Alarmist Tom Yulsman suggests that a guy could predict future climate using a "back of the envelope" calculation

Climate change: it’s the physics, stupid! | CEJournal

In the [1975] paper, Broecker predicted that warming during the 20th Century would amount to 0.8 degrees C. It turned out that he was a little high. The planet actually warmed by about 0.7 degrees C. Not bad for a back of the envelope calculation.

Note that 3/4 of the century was already in the books by the time this paper was published.