[rbavocadotree]

From the 1975 US National Academy of Sciences/National Research Council Report

...we do not have a good quantitative understanding of our climate machine and what determines its course. Without the fundamental understanding, it does not seem possible to predict climate... [0]

The point is that there were a lot of climate models done in the 70's, and only a few of them were accurate. No one was randomly guessing, but the model you gave was not the consensus.

[0] https://skepticalscience.com/ice-age-predictions-in-1970s-in...