Even the "global warming skeptics" aren't actually skeptical about the fact that warming has been happening. Rather, they're skeptical that there's a sudden uptick in the last few decades, and/or that global warming as it exists is human-caused.
It's well understood that several hundred years ago, it was colder (at least in the northern hemisphere), simply because of historical records we have that don't make much sense otherwise. Also, it's well understood (though recently challenged) that it was warmer than twentieth century levels before that for some amount of time.
So the controversy is all over whether humans are causing any significant part of the warming that is currently happening, and whether that can or should be fixed by reducing greenhouse gas levels.
It's well understood that several hundred years ago, it was colder (at least in the northern hemisphere), simply because of historical records we have that don't make much sense otherwise. Also, it's well understood (though recently challenged) that it was warmer than twentieth century levels before that for some amount of time.
So the controversy is all over whether humans are causing any significant part of the warming that is currently happening, and whether that can or should be fixed by reducing greenhouse gas levels.