Oh, it will be noticed.
A one foot rise in sea level is going to create a lot more shallow water basins and tidal flat areas. All that increase in surface area is going to increase evaporation rates. That will result in an increase in atmospheric water vapor, which is one of the more potent greenhouse gases, which introduces a new positive feedback to global warming.
But in turn the increased atmospheric water vapor will, under some conditions, create an increase in clouds, which will lower the insolation of the land and ocean below them and tend to counter global warming. Since evaporation and cloud formation will be regional, there will be a stronger thermal differential between regions, which will make severe weather incidents more frequent and more intense.
People are going to be displaced by storm damage more than by the simple rise in sea level. If every year 3 to 5 port cities on the East Coast of the USA were hit by an incident on the level of Hurricane Katrina, what would that permanent stream of refugees look like? How could even the wealthiest nation keep up the infrastructure repairs needed to keep those cities functional?
No one knows how to model this, so there can be no scientific talk about it yet. All we can know is that somewhere along the way as the seas rise to 21 feet above their current level, these kinds of effects are going to occur. I think the flooding that will happen with a one foot rise will be enough to change the Earth's weather engine. I may be off by a few feet... or by a few inches. We'll have to wait and see.