It wasn't derived by examining past temperature records, noting a rise, and picking on CO2 as the cause. (realclimate.org)
I confused Forster and Gregory 2006 with Gregory and Forster 2008, which does attempt to estimate the Transient Climate Response (TCR) from past temperature records and estimated forcing. (julesandjames.blogspot.ru)
I think this is following on the discussion on the other thread, where it was stated that lack of accuracy in model inputs places limits on how well we can expect models to match past temperature records. (skepticalscience.com)