The Atlantic Meridional Overturning Circulation has a major impact on climate, not just in the northern Atlantic but globally. Paleoclimatic data show it has been unstable in the past, leading to some of the most dramatic and abrupt climate shifts known. These instabilities are due to two different types of tipping points, one linked to amplifying feedbacks in the large-scale salt transport and the other in the convective mixing that drives the flow. These tipping points present a major risk of abrupt ocean circulation and climate shifts as we push our planet further out of the stable Holocene climate into uncharted waters.
I believe that Western Europe’s population at the dawn of the 17th century was about 20-25 million and this did not represent a base case for preindustrial organization – in fact it was quite scaled up and organized in its own preindustrial way, with population having been significantly less at certain places and times before that. So that gives an idea of where things may be headed, and that doesn’t take into account the accumulated damage to the biosphere that we can expect on the downside of the slope which did not have any parallel in the preindustrial world.