So I finally read Hansen et al's article which I asked for last week. (Oh, readers, how I appreciate you so.) Hansen argues that we're much closer to a climate disaster than, for example, the IPCC. Basically, the IPCC doesn't take in to account the latest understandings of ice-sheet collapse, which seriously affects their estimates of sea-level rise. Hansen, on the other hand, relies on the historical data: the last time the temperature of the Earth was where it's headed if we don't change course the sea level was at least 45 feet higher, and may have been as much as 90 feet higher. My understanding is that for anything more than 15 metres of sea-level rise we'd have to melt East Antarctica (which so far seems stable, praise Buddha) but just melting Greenland and Antarctica would be more than enough to, uh, destroy the global economy.
To take my favorite example, global container shipping requires massive, specialized ports which a) require billions of dollars of investment, and b) are (duh) all built at the current sea level. It wouldn't take anything close to the 15-metre rise that Hansen is worrying about to make global shipping a total mess. And ask yourself: what happens when we can't get anything from China anymore?
The IPCC has downplayed the threat of sea level rise, and it kind of went out of the headlines for a while. But it is the single biggest economic threat posed by climate change -- global infrastructure at risk measures in the tens of trillions of dollars, and the global population at risk, directly and not, measures in the billions of people.
The key to worry about here is positive feedback: the Earth has, historically, warmed up very quickly and taken much, much, much, much, much longer to cool off. A warming phase might only last centuries, or even decades, but the cooling phases last millennia. The reason for this is simple: all the mechanisms to cool the planet off (and there are many) operate on much slower cycles than the mechanisms that exacerbate warming. The natural process for sequestering CO2 in ocean sediments, for example, is much slower than the ice/water albedo flip, where liquid water retains 90% of the energy it absorbs when, as ice, it would have reflected 90% of the energy away.
So if we get to the point where the Arctic is ice-free in the summer, the Greenland ice sheet is probably doomed. A Greenland that is surrounded by a liquid ocean is going to be a Greenland that is bathing in much warmer water than can support the ice sheet, and the collapse (while it won't be "fast" by human terms) will probably be unavoidable at that point.
There are some extremely speculative ways we might avoid disaster if faced with rapid sea-level rise. The weirdest one is simply pumping the water in to high basins -- the Taklamakan desert in China, for example. This would basically abandon any idea of "fighting" climate change, though -- the creation of a new, inland Mediterranean in North Asia most certainly qualifies as catastrophic climate change. And I'd hate to think of the energy bill to move all that salt water. So basically, the best option for dealing with sea level rise is to stop it before it starts.