Oh but wait why stop there? Medieval? Nope.
More like the planet could very well turn into Venus. Methane is much more destructive and is now in a loop cycle as the planet warms. More warmth, more methane, more warmth…
Water is already running out, but it could very well become poisoned by chemical spills from large storms, fires, etc. or from nuclear plants melting down. Also, microbes do not care. They will just move in, and that means diseases unheard of before.
I could go on, but the spiral leads to collapse, and if that happens then the ecology changes so drastically there are no cell phones, and certainly no society as advanced as Medieval Europe.
There are no reins. No one group or person controls this mess. Cities around the world all now use the same resources. We all wear the same clothes, drive the same cars, live in the same apartments, etc. and the world market is based on making more and more stuff so people can have kids and go shopping.
Habitat overshoot. Look it up. We are not the only species that does this.