@Dutchess_III is right, it was the Industrial Revolution that brought it on. The global temperatures in that period aren’t particularly relevant though.
Basically the emission of greenhouse gasses causes global warming. But the two factors don’t linearly correlate. When the Industrial Revolution kicked off, the oceans absorbed much of the carbon dioxide. Their vast volumes meant that there was probably little to no effect on the environment during this period, besides the occasional acid rain.
We’ve been burning so much for so long though, that the oceans aren’t absorbing as much as they once did. More is going into the atmosphere, and the effect compounds. We’ve also lost huge amounts of forrest, which has again compounded the effect. We’re going past the tipping point. There was no great environmental impact until a few decades ago. But we’ve been hurtling towards it since the Industrial Revolution.