The estimation of historical CO2 trajectories is indeterminate:
It is somewhat technical but it is obvious that the IPCC used the highest possible (exponential ) curve fitting method....for modeling the catasprophe caused by man ( their intent).
Web-info about CO2 and the "Greenhouse Effect" Doom; by Tom V. Segalstad
Tom V. Segalstad
Associate Professor of Resource- and Environmental Geology; and
Former Head, Geological Museum of
Natural History Museum, University of Oslo, Norway
Former Expert Reviewer with the UN IPCC,
the United Nation's Intergovernmental Panel of Climate Change.
Created 29 June 1997; established at www.CO2web.info 15 March 2008; updated 9 March 2010.