From the: Greenhouse Bulletin No. 120 Feb 1999
A very interesting piece.
Excerpted from the conclusion:
"It is back to the drawing board for carbon cycle models. Atmospheric carbon dioxide concentration varies in a manner which has not been predicted successfully by existing models. There was significant variability before there could have been a human contribution. This variability appears to have followed temperature changes, rather than being responsible for them..."
There are some intriguing leads for follow-up, as well.
We'll keep digging.