Four meters of mean sea level rise is plenty to completely disrupt global shipping and make many densely populated coastal cities uninhabitable. In some places

Four meters of mean sea level rise is plenty to completely disrupt global shipping and make many densely populated coastal cities uninhabitable. In some places, nothing will change. In some places, sea level may decrease. In others, it will be very substantial. Never mind the effect that the decrease in albedo and the increased heat capacity of liquid water would have. In terms of timescales - it won't be instant. If the shelf collapses then it'll allow the rapid outflow of the glaciers behind it, but when we say rapid we mean 30km/yr (total guess, depends on so much, could be less, could be much more) rather than 4km/yr. So far, however, everything has gone worse than our worst published models. I think we're going to see some nightmarish stuff in our lifetimes.