An international team, made up of scientists from the United States, the UK, Denmark, Germany and Switzerland, analysed the sediment cores recovered beneath the floating Pine Island Glacier (PIG) ice shelf. The team concluded that the date at which the grounding line retreated from a prominent seafloor ridge was 1945 at the latest. They also found that the final ungrounding of the ice shelf from the ridge occurred in 1970.
These results are not only unprecedented but also alarming – not only is the glacier going backwards, it is also thinning fast, losing more than 2 metres in elevation every year. Other field studies have suggested that a runaway collapse might even be possible over the next few years. Estimates have been made that suggest the PIG alone could add up to 10 mm to sea levels over the next couple of decades.
The PIG is a colossal feature that drains a region of the West Antarctic Ice Sheet which is approximately two-thirds of the size of the UK. It dumps around 130 billion tonnes of ice into the ocean every year. It is also a marine-terminating glacier, which means that its front flows off the land and pushes out into the ocean along the seafloor until its mass begins to lift up and float. Eventually, the buoyant section breaks up to form icebergs.
‘Our results suggest that, even when climate forcing (such as El Niños, which create warmer water) weakened, ice-sheet retreat continued,’ commented James Smith of the British Antarctic Survey and lead author of an article that has appeared in the most recent issue of the journal ‘Nature’. One of the possible explanations as to why the glacier began its retreat in the mid-1940s is that there was a particularly powerful El Niño event between 1939 and 1942.
Another factor as to why the research team can be sure of when the glacier’s retreat began is by seeing when lead and plutonium traces began appearing in the glacier’s sediment layers. These are a telltale throwback to the atomic bomb tests that began in earnest after the Second World War and peaked in the 1960s. Analyses of trace levels of global fallout plutonium in the sediment were performed by a high-precision mass spectrometry at Lawrence Livermore National Laboratory in California.
Although El Niño events have waxed and waned in the decades since, the glacier’s retreat has relentlessly continued. ‘Despite a return to pre-1940s climatic conditions in the ensuing decades, thinning and glacier retreat is unlikely to be reversible without a major change in maritime or glaciological conditions,’ Smith argues.
Dr Anna Hogg from Leeds University, UK, monitors PIG on a daily basis using Europe’s Cryosat and Sentinel satellites. She commented to the BBC: ‘We know from satellite observations that the PIG has sped up and retreated episodically since the late 1970s, so it’s interesting to see that the sediments beneath the glacier record similar levels of variability dating back to the 1940s. This erratic behaviour suggests that we should not these colossal glaciers to respond in a steady way in the future, making continuous monitoring increasingly important.’