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.
