The real question is what will it take to overcome inertia and start the big meltdown. That is probably not something that can be known before it happens. If you're feeling pessimistic you can look at that accelerating curve and think that inertia is breaking now. Or you can feel more phlegmatic if you want.
In the last big interglacial meltdown it went 5m/century, 5cm/year. Should that get underway there will be panicked migration from coasts.

Hi Garth,
take a look on page 33/34 of the referred document. It says:

The end of the last ice age provides information about the possible rate of sea-level rise. At that time the global average temperature rose by around 4-7 °C, an amount that is also reached in pessimistic scenarios for the future. But the warming at that time took around 5000 years, which is much slower than the present trend. From 15,000 to 10,000 years ago sea level rose by around 80m, an average of 1.6m per century (Fairbanks, 1989). During some intervals rates of up to 5m per century were reached (Clark et al., 2004).

These values cannot simply be applied to today's situation. The ice sheets at that time were considerably larger, which means the melting regions on the margins were greater, allowing a greater flow of meltwater. In addition, due to Earth's orbital cycles around the sun (Milankovich cycles; Ruddiman, 2000), the incoming solar radiation at high latitudes of the Northern Hemisphere was considerably stronger, a situation that cannot be directly compared with the global increase in greenhouse gas concentrations. These two factors suggest higher melting rates at the end of the ice age than during the present warming. The much slower warming at that time, by contrast, would suggest lower melting rates. In fact, the disappearance of ice sheets at that time for the most part kept pace with the gradual climate warming, so the assumption that ice masses would have melted significantly more rapidly with faster warming
is quite plausible.

Two conclusions can be drawn from this discussion. Firstly, rates for sea-level rise of up to 5m per century are documented, and these probably do not represent an upper limit. Thus climate history shows that a much more rapid rise than that expected by the IPCC for the 21st century is possible. Secondly, such rates of sea-level rise suggest dynamic melting processes of the ice sheets, also taking account of the conditions at the end of the last ice age. This means there can be not only a simple melting through contact with warmer air, but also an accelerated flow of
the ice into the sea.

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In daily live we are familiar with the last type of melting, when we clean up ice from our freezer. I'm aware that the boundaries of the cases are different, but it explains well why we can expect higher melting levels. Already are some glaciers increasing their flow speed.

Bart