Do you have any idea how much of the earth's land surface (to say nothing of its enormous oceanic heat sink) was covered by temperature observation before 1979? We're talking about a few times a day at a few hundred locations, over a few hundred years. Most of the temperature data before 1800 is from SE England; from 1900 onwards, the only better data are from the continental US. All amounting to a miniscule fraction of the data needed for a model...if the system were closed. It's not. No one has any idea what the sun (chief driver of earth's climate) will do next.