A new way has been identified for global warming to strip the Earth of its remaining ice. Meltwater lakes on ice shelves cause the ice nearby to twist under the water's weight, hastening the shelves' collapse.
Computer models have suggested that when it gets warm enough for water to pool on ice shelves, such as those around Antarctica, the weight – up to 2 million tons – will depress the ice beneath. Refreezing come winter makes no difference, but if an opening drains the lake, the sudden weight loss should raise the shelf beneath. Like any other solid object, repeated flexing could produce cracks, which may contribute to the shelf breaking up.
These ideas are all theoretical, however, and we've discovered recently our understanding of the behavior of large quantities of ice is often not all that good. Dr Alison Banwell of Cambridge University attached pressure sensors to the McMurdo ice shelf to see how it is really behaving.
"Scientists have been predicting and modeling this process for some time, but nobody has ever collected field data that showed it happening until now," Banwell said in a statement.
In Nature Communications, Banwell reports that after controlling for those movements induced by the tides, sensors situated near 2-meter-deep (7 foot) depressions shifted as lakes formed and drained. More distant sensors remained stable.
The results confirm Banwell's modeling, and while good news for the authors, this is bad for humanity. Banwell's models indicate bending in parts of the shelf, while nearby areas were unmoved, could have caused the 2002 break up of 3,250 square kilometers (1,250 square miles) of the Larsen B Ice Shelf.
Antarctic expeditions described water pooling on ice shelves more than a century ago, but warmer temperatures are seeing it happen more often, meaning other shelves could soon follow Larsen B.
Ice shelves sit over water, rather than on land like ice sheets. Their loss doesn't directly affect sea level rise, since most of their volume is already displacing water.
Unfortunately, the disappearance of ice shelves does have two indirect, but very large, effects on ocean height. For one thing, ice shelves reflect most sunlight back into space, while the dark water that replaces them captures this energy, accelerating the rate of global warming. Furthermore, ice shelves often act as plugs, blocking glaciers' flow to the sea. As a result, they slow down the rate at which Antarctic ice is lost, and their removal could accelerate that process. With three-quarters of Antarctica bordered by ice shelves, their survival is essential to keeping the rate of land ice melt, and therefore sea level rise, to something we can manage.