This is interesting – here is the latest paper from James Hansen and coauthor Miki Sato Paleoclimate Implications for Human-Made Climate Change. If you are up to reading climate science papers it’s highly recommended (I’m a little slow in getting to it – the press release was Dec 8th 2011 but I just got to reading it yesterday and today).
A little background is in order – one of the serious scientific debates in the climate science community over the last decade has been the implications of the unexpectedly large acceleration of glacier discharge in Greenland and Antarctica and in particular a discovery by Zwally et al in 2002 that surface melt water can get down the base of a glacier and lubricate its motion. Prior to the early 2000s it was assumed that ice sheets would decay mainly by melting on the surface and climate models all assumed that they would decay only very slowly in a warmer world – it was a surprise to realize that the most important breakdown mode was actually basal lubrication and sliding down into the ocean.
Hansen in particular became the leading spokesman for the view that the ice sheets on Greenland and parts of Antarctica would prove quite unstable under Anthropocene conditions and might break down in a rapid non-linear manner and cause very large levels of twenty-first century sea level rise. See for example this essay from 2005 in which he says:

Consider the situation during past ice sheet disintegrations. In melt-water pulse 1A, about 14,000 years ago, sea level rose about 20 m in approximately 400 years (Kienast et al., 2003). That is an average of 1 m of sea level rise every 20 years. The nature of glacier disintegration required for delivery of that much water from the ice sheets to the ocean would be spectacular (5 cm of sea level, the mean annual change, is about 15,000 cubic kilometers of water). “Explosively” would be an apt description, if future ice sheet disintegration were to occur at a substantial fraction of the melt-water pulse 1A rate.
Are we on a slippery slope now? Can human-made global warming cause ice sheet melting measured in meters of sea level rise, not centimeters, and can this occur in centuries, not millennia? Can the very inertia of the ice sheets, which protects us from rapid sea level change now, become our bete noire as portions of the ice sheet begin to accelerate, making it practically impossible to avoid disaster for coastal regions?

This kind of nigh-apocalyptic rhetoric from a very senior and respected climate scientist provoked a flurry of papers in response seeking to analyze the situation. Most of these suggested various reasons why 21st century sea level rise, while likely worse than previously projected (for example in the 3rd IPCC report in 2001), would not be as bad as the worst fears of Hansen. Hansen and Sato’s own description of this new literature seems fair to me:

Rahmstorf (2007) made an important contribution to the sea level discussion by pointing
out that even a linear relation between global temperature and the rate of sea level rise, calibrated with 20th century data, implies a 21st sea level rise of about a meter, given expected global warming for BAU greenhouse gas emissions. Vermeer and Rahmstorf (2009) extended Rahmstorf’s semi-empirical approach by adding a rapid response term, projecting sea level rise by 2100 of 0.75-1.9 m for the full range of IPCC climate scenarios. Grinsted et al. (2010) fit a 4-parameter linear response equation to temperature and sea level data for the past 2000 years, projecting a sea level rise of 0.9-1.3 m by 2100 for a middle IPCC scenario (A1B). These projections are typically a factor of 3-4 larger than the IPCC (2007) estimates, and thus they altered perceptions about the potential magnitude of human-caused sea level change.
Alley (2010) reviewed projections of sea level rise by 2100, showing several clustered around 1 m and one outlier at 5 m, all of these approximated as linear in his graph. The 5 m estimate is what Hansen (2007) suggested was possible under IPCC’s BAU climate forcing. Such a graph is comforting – not only does the 5-meter sea level rise disagree with all other projections, but its half-meter sea level rise this decade is clearly preposterous.
However, the fundamental issue is linearity versus non-linearity. Hansen (2005, 2007) argues that amplifying feedbacks make ice sheet disintegration necessarily highly non-linear, and that IPCC’s BAU forcing is so huge that it is difficult to see how ice shelves would survive. As warming increases, the number of ice streams contributing to mass loss will increase, contributing to a nonlinear response that should be approximated better by an exponential than by a linear fit. Hansen (2007) suggested that a 10-year doubling time was plausible, and pointed out that such a doubling time, from a 1 mm per year ice sheet contribution to sea level in the decade 2005-2015, would lead to a cumulative 5 m sea level rise by 2095.
Nonlinear ice sheet disintegration can be slowed by negative feedbacks. Pfeffer et al.
(2008) argue that kinematic constraints make sea level rise of more than 2 m this century
physically untenable, and they contend that such a magnitude could occur only if all variables quickly accelerate to extremely high limits. They conclude that more plausible but still accelerated conditions could lead to sea level rise of 80 cm by 2100

