With Japan’s tsunami still fresh in the minds of many, researchers remind us that earthquakes are not the only source of these giant ocean waves—-collapsing icebergs can cause them too. In an increasingly warmer world, where it is more likely for a mass of ice to break off and fall into the sea, should we be worried about ‘glacial tsunamis’?
The splitting and shedding of parts of a glacier can cause water levels to surge. Tsunami risk is higher if the infall occurs from the elevated areas of ice streams ending at the sea or bodies of freshwater. Last year, a glacier broke apart in Peru triggering a powerful tsunami in a lake in the Andes. In 1995, damage in a Greenland fjord caused destruction in a nearby harbor.
But a recent article in Annals of Glaciology, highlighted in Nature last week, tells of yet another ‘glacial source’ of tsunamis: the capsizing of giant masses of ice. The icebergs that result from calving—-or splitting off—-from a glacier vary in shape and size. If their height is larger than their width, they can be unstable following break off and roll over, possibly creating a giant wave.
Iceberg capsizing “poses the least-recognized practical hazard to humans” of the tsunami-causing mechanisms related to calving of ice masses, the authors write in the paper. However, the energy released when icebergs roll over “can be as large as that of an earthquake of magnitude 5-6,” to quote Anders Levermann, from the Postdam Institute for Climate Impact Research in Germany, writing for Nature.
The energy estimate done by the authors of the Annals of Glaciology paper, researchers at the Chicago and Princeton Universities, is based on “a simplified, but not completely unrealistic, rectangular geometry,” according to Levermann. Using this model shape, they calculate the potential energy (due to the Earth’s gravity and water pressure) stored in the floating iceberg before and after roll over. The difference provides the energy released due to capsize, which can amount to the explosive energy of several thousands tons of TNT.
While some of it is dissipated through rocking motions and other processes, a significant proportion of the released energy can create a tsunami with a crest reaching up to one percent of the initial height of the iceberg. For an average floating ice mass from Antarctica, this corresponds to a 4-meter wave. But the tsunamis created by the tallest icebergs on Earth could be as high as 10 meters. “These numbers are comparable to the open-ocean crest heights of the devastating tsunami in the Indian Ocean in 2006 and the recent event in Japan,” Levermann writes.
As the Chicago and Princeton researchers conclude in their paper, “icebergs can produce tsunamis that are sufficiently energetic to pose a threat to life and property.” While this damage is typically limited to icy areas with few inhabitants, tsunamis can travel large distances and eventually reach more populated regions.
Whether the risk of ‘glacial tsunamis’ will be higher in the warmer world of tomorrow is uncertain. But given that most of Greenland’s and Antarctica’s ice loss, which has accelerated over the last 20 years, is due to calving, these tsunamis may need to be added to the list of future climate change threats.