The IPCC AR4 estimates explicitly exclude the influence of the melting of ice sheets. These ice sheets include most notably the Greenland ice sheet, and both the east and west Antarctic ice sheets, as well as numerous glaciers. This may result in a major underestimate of the upper limit for sea level rise in the long term. Due to Arctic melting the Greenland ice sheet is particularly vulnerable, and a study by climatologist James E. Hansen states that "we cannot rule out large changes on decadal time-scales once wide-scale surface melt is underway.", the melting of the Greenland ice sheet would result in an increase in sea level rise of over 7m. Melting of the west Antarctic ice sheet would cause a similar, if slightly smaller rise in sea levels due to being grounded below sea level, whilst the effect of the melting of the east Antarctic, although less probable would be an order of magnitude greater.