause. There was no reason to suspect CO2 as the root cause because everyone knew CO2 concentrations had been stable. Except maybe they weren’t. Let’s take a look at that graph again and check out CO2 levels before 1950. Uh oh.
If Beck is right, then we have an explanation for what has been called the Divergence Problem. As temperatures went up from 1880 or so on, tree growth went along with it. But more growth means you need more CO2, and when CO2 levels suddenly fell, the tree growth could no longer take advantage of the temperature increase. Variation for other limiting factors would make this sporadic in different areas. Wikipedia quotes 1950 as the date the issue appeared, but in the tree ring studies above, 1961 and 1981 were the years. These were series from far northern climates, so temperature had to go up more before the growth it produced surpassed the ability of the CO2 levels to support it.
But wait… CO2 levels according to Beck were higher in the early 1800’s, and current CO2 levels are also higher than that patch from 1920 to 1950. Shouldn’t we be seeing the Divergence Problem appear again? Shouldn’t it have existed before? The answer is yes to both. These and other reports on “unprecedented” growth in cold climate bristle cone pines that is uncorrelated to temperature have recently been appearing:
For the past record, since no one was looking for the Divergence Problem as it was only discovered recently, it may have been overlooked. However, a search does in fact turn up studies such as this one complaining of a divergence problem in…. the early 1800’s
Not to mention that the tree ring studies themselves show a large bump around 1800, which suggests that perhaps some of the growth should be attributed to CO2, not temperature.
Conclusion: I have long held tree rings as a weak proxy for temperature. The tree has no way of telling us what temperatures were like in the winter, their growth is heavily influenced by moisture and nutrients as well, which we cannot measure. But it seems there is enough evidence from the tree ring record to suggest that the Divergence Problem is related primarily to CO2 concentrations, and that in turn, the criticism levelled by Zbigniew Jaworowski regarding ice core data, and by Beck on past CO2 levels is well justified.