of any geological modelwe knew something was missing dr deconto saidthe new idea came from dr alley he urged his colleagues to consider what would happen as a warming climate attacked huge shelves of floating ice that help to protect and buttress the west antarctic ice sheetsmaller nearby ice shelves have already started to disintegrate most spectacularly in 2002 when an ice shelf the size of rhode island the larsen b shelf broke apart in two weeksthe west antarctic ice sheet sits in a sort of deep bowl that extends far below sea level and if it loses its protective fringes of floating ice the result is likely to be the formation of vast sheer cliffs of ice facing the sea these will be so high they will become unstable in places dr alley said in an interview and the warming atmosphere is likely to encourage melting on their surface in the summer that would weaken them furtherthe result dr alley suspected might be a rapid shrinkage as the unstable cliffs collapsed into the water something like this seems to be happening already at several glaciers including at least two in greenland but on a far smaller scale than may be possible in west antarcticawhen dr deconto and dr pollard drawing on prior work by j n bassis and c c walker devised some equations to capture this icecliff instability their model produced striking results in contrast to many prior attempts it suddenly had no difficulty recreating the high sea levels of past warm periodsthe obvious next step was to ask the model what might happen if human society continues to warm the planet by pouring huge amounts of greenhouse gases into the atmospherethe answer the scientists got is described in their paper in the dry language of science but it could easily serve as the plot device of a hollywood disaster movie they found that west antarctica which is already showing disturbing signs of instability would start to break apart by the 2050svulnerable parts of the higher colder ice sheet of east antarctica would eventually fall apart too and the result by the year 2500 would be 43 feet of sea level rise from antarctica alone with still more water coming from elsewhere the computer estimated in some areas the shoreline would be likely to move inland by milesthe paper published wednesday does contain some good news a far more stringent effort to limit emissions of greenhouse gases would stand a fairly good chance of saving west antarctica from collapse the scientists found that aspect of their paper contrasts with other recent studies postulating that a gradual disintegration of west antarctica may have already become unstoppablebut the recent climate deal negotiated in paris would not reduce emissions nearly enough to achieve that goal that deal is to be formally signed by world leaders in a ceremony in new york next month in a united nations building that stands directly by the rising water