A new study challenges as highly unlikely an alarming prediction of sea-level rise that -- while designated as low likelihood --earned a spot in the latest UN climate report for its projection that the collapse of polar ice sheets could make the world's oceans up to 50 feet higher by 2300. But researchers found that the model is based on inaccurate physics of how ice sheets retreat and break apart, though they stress that the accelerating loss of ice from Greenland and Antarctica is still dire.