Originally posted by Darkslash
We can't possibly know that! Any time in history? Say, perhaps, the first century of this planet's existence? Nobody was around to take a measurement. So all we know is that for the time period we have been taking temperatures, we have seen the sharpest rise only recently.
Well, if you want to get technical about it the first century of this planet's existance would be referred to as Pre-historic. But I don't feel like getting into the semantics of "history".

But your graph is interesting.