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ACSL Key Findings

              ABRUPT CHANGES IN THE EARTH’S CLIMATE SYSTEM

           ABRUPT CHANGE IN SEA LEVEL




                                                                 Key Findings

 -    Since the mid-19th century, small glaciers (sometimes called " glaciers and ice caps;”) have been losing mass at an average rate equivalent to 0.3 to 0.4 millimeters per year of sea level rise.

-    The best estimate of the current (2007) mass balance of small glaciers is about -400 gigatons per year (Gt a-1), or nearly 1.1 millimeters sea level equivalent per year.-    The mass balance loss of the Greenland Ice Sheet during the period with good observations increased from 100 Gt a-1 in the mid-1990s to more than 200 Gt a-1 for the most recent observations in 2006.  Much of the loss is by increased summer melting as temperatures rise, but an increasing proportion is by enhanced ice discharge down accelerating glaciers.

-    The mass balance for Antarctica is a net loss of about 80 Gt a-1 in the mid-1990s, increasing to almost 130 Gt a-1 in the mid-2000s.  There is little surface melting in Antarctica, and the substantial ice losses from West Antarctica and the Antarctic Peninsula are very likely caused by increasing ice discharge as glacier velocities increase.

-    During the last interglacial period (~120 thousand years ago) with similar carbon dioxide levels to preindustrial values and arctic summer temperatures up to 4 ºC warmer than today, sea level was 4-6 meters above present.  The temperature increase during the Eamian was the result of orbital changes of the sun.  During the last two deglaciations, sea level rise averaged 10-20 millimeters per year with large "meltwater fluxes” exceeding sea level rise of 50 millimeters per year lasting several centuries.

-    The potentially sensitive regions for rapid changes in ice volume are those with ice masses grounded below sea level such as the West Antarctic Ice Sheet, with 5 to 6 meters sea level equivalent or large glaciers in Greenland like the Jakobshavn Isbrae, also known as Jakobshavn Glacier and Sermeq Kujalleq (in Greenlandic), with an over-deepened channel reaching far inland; total breakup of Jakobshavn Isbrae ice tongue in Greenland, as well as other tidewater glaciers and ice cap outlets, was preceded by its very rapid thinning.

-    Several ice shelves in Antarctica are thinning, and their area declined by more than 13.500 square kilometers in the last 3 decades of the 20th century, punctuate by the collapse of the Larsen A and Larsen B ice shelves, soon followed by several-fold increases in velocities of their tributary glaciers.

-    The interaction of warm waters with the periphery of the large ice sheets represents a strong potential cause of abrupt change in the big ice sheets, and future changes in ocean circulation and ocean temperatures will very likely produce changes in ice-shelf basal melting, but the magnitude of these changes cannot currently be modeled or predicted.  Moreover, calving, which can originate in fractures far back from the ice front, and ice-shelf breakup, are very poorly understood.

-    Existing models suggest that climate warming would result in increased melting from coastal regions in Greenland and an overall increase in snowfall.  However, they are incapable of realistically simulating the outlet glaciers that discharge ice into the ocean and cannot predict the substantial acceleration of some outlet glaciers that we are already observing. 
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