I'm curious as to why your equations, if linear, don't go backward in the same way that they go forward.
"the earth is warming on average of one degree per century."
If this were true, and the rate were linear, that is if today's average earthly temperature were 60 deg F, and 1000 years ago it were 10 degrees cooler, 2000 years ago, 20 degrees, 3000 years ago, 30 degrees cooler, you could go back less then 10,000 years and expect to lower the average earth temperatures to a level that would have all the water frozen except for the tropical areas, 60+ -100 = -40, yet that is not a case supported by any historical geology. If you stayed linear, you could go back less then 50,000 years, lowering average temperatures by -500 degress for the sake of argument, and cross the point of absolute zero.
Good luck figuring out how the hell to explain that scientifically.
Nor would the assumption that the oceans would raise as the polar ice caps melted with an increasing temperature, match up with the concept you suggest. Would the oceans be lower because more and more water was locked up in the polar ice cap regions as you went back in time and water was more solid then liquid because of lower average temperatures?
What science and geology do suggest is that there are ice ages and warming periods. Also that when insignificant data exists to draw reasonable conclusions, the conclusions drawn in the absence of such data is usually wrong. [Law of probability]
Average temperatures, are more like the Dow Jones, up and down, up and down, trending over time. If you just take the reading between 9:15 and 9:17 AM you can hardly claim to know if the trend is up or down for the entirety of the day, or the Earth for that matter.