the West Antarctic Ice Sheet is losing mass at an increasing rate come from the Amundsen Sea sector, and three glaciers in particular: Pine Island Glacier, Thwaites Glacier and Smith Glacier. Data reveals these three glaciers are losing more ice than is being replaced by snowfall. According to a preliminary analysis, the difference between the mass lost and mass replaced is about 60%. The melting of these three glaciers alone is contributing an estimated per year to the rise in the worldwide sea level. There is growing evidence that this trend is accelerating: there has been a 75% increase in Antarctic ice mass loss in the ten years 1996–2006, with glacier acceleration a primary cause. As of November 2012 the total mass loss from the West Antarctic Ice Sheet is estimated at mainly from the Amundsen Sea coast. Satellite measurements by ESA's CryoSat-2 revealed that the West Antarctic Ice Sheet is losing more than of ice each year. The loss is especially pronounced at grounding lines, the area where the floating ice shelf meets the part resting on bedrock, and hence affects the ice shelf stability and flow rates. Potential collapse Large parts of the WAIS sit on a bed which is both below sea level and sloping downward inland. This slope, and the low isostatic head, mean that the ice sheet is theoretically unstable: a small retreat could in theory destabilize the entire WAIS, leading to rapid disintegration. Current computer models do not account well for the complicated and uncertain physics necessary to simulate this process, and observations do not provide guidance, so predictions as to its rate of retreat remain uncertain. This has been known for decades. It is considered one of the tipping points in the climate system. In 2000s The possible disastrous outcome of a disintegration of the WAIS for global sea levels has been mentioned and assessed in the IPCC Third Assessment Report from 2001. However, it was not included in the IPCC Fourth Assessment Report. Jessica O'Reilly, Naomi Oreskes and Michael Oppenheimer discussed the case in a Social Studies of Science paper 2012. According to them, IPCC authors were less certain about potential WAIS disintegration not only due to external new science results. As well pure internal "cultural" reasons, as changes of staff within the IPCC and externally, made it too difficult to project the range of possible futures for the WAIS as required.