The depression is 19,605 km2 by (say) 100 m deep or 1960 cubic km. Greenland has 2,800,000 cubic km of ice which would produce less water, although Greenland's ice is compressed so wouldn't decrease that much. Anyway, you are short by 3 orders of magnitude in that statement. But you're right, even if it did warm, it would take 1000's of year to lose that ice.
Furthermore, Greenland's ice caps are not all fully compressed. They are a combination of porous and compressed layers depending on the climate cycles over the last 18,000 years. The weight of the glacial layer accumulated over those years would put much of the subcontinent of Greenland under water depending, of course, on the rate of any melt.
The other lunacy of the global warming crowd is failing to account for storage of moisture in vegetation. As recently as Hannibal's time (a mere 2300 years ago), most of coastal north Africa was a lush tropical rain forest and most of the northern Sahara was savanna and semiarid tillable land (much as the coastal regions are today). His famous elephants were native to what is modern-day Algeria (then Numadia) whereas today, you would have to go south of the Sahara to even find wild elephants. In general, the world was much warmer then than it is today and we still had ancient civilizations, including small islands which were above water during or shortly before that time which are under water today. We also have islands which were born or disappeared since-- the volcanic ring islets around Iceland being well known examples of the former and Krakatoa being perhaps the best known example of the later.
The links on my post discuss the theory and calculations further. While I believe the calculation which I have posted is realistic, I do not discount the possibility it could be way off. My sources are genuine climate scientists who, unlike the global warming crowd, do not claim infallibility.
But We both agree on the main point: even if we entered into a long warming cycle, it would still take 1000'2 of years to lose that ice.