We don't have as goos an idea of what the temperature was anywhere back then, but we can infer from vegetation that nearly all or the Earth was warmer, and it was considerably warmer in some areas.
Well, the paper that contradicts that assessment is
here, but if we're not going to trust the data from that far back, let's stop bringing up the Medieval Warm period?
You can believe that if you like, but it is an unproven assertion that contains some assumptions that are almost certainly false. Since we don't have a collection of Earths on which we can experiment about climate, We don't actually know what the Earth's temperature woul be now, if there were less CO2 in the atmosphere.
We do have satellites, however, which for 40 years have been watching the greenhouse effect happening in real time - more longwave radiation being trapped in our atmosphere, and at precisely the wavelengths that CO2 and methane absorb. These are measurable, empirical effects - CO2's influence is not an unquantifiable mystery.
We've also got really good models for things like radiative forcing and climate sensitivity to particular gases. How do we know they're good? There's a test, called 'hindcasting'. You set up the model as if we were back in 1900, and then try to predict how the last century would go. What happens is, the predictions match up really well with the observed facts. If anything, the predictions turn out a bit more conservative and less alarming than reality. So although we don't
know what things would be like with less CO2, and we can't experiment on a
real alternate earth, we have the next best thing. I think we can look at our models with a lot of confidence.