For the past decade, scientists have debated whether a massive Antarctic ice shelf is thinning from warmer air above or warmer oceans below. Today, the mystery is solved: It's both.
Using a new technique to analyze satellite data and radar surveys, researchers with the British Antarctic Survey tried to determine what caused the Larsen C ice shelf to drop in surface elevation by about a metre between 1998 and 2012. The team's paper, published in the journal The Cryosphere on May 13, found that surface melting and snow compaction from above contributed to the drop in roughly equal measure to melting on the underside of the shelf.
"So we know that Larsen C is being thinned from two angles at once," said lead author Dr. Paul Holland with the British Antarctic Survey in an email to Motherboard. "This resolves the debate, but is also quite surprising. In other parts of Antarctica ice shelves are only thinned by ocean melting because those ice shelves do not melt much on their surface."
The finding is noteworthy because the ice shelf is currently performing the important task of preventing massive glaciers from flowing into the ocean, where they would contribute to a significant rise in sea level.
Ice shelves are floating masses of ice and snow that are attached to continental ice but sit out over the ocean. Larsen C's neighbours, Larsen A and B, broke away and collapsed spectacularly in 1995 and in 2002, respectively. Larsen B had been stable for at least 10,000 years. Here's an animation of the what the Larsen B collapse, which occurred over the span of about a month, looked like from NASA's Terra satellite.
Ice shelves alone do not contribute to sea level rise when they melt or even when they collapse, because they are already floating. However, their absence clears the way for continental ice to creep into the ocean, which does affect the water level.
Larsen B was about 3,250 square kilometres in size, while Larsen C is about 50,000—nearly the size of Nova Scotia. It is the largest ice shelf on the Antarctic peninsula. The glaciers behind Larsen A and B, now freed from their icy restraints, are flowing into the ocean at an accelerated rate. Together those glaciers contribute about a quarter of Antarctica's contributions to sea level rise, Holland said.
"Larsen C is much bigger and so it collapsing is a worry. In total, a collapse of Larsen C could cause sea-level rise of a few centimetres," he explained. That's equivalent to more than ten years' worth of global sea level rise at the current rate, according to brand new research published in Nature Climate Change.
The surface melting is of less concern to the shelf's integrity, for now. When snowpack melts, it seeps down into the shelf and refreezes, Holland said. This compresses and lowers the profile of the shelf but does not reduce its mass. If the snowpack is fully depleted, though, this becomes a greater concern, as meltwater is likely to pool and carve deep crevasses. This was likely a very significant contributor to Larsen B's collapse. Larsen C still has about 10 metres of snowpack. At the current rate of loss, it will be 250 years before it's all gone, according to the research.
Holland's team evaluated the potential for Larsen C's collapse, and found an "imminent risk" from at least two sources of stress. For one, ocean melt and iceberg calving—in which chunks of a glacier break off and fall into the water—could destabilize the arch-like structure of the shelf, causing a progressive collapse from the outside in. Researchers are watching one rift in the southern part of the shelf particularly closely. According to the paper, it is at risk of threatening the shelf's structure within a few years.
Secondly, the shelf could also break away from where it meets continental ice at the Bawden Ice Rise in the near future. "A large calving occurred south of Bawden between late December 2004 and early January 2005, and the ongoing thinning and acceleration in this region could indicate that ungrounding from Bawden is underway," according to the study. (Ungrounding is when an ice shelf turns free-floating, having detached from the ground beneath.)
Sea level rise is one of the most frightening threats of climate change. The Intergovernmental Panel on Climate Change currently projects average levels to go up between half a metre and one metre within the 21st century if drastic measures are not taken to reduce carbon emissions. One study estimated that 10 percent of the world's population lives at elevations of 10 metres or less above sea level. About 40 percent of those at risk live in low-income countries, and 75 per cent in Asia. Twenty one countries have more than half of their population in low-lying areas, leaving them particularly vulnerable to threats of bigger storms, more severe flooding, contaminated drinking water, erosion and environmental destruction.
"To understand the future rise in sea level we need to add up the contributions from all the contributions worldwide, which includes that of individual glaciers and ice shelves," said Holland. "To do that, we need to first understand really basic aspects of their behaviour, such as whether they are being destabilized by ocean forcing or atmospheric forcing."