A sheet of ice extending over the sea from a land base, fed by snow falling on it, or from glaciers on the land. Smith et al. (2007) Antarc. Sci. 19, 1 record the constant, but minor, retreat of George VI ice shelf, Antarctica, since the mid-1980s, concluding that while atmospheric warming has been critical in driving this retreat, other mechanisms and feedbacks, such as changes in sea ice extent or oceanography, may be involved.