Here’s how the skylines of Toronto, Chicago, Boston and Montreal compare against the icesheets covering those locations during the last ice age (XKCD).
If the ice slabs of our current ice age were suddenly as thick and extensive as those of Earth’s last major glaciation, anyone living in an area colored white would be instantly crushed and buried forever (or at least until the region thawed out):
The above map comes from “The Laurentide and Innuitian ice sheets during the Last Glacial Maximum” by Dyke et al. (Quaternary Science Reviews), which Munroe used as reference for the webcomic. The units on the figure’s topographical high points are given in meters.