State of the Cryosphere

Like a glacier, an ice sheet forms through the accumulation of snowfall, when annual snowfall exceeds annual snowmelt. Over thousands of years, the layers of snow build up, forming a flowing sheet of ice thousands of feet thick and tens to thousands of miles across. As the ice thickens, the increasing height of snow and ice causes the ice sheet to deform and begin to flow.

Global sea level rose by about 120 meters during the several millennia that followed the end of the last ice age (approximately 21,000 years ago), and stabilized between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century when the instrumental record of sea level change shows evidence for an onset of sea level rise. In the mid-19th century, the rate of sea level rise probably started exceeding the mean rate from the previous 2,000 years.

An ice shelf is a thick slab of ice, attached to a coastline and extending out over the ocean as a seaward extension of the grounded ice sheet. Ice shelves range in thickness from about 50 to 600 meters, and some shelves persist for thousands of years. They fringe the continent of Antarctica, and occupy a few fjords and bays along the Greenland and Ellesmere Island coasts.

Antarctic September Concentrations, 1979-2014. Image by Matt Savoie, National Snow and Ice Data Center, University of Colorado, Boulder.[/caption]

Last updated: 6 October 2014

Antarctic March Concentrations, 1979-2014. Image by Matt Savoie, National Snow and Ice Data Center, University of Colorado, Boulder.

Last updated: 6 October 2014

Arctic March Concentrations, 1979-2014. Image by Matt Savoie, National Snow and Ice Data Center, University of Colorado, Boulder.

Last updated: 6 October 2014

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