[Always Right]

It was my understanding that dendrochronology is used for dating the rings, not for calculating temperature. The size of the rings is used to obtain climate information, which doesn't really give you a temperature so much as tells you how the overall growing season was which is based on amount of rainfall and numerous other variables.

[Wonder Warthog]

"It was my understanding that dendrochronology is used for dating the rings, not for calculating temperature. The size of the rings is used to obtain climate information, which doesn't really give you a temperature so much as tells you how the overall growing season was which is based on amount of rainfall and numerous other variables."

Which is why you need the isotope ratio, as that is independent of rainfall. The tree ring pattern gives you the dates, the isotope ratios in the rings give you the temperatures for the year of a specific date.

I suspect they also cross check these against the same isotope ratios in seashells deposited in silt layers (again, the silt layer gives you the year, and the isotope ratio again gives temperature).

You have to remember that these measurements always involve many, many data points--they are not trying to get one temperature from a single tree ring.

[blam]

FYI, The tree ring chronology is now over 10,000 years long. This guy, Mike Baillie, has done some amazing work with this data.