Image: an 18 mile crack across Pine Island's floating tongue, the site of departure for a future iceberg/llnl.gov
Scientists have watched with awe the deterioration of the Pine Island Glacier, one of the major outlets of the West Antarctic Ice Sheet and the fastest moving glacier on the continent. Last summer, the glacier deposited an iceberg eight times the size of Manhattan into the Amundsen Sea. According to data gleaned via NASA’s IceBridge aerial survey, the glacier is giving up 46 gigatons of water every year, or about a single Chesapeake Bay every two years. That’s projected to increase up to and above 100 gigatons a year in the coming decades. The ice mass itself moves at about four kilometers per year into the ocean, a dramatic acceleration of the glacier’s natural role as an outlet channel for the Antarctic ice sheet.
Climate change? A study last month in Science didn’t draw a neat cause and effect, but said something just as significant: the uniqueness of the glacier and its surrounding geology makes it especially vulnerable to climate variability (climate change, if you will), which is delivered in the form of La Nina cycles in the Pacific. La Nina and El Nino ocean temperature periods help push warmer and (lately, via La Nina) cooler water into and out of the pocket of water that exists underneath the glacier’s deepest ice, which is thought to function as a sort of lubricant for its ongoing trek to the sea.
Research published last spring in Nature Climate Change made a convincing case that that trek is now engaged in a 40 km retreat, a movement that will only continue to accelerate as warm water laps at the shallow leading edge of the glacier and as the glacier very slowly increases its angle of descent off of the continent's land shelf. The thin hope for Pine Island is that climate change will boost the frequency of La Nina events, which should in turn slow down the glacier’s progression by injecting cooler water from the water mass known as the Circumpolar Deep Water.
I say “thin hope” above in part because of a study out last week in Science examining Pine Island’s very long-term history. Researchers centered at the Lawrence Livermore National Laboratory examined beryllium-10 dated glacial rocks to chart the glacier’s historical trends. What they found is that 8,000 years ago Pine Island was in remarkably similar shape to what we see today, with a similar rate of acceleration into the ocean. The concern for us now is that that rate of glacial retreat continued for decades or even centuries.
If the entire West Antarctic Ice Sheet were to follow the Pine Island Glacier into the sea, the resulting melt would boost worldwide sea levels by over three meters. The deep worry is that the glacier system will be pushed beyond a certain tipping point where such a thing becomes if not just feasible then likely, with assistance from progressing climate change and its assorted feedback loops. We’re in a period of unique instability on Earth, and that instability is directly tied to the inherent instability of ice formations that keep sea level doom at bay. It's not exactly a comforting thought.