I had been following this debate and reading the papers in question and had been somewhat reassured that 21st century sea level rise would be not too problematic for civilization at large (though it clearly would be very painful for coastal property owners and jurisdictions).
(Before we go on it’s worth emphasizing the important aside – hardly any climate scientists doubt that huge quantities of the Greenland and Antarctic ice sheets would eventually melt and cause tens of meters of sea level rise as a result of human climate modifications – the debate is solely about how much of the consequences of our actions we will experience in the 21st century).
However, Hansen is not reassured by these new papers and is doubling down:

The kinematic constraint may have relevance to the Greenland ice sheet, although the assumptions of Pfeffer at al. (2008) are questionable even for Greenland. They assume that ice streams this century will disgorge ice no faster than the fastest rate observed in recent decades. That assumption is dubious, given the huge climate change that will occur under BAU scenarios, which have a positive (warming) climate forcing that is increasing at a rate dwarfing any known natural forcing. BAU scenarios lead to CO2 levels higher than any since 32 My ago, when Antarctica glaciated. By mid-century most of Greenland would be experiencing summer melting in a longer melt season. Also some Greenland ice stream outlets are in valleys with bedrock below sea level. As the terminus of an ice stream retreats inland, glacier sidewalls can collapse, creating a wider pathway for disgorging ice.
The main flaw with the kinematic constraint concept is the geology of Antarctica, where large portions of the ice sheet are buttressed by ice shelves that are unlikely to survive BAU climate scenarios. West Antarctica’s Pine Island Glacier (PIG) illustrates nonlinear processes already coming into play. The floating ice shelf at PIG’s terminus has been thinning in the past two decades as the ocean around Antarctica warms (Shepherd et al., 2004; Jenkins et al., 2010). Thus the grounding line of the glacier has moved inland by 30 km into deeper water, allowing potentially unstable ice sheet retreat. PIG’s rate of mass loss has accelerated almost continuously for the past decade (Wingham et al., 2009) and may account for about half of the mass loss of the West Antarctic ice sheet, which is of the order of 100 km^3 per year (Sasgen et al., 2010).
PIG and neighboring glaciers in the Amundsen Sea sector of West Antarctica, which are also accelerating, contain enough ice to contribute 1-2 m to sea level. Most of the West Antarctic ice sheet, with at least 5 m of sea level, and about a third of the East Antarctic ice sheet, with another 15-20 m of sea level, are grounded below sea level. This more vulnerable ice may have been the source of the 25 ± 10 m sea level rise of the Pliocene (Dowsett et al., 1990, 1994). If human-made global warming reaches Pliocene levels this century, as expected under BAU scenarios, these greater volumes of ice will surely begin to contribute to sea level change. Indeed, satellite gravity and radar interferometry data reveal that the Totten Glacier of East Antarctica, which fronts a large ice mass grounded below sea level, is already beginning to lose mass (Rignot et al., 2008).

However, probably their main point is that the data we have on the Antarctic/Greenland meltdown is relatively short and is consistent with the idea that it’s doubling with a relatively short (decade or less) timescale and if you extrapolate that out over the 21st century you get to very large values of sea level rise (a point I made in a blog post back in 2006). This leads them to include this figure (which I take to be a conceptual sketch rather than an exact forecast):

The picture that emerges is a relatively slow manageable sea level rise in the first part of the century followed by increasingly catastrophic levels of change in the latter part of the century as the rapid breakdown of the ice sheets overwhelms everything else.
I take Hansen’s opinions very seriously. It’s certainly true that there isn’t enough data to rule out this scenario yet (though another decade of data should help a lot). Obviously at this point he hasn’t succeeded in persuading most of his colleagues, but neither have they persuaded him. Only more data is likely to resolve the situation.