The mass balance, or difference between accumulation and ablation (melting and sublimation), of a glacier is crucial to its survival. Climate change may cause variations in both temperature and snowfall, resulting in changes in mass balance. A glacier with a sustained negative balance loses equilibrium and retreats. A sustained positive balance is also out of equilibrium and will advance to reestablish equilibrium. Currently, nearly all glaciers have a negative mass balance and are retreating. About 99 percent of all freshwater ice is in the great ice sheets of polar and subpolar Antarctica and Greenland. These continuous continental-scale ice sheets, 3 km (1.9 mi) or more in thickness, cap much of the polar and subpolar land masses. Like rivers flowing from an enormous lake, numerous outlet glaciers transport ice from the margins of the ice sheet to the ocean.
In Greenland, glacier retreat has been observed in outlet glaciers, resulting in an increase of the ice flow rate and destabilization of the mass balance of the ice sheet that is their source. The net loss in volume and hence sea level contribution of the Greenland Ice Sheet has doubled in recent years from 90 km3 per year in 1996 to 220 km3 per year in 2005. Researchers also noted that the acceleration was widespread affecting almost all glaciers south of 70N by 2005. The period since 2000 has brought retreat to several very large glaciers that had long been stable. Three glaciers that have been researched—Helheim Glacier, Kangerdlugssuaq Glacier, and Jakobshavn Isbræ—jointly drain more than 16% of the Greenland Ice Sheet. In the case of Helheim Glacier, researchers used satellite images to determine the movement and retreat of the glacier. Satellite images and aerial photographs from the 1950s and 1970s show that the front of the glacier had remained in the same place for decades. In 2001 the glacier began retreating rapidly, and by 2005 the glacier had retreated a total of 7.2 km, accelerating from 20 m (66 ft) per day to 35 m (115 ft) per day during that period.
Antarctica is intensely cold and arid. Most of the world's freshwater ice is contained within its sheets. Its most dramatic example of glacier retreat is the loss of large sections of the Larsen Ice Shelf on the Antarctic Peninsula. The recent collapse of Wordie Ice Shelf, Prince Gustav Ice Shelf, Mueller Ice Shelf, Jones Ice Shelf, Larsen-A and Larsen-B Ice Shelf on the Antarctic Peninsula has raised awareness of how dynamic ice shelf systems are. The Antarctic sheet is the largest known single mass of ice. It covers almost 14 million km2 and some 30 million km3 of ice. Around 90% of the fresh water on the planet's surface is held in this area and if melted would raise sea levels by 58 metres. The continent-wide average surface temperature trend of Antarctica is positive and significant at > 0.05 °C/decade since 1957.
