In October 2018, an image floating around the internet caused a stir. The photograph appeared to show something statistically implausible, if not impossible — a perfectly rectangular iceberg like a tabletop, serenely circling Antarctic waters:
— RTÉ News (@rtenews) October 25, 2018
Although the oddly-shaped piece of ice looks like it either is the product of digital editing software or perhaps some unusually motivated pranksters (or aliens), the truth is at once more interesting and more pedestrian than the theories, as it usually is. The original photograph appeared on October 17th, courtesy of the National Aeronautics and Space Administration and photographer Jeremy Harbeck:
From yesterday’s #IceBridge flight: A tabular iceberg can be seen on the right, floating among sea ice just off of the Larsen C ice shelf. The iceberg’s sharp angles and flat surface indicate that it probably recently calved from the ice shelf. pic.twitter.com/XhgTrf642Z
— NASA ICE (@NASA_ICE) October 17, 2018
There are two types of icebergs, tabular and non-tabular. Tabular icebergs, as their name implies, have steep sides and a flat top, like a table, according to the United States Coast Guard’s Navigation Center’s terse description:
An iceberg with steep sides and flat top having a length-to-height ratio greater than 5:1. Many show horizontal banding.
By comparison, non-tabular icebergs (also known as everything else) are described as follows: Dome, pinnacle, wedge, dry-dock and blocky. Different sizes have different designations, as well. Glaciers and icebergs are well-known descriptors, but the smaller chunks of ice have the most delightful names: bergy bits, for bergs between one and four meters tall, and growlers, which are a meter high or less.
But tabular icebergs are common, even prosaic. Whereas non-tabular icebergs often come into the world as the result of a spectacular calving or crashing, tabular icebergs tend to split relatively quietly away from ice shelves, which are (again, as the name suggests) long, flat sheets of ice connected to landmasses:
Most of the world’s ice shelves hug the coast of Antarctica. However, ice shelves can also form wherever ice flows from land into cold ocean waters, including some glaciers in the Northern Hemisphere. The northern coast of Canada’s Ellesmere Island is home to several well-known ice shelves, among them the Markham and the Ward Hunt ice shelves.
Given that these “ice shelves” are already elongated and flattened by nature, it makes sense that chunks break off at angles:
Sometimes, those ice chunks break into perfectly aligned right angles that look as though they might have been sculpted before they are eroded away by water, sunlight, marine creatures, and other pressures:
Wow, it’s been amazing to see what a splash our photo of a tabular Antarctic iceberg, by #IceBridge’s Jeremy Harbeck, has made. Fly toward the berg with @NASAArmstrong‘s DC-8 forward camera. More: https://t.co/kADuUL455F pic.twitter.com/tm4Rydh8V3
— NASA ICE (@NASA_ICE) October 23, 2018
But it’s not all good news. Climate change has so accelerated the warming process that ice shelves are disintegrating at a record pace:
Most ice shelves are fed by inland glaciers. Together, an ice shelf and the glaciers feeding it can form a stable system, with the forces of outflow and back pressure balanced. Warmer temperatures can destabilize this system by increasing glacier flow speed and—more dramatically—by disintegrating the ice shelf. Without a shelf to slow its speed, the glacier accelerates. After the 2002 Larsen B Ice Shelf disintegration, nearby glaciers in the Antarctic Peninsula accelerated up to eight times their original speed over the next 18 months. Similar losses of ice tongues in Greenland have caused speed-ups of two to three times the flow rate in just one year.
While calving or disintegrating ice shelves don’t raise ocean level, the resulting glacier acceleration does, and this poses a potential threat to coastal communities around the globe. Worldwide, more than 100 million people currently live within 1 meter of mean sea level. Greenland contains enough ice to raise sea level by 7 meters, and Antarctica holds enough ice to raise sea level by 57 meters. While these ice sheets are unlikely to disappear anytime soon, even partial loss of the grounded ice could present a significant problem. In the coming decades of a climate warming era, ice shelves and ice tongues are likely to play a prominent role in changing the rate of ice flow off the major ice sheets.
Waters around the Antarctic Peninsula has warmed 2.5 degrees Celsius, or 4.5 degrees Fahrenheit, since 1950, alarming climate scientists and accelerating other changes. However, whatever fate of this hunk of ice might meet in the end, it is real and formed not by pranksters or playful extraterrestrials, but by natural forces.