We recently covered some research on Greenland’s Jakobshavn Ice Stream, the world’s fastest glacier. While nothing on Antarctica can match that speed, the continent has ice streams of its own. Many have been shrinking, too—retreating and thinning as melting at the coast pulls continental ice out to die in the sea.

Of particular note is the Pine Island Glacier (PIG, for short). This massive glacier flows into the Amundsen Sea, and was the source of the 35km wide iceberg that made news last November. The hard-to-access Pine Island Glacier and its even larger neighbor, the Thwaites Glacier, are currently responsible for carrying about two-thirds of the ice lost from the West Antarctic Ice Sheet.

Trying to figure out exactly what the future holds for these glaciers is hard work. Studying the topography beneath them and the warming waters off the coast can enable computer models to more realistically simulate their behavior. Another way to get indications of future behavior is to exame evidence of past behavior.

But uncovering that history isn’t easy, either. After all, when part of a glacier melts, it’s gone. That leaves us looking for less ephemeral debris associated with the glacier's location. Shifts in the sediment deposited at the end of a glacier can record details of where it has been and what it has been eroding

One useful technique can enable researchers to determine the past activities of glaciers. Subatomic particles are constantly raining down on the Earth as a result of cosmic rays interacting with molecules in the atmosphere. Some of those particles can collide with atoms at the surface of the ice and transform them into different elements. Oxygen atoms in quartz, for example, can become a specific isotope of beryllium that would not naturally be present. The longer a quartz crystal has been at the surface (a meter of rock is enough to shield the crystals beneath from this bombardment), the more beryllium accumulates within it. The beryllium is a bit like a sunburn—measure how red your skin is and you can calculate how long you’ve been sitting out.

When a glacier picks up a chunk of bedrock or grinds down a boulder and drops it somewhere, “fresh” quartz gets exposed to this subatomic rain for the first time. Measure its beryllium levels, and you can estimate how long it’s been since ice was present to put that rock there.

The British Antarctic Survey’s Joanne Johnson and her collaborators sampled boulders from a pair of nunataks (peaks that stick up above the ice) near the end of the Pine Island Glacier. As glaciers shrink, they thin, as if deflating. That leaves more and more of the nunataks exposed. The researchers wanted to see what they could learn about past thinning, so they sampled boulders at various elevations above the ice.

The results showed that most of the rocks had been exposed for right around 8,000 years, with the exception of the three lowest elevation rocks at one of the nunataks, which have only been uncovered for 6,000 to 7,000 years. All the 8,000-year ages were within error bars of each other, meaning that they all emerged from the ice within about 100 years. Given their locations on the nunataks, that means the ice surface must have been dropping as the glacier thinned by around 1.5 meters per year.

(Incidentally, that time period appears to correspond to the warmest period of the Holocene, the current interglacial period.)

Why would that be? Like the present day, the researchers believe that the loss of ice from the (then much larger) shelf in front of the Pine Island Glacier following the end of the last ice age caused the glacier to speed up and thin. The rapid thinning must have lasted from a few decades to a few centuries. After that, it apparently remained pretty stable as the climate gradually cooled—until the recent warming.

This evidence of the Pine Island Glacier’s past behavior, the researchers write, “suggests that the PIG system can respond quickly to environmental change by abrupt, discontinuous, and stepwise retreat. Continued thinning may lead to an even more dramatic response if a dynamic threshold, such as a critical ice shelf thickness or ice flow rate, is exceeded.”

It’s that kind of dynamic change in flow like that makes the future sea level rise contributions from large glaciers like these hard to predict. They're like wildcards; if the flow rate increases because the glacier becomes unstable, it could lose a lot of ice rather quickly.

The estimated rate at which the Pine Island Glacier was thinning about 8,000 years ago is similar to the rate that models suggest it will continue thinning over the coming century. In those simulations, the PIG alone contributes up to a few centimeters—possibly even more than ten—to 21st century sea level rise.

Science, 2014. DOI: 10.1126/science.1247385 (About DOIs).