When Greenland's melting glaciers lose large chunks of ice, it's a violent process. Last year, for instance, scientists documented that gigantic glacial earthquakes are triggered by the rolling and tumbling of billion-ton icebergs as they break away and hit the glaciers to which they once belonged – hard.
But large masses of ice falling into the waters of Greenland's fjords do something else, too. Depending on the mass of ice lost and the particular configuration of the water and the fjord into which it surges, these events can also create destructive tsunamis, albeit of a relatively small scale (compared with how big open ocean tsunamis can get). And now, a recent study has found that at least one notable Greenland glacier, these tsunamis appear to be getting worse as melting advances.
Martin P. Lüthi and Andreas Vieli of the University of Zurich in Switzerland studied what they call an "exceptionally well-documented" tsunami event that happened in July of 2014 in the fjord that terminates at the glacier Eqip Sermia. It's one of the many large ocean-terminating glaciers of southwest Greenland and a popular tourist spot, since it has the advantage of being relatively close to the town of Ilulissat and reachable in a few hours by boat.
Not only did the researchers have tide gauges and other instruments set up in the area around Eqip Sermia – a tourist boat was 800 meters away from the glacier when the ice collapse that triggered the tsunami happened. So the scientists could also analyze video from the boat that was published on YouTube (and embedded here):
The new study of these tsunamis, recently published in the open access journal The Cryosphere, contains some startling findings about the scale of these events.
In the closely studied 2014 event, Eqip Sermia lost what the researchers calculate to be some 900,000 cubic meters of ice in a landslide from its 200 meter (656 feet) tall ice cliff face. For a Greenland glacier, 900,000 cubic meters is not actually a very big loss. The monstrous nearby Jakobshavn glacier can lose well over a square mile in glacier area (as measured at its surface, in two dimensions) at a single time.
Nonetheless, when this particular ice mass fell into the fjord, whose waters are quite shallow near the front of the glacier, it created a sudden tsunami wave of some 50 meters (164 feet) in height, based on the researchers' analysis of the video above, taken from the tourist boat. (This means that wall of water was much larger than it may have appeared in the video.)
"This is like a bulldozer at 30 meters a second, going into this water, and pushing it forward," said Lüthi.
Fortunately, in this case the boat was far enough away that by the time the swell reached it, it was not damaged (though the boat certainly bobbed up and down quite a bit as the tsunami traveled by).
However, at the far shore 4.55 kilometers (2.33 miles) away from the glacier front, it was another story. Here, the resulting wave, arriving 160 seconds later, reached heights of 10 to 15 meters (32 to 49 feet).
That wave seems not to have been recorded on video, but a year earlier, another one at the same location was:
Greenland's 200 or so outlet glaciers have always calved and lost ice, so what makes this behavior at Eqip Sermia so unique?
The researchers say that Eqip Sermia – which is only "medium sized" in Greenland terms, with an ice front of 4 kilometers (just under 2.5 miles) in width – has been undergoing changes lately that seem to prompt worse tsunami events.
In particular, the glacier has sped up its losses and its retreat inland toward the massive center of the Greenland ice sheet. In the process, as more ice has been lost, the retreating glacier formed an unusually high cliff face, extending between 150 and 200 meters out of the water of the fjord, around 2012. It is this sheer height that appears to have allowed more ice to collapse into the water at one time, and thus "induced calving events that lead to 15m tsunami waves upon landfall which had not been observed before."
"What's exceptional for that glacier is the height of the front, it's 200 meters, it's really really high," said Lüthi. "And I've never seen any glacier with such a high front. This is just not stable, that's why it collapses constantly." Lüthi thinks this is only a temporary configuration of the glacier – with further retreat, it might lose some of this height and, with it, the proclivity to generate such alarming tsunamis.
The researchers add that before 2012, Eqip Sermia's cliff face was only 50 meters high, and while collapses still produced tsunamis, they were considerably smaller.
"This is really special, it probably will stay like this for a couple more years, causing crazy waves like that, and after that, it will be different," said Lüthi.
But for now, on a weekly basis during the summer, calving events and tsunamis occur, the study says, creating hazards for those visiting the area. Additional evidence that this is unique and new, at least in its scale, is that "old vegetation including birch bushes high on the shores" has now been hit by and "eroded" by the waves, the paper says. Presumably the vegetation had survived in this location for a long time.
"This supports the notion that such high tsunami waves are a novel phenomenon on a decades-to-century timescale," the study notes.
Again, the good news is that this one spot in Greenland seems to be unique. But while it's a special instance, it can also be interpreted in the context of a much broader tapestry of transformation.
"The glaciers are changing like crazy, really rapidly," said Lüthi. "Everything changes, and people cannot rely on their experience from generations. Suddenly things happen that nobody thought of before."