ScienceNOW: Earth continues to hit temperature and greenhouse gas milestonesjust a couple of months ago, multiple stations measured carbon dioxide concentrations in the atmosphere of 400 parts per million, the highest in several million years. Many studies have tried to estimate how much and how rapidly the two great ice sheets covering Greenland and Antarctica might meltand the one reassuring point has been the apparent relative stability of the eastern (and, by far, larger) half of the Antarctic Ice Sheet....