5000 ppm would be significantly worse than that! According to a 2013 paper by James Hansen, burning all fossil fuels would result in 12 degrees C of warming, w

5000 ppm would be significantly worse than that! According to a 2013 paper by James Hansen, burning all fossil fuels would result in 12 degrees C of warming, which would be hot enough that anyone living in the tropics or close to them would die immediately of heatstroke. So the number of people who would have to move would be in the billions. Also of course agriculture would collapse pretty much everywhere so you’d be looking at a planet that could support a few hundred million people.