So what makes tree ring data such a reliable source of past climate?What makes tree ring data such a reliable source of past climate?
We've known about the problems with tree rings for some time. Climategate revealed how some of the AGW proponents responded to the problems.
Simply put, tree ring widths didn't continue to increase after 1980, when we know that the temperature went up. The fact that they didn't increase in the 1100s and 1200s had been argued to mean that the temperature didn't go up then. But now that we know that the linear relationship between tree ring width and temperature ends, we know that lack of tree-ring-width-increase during the peak of the MWP doesn't mean that it didn't continue to warm.
Keep in mind that this is on top of the fact that tree ring width is affected by myriad other factors such as moisture, pests, competing plants, soil quality at various depths, etc....
It is information but not particularly reliable.
I seem to remember that tree rings became popular to date things. By taking very old and long dead trees they were able to reconstruct "climate" back thousands of years. These reconstructed climates were really patterns of growth over many years. They could and did use tree rings in archeological sites where wood was found and they could sometimes use the pattern to date a particular site. Obviously that has some limitations. Growth can be affected by many things such as water, sun, heat, cold, dust, etc. You can look at tree rings in various places and they are not consistent with each other and often recent ones don't even reflect the actual measured climate. This is one reason why Mann's reconstructed climate was such a fraud. In addition those tree ring proxies with a hockey stick were favored.What makes tree ring data such a reliable source of past climate?
On the one hand, I've never used tree ring data to gauge temperature, although it might reflect extreme shifts in the temperature at that time and place. However, you get very good positive correlation of tree-ring growth with rainfall data.
What you do is run the correlation where you have both sets of data. If the correlation is good, you can feel safe in extrapolating it back to times for which you have only the tree rings.
Tree rings do not make good thermometres. From 1960 on there was a divergence problem. Temperatures went up, while the tree ring data showed temperatures declining. The problem is if there is one known divergence problem, what makes people so sure that there are not any more going back further in time.What makes tree ring data such a reliable source of past climate?
tree growth is spurred because of (amongst other things) a rise in Co2 - which is why it is misleading to assume that you can use dendrochronology as a proxy for temperature.
Apparently trees may not be that great for preventing global warming anyway, at least in some environments: http://www.sciencedaily.com/releases/201鈥?/a>
Unless its a long living redwood or oak it may not be a good guage. People must be able to read these rings as yearly climate effects can change the appearence of some of the rings.
http://www.csmonitor.com/Environment/Bri鈥?/a>
Good question. Apparently nothing. I can't understand we're still having this discussion. And why is Michael Mann taken seriously by anyone?
The data collected from tree rings is incredibly noisy. An initial exponential decay function is typically used to smooth out the age related decline in tree growth (that is younger trees put on more girth per unit stem diameter in a year than older trees). Then any number of different functions can be used, from sin waves to moving averages, to further smooth the data and removed stand dynamics, species and local site variations.
There is an unusual amount of subjectivity involved in the process.
Be that as it may, there is a strong correlation between tree ring chronologies and past climate up until the 1960's-1970's (that is past climate from direct measurements but also other proxy data) when the two diverged and tree rings no longer served as a reliable proxy.
This divergence can be for any number of factors from a change in precipitation patterns, lowered water tables or even a change in wind direction/velocity.
So it certainly seems that they are acurate for a period of the temperature record but the divergence should be a great cause for concern and should cast a speculative eye on older data that lacks a true (not proxy) temperature record to compare to.
No comments:
Post a Comment