That's correct. Our planet was significantly warmer during the 12th and 13th c., so much so that many parts of Northern Europe like Scandinavia and even Greenland were a lot more hospitable than they are today (despite the "global warming" - does that suggest anything to you?).

In the 14th c. the climate changed gradually and we had what is now sometimes called the Little Ice Age. It did have a very considerable impact on the population of Europe at the time - harsher living conditions, less arable land, more diseases and in many places depopulation or at least demographic stagnation.

That the climate is changing is a fact of life. It has been changing all the time anyway. Our actions certainly have an impact on the environment but by and large, all the major factors are unpredictable and beyond our control. On the other hand, the human race is very adaptable. We have survived things as bad as the Ice Age. Not that it's easy, but it can be done.