Credit: WRAGG/BRITISH ANTARCTIC SURVEY

Over three-quarters of Earth's fresh water is locked in the great ice sheets covering Greenland and Antarctica. The stability of these ice sheets is the single largest unknown factor in attempts to predict how sea level may rise in the future, as even a small change in the mass balance of this ice would result in a significant change in global sea level. The recent break-up of some ice shelves on the Antarctic Peninsula (see Nature 379, 328–331; 1996), and the recognition that large changes have occurred over the past few decades in ice streams feeding the West Antarctic ice sheet (Science 279, 689-692; 1998), has generated a great deal of public interest in the problem.

Although the break-up of ice shelves does not directly affect sea level (by definition, ice shelves are the floating parts of ice sheets, and so already displace an equivalent volume of sea water), it could be that their disintegration would seriously affect the discharge rate of grounded ice from inland regions.

Elsewhere in this issue (Nature 391, 778–780; 1998), Christopher Doake and colleagues describe an attempt to establish a stability criterion for ice shelves through modelling of the two northern sections of the Larsen ice shelf on the Antarctic Peninsula. Shown here is the northern section of this ice shelf, Larsen A, which disintegrated within a few days in January 1995 (just before this photograph was taken); in the upper left-hand corner can be seen the edge of Larsen B, which is still intact, although retreating. Through finite-element computer modelling of the strain rates for these two sections of ice shelf from 1986 to 1997, Doake et al. discovered that only the initial and final ice front positions of Larsen A were stable, and that if Larsen B were to retreat only a few more kilometres it too is likely to disintegrate.

The hope is that these stability criteria can be used elsewhere, to help distinguish which ice shelves are in danger of collapse.