by Katherine Dempsey
Nov 18, 2014
Global warming is driving increased calving of the Greenland glaciers, causing icebergs the size of Central Park to crash into the sea. But low tides might be contributing to the calving crisis and helping to trigger huge earthquakes created by the massive icebergs, according to seismologist Meredith Nettles.
Nettles, of Columbia University, shared her latest findings with other leading climate scientists at the Comer Abrupt Climate Change Conference held in southwestern Wisconsin this fall. She showed her latest data about calving, the process of massive icebergs collapsing off of glaciers with increasing losses due to climate change. Nettles studies calving in Greenland and the “glacial earthquakes” that radiate seismic signals all the way to New Mexico.
Glacial earthquakes, which Nettles and a colleague first discovered in 2003, happen when a massive calving event occurs. In 2013, a record number of glacial earthquakes rocked Greenland compared to records kept since 1993. These magnitude-5 quakes can be detected even farther south than New Mexico, Nettles said. The seismic signals provide a sort of clock to indicate when calving takes place and Nettles also measures the velocities of glaciers with GPS receivers.
“One thing that’s been really useful about the earthquakes is to be able to say we know to the minute when this calving event happened,” Nettles said. “We can say, ‘Okay, the calving happened at this time and the speed-up happened at this time.’”
Enter the tides. Evidence suggests that calving affects how quickly glaciers overall weaken and move forward into the ocean, so it’s key to figure out what’s involved in the process, Nettles told conference scientists. And that’s what she is doing.
She documented observations at Helheim Glacier in 2009, using six seismic stations positioned on rock, to pick up small earthquakes – presumably caused by the ice breaking – in 30-second intervals. She and her team noticed that the earthquakes occurred in 12-hour cycles, similar to the tide. Comparing the occurrence of the earthquakes to tides revealed an anti-correlation – a low tide corresponded to a greater number of earthquakes than with a high tide.
Tides result from forces applied by the moon and sun. Most coastal places encounter two high and two low tides each lunar day. A lunar day – 24 hours and 50 minutes – signifies how long it takes for a location on Earth to rotate from a point under the moon and return to that spot under the moon. High tides happen every 12 hours and 25 minutes.
The parts of the earth pulled by the gravity of the moon as it orbits experience high tide. When the gravitational pull subsides, the tides retreat to low levels.
Although Nettles is still working to determine the relationship between the low tides and the earthquakes, low tide does add more stress to pull on the glacier, she noted. It lets the ice stretch more, and stretching any material makes it break.
Nettles pointed out that the flow of the glaciers changes as the tide changes. “When the tide is high, we have the position of the glacier retarded compared to mean,” she told scientists. When a low tide occurs, though, the glacier moves in advance of its average location.
Lots of stress from the tide creates strain in the glacier, she said. That strain may encourage calving. Calving models indicate that fragmenting the ice is necessary for an iceberg to separate, Nettles said, and small earthquakes seem to come from ice fracture or slip on shear faults.
Sea level increases as the ice sheet becomes smaller and its ice enters the ocean. That can create flooding and more severe storm surges, Nettles said.
A warmer world is linked to the shrinking ice sheets – more calving occurs when warmer water in the fjord makes contact with the glacier, she said. Understanding the process is critical as glaciers in the Arctic and Antarctic as well as in Greenland break apart.
If the West Antarctic Ice Sheet collapses, sea level would rise by about 11 feet on average across the earth. “There’s no question at all that the number of people and the amount of stuff that we have close to sea level is huge,” glaciologist Richard Alley said. “What happens to the beach, what happens to our shipping, what happens to our naval base?”
Alley – a top climate change researcher – discussed at the conference whether losing the West Antarctic Ice Sheet is in our future. Right now, the ice sheet is getting smaller, which drives sea level rise. Snowfall hits the surface of the sheet, and it spreads under its own weight and flows into the ocean. Typically, the ice does not calve off and create icebergs when it starts floating. It usually creates ice shelves that occupy coastal bays and fjords, he explained. Sea-floor high points and the fjord walls rub against the sides of the ice shelves, producing friction and slowing down how quickly the non-floating ice flows into the shelves, he noted.
But melting, linked partially to climate change, is accelerating.
“Recently, warmer ocean waters have been circulating beneath some of the ice shelves, increasing melting and iceberg calving from them, reducing their friction and thus allowing the non-floating ice to flow more rapidly into the ocean, raising sea level,” Alley pointed out. Warmer waters have moved under the ice shelves due to varying winds, which probably result from global warming, the Antarctic ozone hole, and “natural variability,” he said.