Indeed it is a daunting task!
And satellites should make it a lot easier.
As far as how you get from millimeters to the "20 foot rise", well there is some reason to expect acceleration. What is happening to glaciers around the world and to permafrost, has to show up somewhere eventually. And these are phenomenon that have only been remarked on (in my memory, anyway) in this century. They need some time to show up in sea-level increase, in my opinion.
Note that the referenced papers covered periods that were years in the past, too.
Note also, that my participation in this thread was only to refute something silly that Johannes said. I really did not intend some sweeping statement about climate. Or were you taking Johannes' side?