Driving ice-shelf melt is important because when ice shelves lose mass, they speed up flow of land-bound glaciers that feed them, moving ice from continent to ocean, & contributing to global sea level rise.
Fast Melt of Ice Shelves
Study, published on 13th June in journal Science, Antarctica՚s ice shelves are thinning by abt 1.6 feet (50 cm) per year. Some of them are thinning much more quickly, by as much as 328 feet (100 meters) annually.
Continent՚s glaciers are losing mass 2 to 3 times faster than it՚s being replenished.
Ice shelves are formed when glaciers meet ocean & begin flowing out to sea.
Their mass comes from snow that falls on top, which becomes compacted to form frozen water.
2 Fold Causes of Melting
Ice melts when ocean water beneath it warms sufficiently. In this case, melting isn՚t driven primarily by gradual warming of ocean water due to climate change, but rather by change in ocean circulation that is bringing up warmer water from offshore.
Atmospheric warming in Southern Hemisphere has led to slightly stronger “zonal” winds that whip clockwise around Antarctica & this, at least in certain areas, has pushed cold surface waters away from continent, allowing slightly warmer water to rise to surface & melt ice shelves.
This phenomenon may have helped melt Larsen B Ice Shelf, most of which collapsed in 2002.
Glaciers that were slowed by shelf՚s enormous mass have sped up, flowing to sea up to 8 times faster than previously.
If all ice shelves were entirely removed, glacier speeds would increase by up to tenfold on average.
Measuring Ice Melt
This is 1st comprehensive estimate of all Antarctic ice-shelf melting & calving.
Rignot and his co-authors calculated mass of ice shelves w⟋satellites & airplane radar readings that can penetrate ice & tell how thick it is.
Main problem is lack of data in certain areas - imagine difficulty of obtaining measurements beneath skyscraper-thick ice in Antarctica - as well as short record of accurate measurements.
They used data from NASA՚s IceBridge mission, 6-year campaign to survey & monitor Earth՚s polar ice sheet.
Scientists calculated snowfall by using computer models that show flux of moisture from atmosphere to ground.
By combining data, scientists figured out what fraction of Antarctic glacial ice is lost thru icebergs, and what is lost thru ocean melting.
Small changes, like slight increase in zonal winds - can cause major changes in ice system. It՚s a sensitive system.