It’s Called Isostatic Rebound
Greenland and Antarctica shed record amounts of ice during the past several years, victims of a warming planet. On a single day in the summer of 2019, Greenland lost 12.5 billion tons of ice. A 2020 study by scientists at Ohio State University (King et al.) used four decades of satellite data to trace Greenland’s ice-loss history. The study found that from 2000 forward, the continent lost more ice each year than was replaced by snow. Greenland is now the largest contributor on the planet to sea-level rise. As the ice melts, the continent rises.
Terra firma provides us with a sense of stability when we stand firmly on solid rock, but our sense of unchanging stability is an illusion. From a geological perspective, Earth’s surface is a shifting landscape of continents sliding past each other and sometimes violently colliding. When continents collide, mountains like the Himalayas rise upward towards the heavens. Terra firma is only a fleeting moment in the dynamic age-old dance of plate tectonics — continents are always in motion.
Despite its solid appearance, Earth’s surface is also mobile, flexing and bending in response to the waxing and waning geological stresses, including glacial ice sheets. The simplest analogy is a cork floating in a bowl of water. Press the cork with your finger, and it submerges, but remove the weight of your finger, and it pops back above the surface again.
The worlds thickest ice sheets are in Antarctica, where the ice is up to three miles deep. Pressure at the base of this ice exceeds 6,000 pounds per square inch or 432 tons per square foot. Ice has weight, and massive ice sheets press on the underlying continent, forcing it to sink like the cork in water.
Isostatic Rebound
Melting ice sheets reverse the process and reduce vertical pressure as the meltwater flows off the continent and into the ocean. Once relieved of the weight from the overlying ice, the continental crust rises upward in a process called isostatic rebound. While the cork in our experiment instantaneously popped back to its original level, continental rebound takes thousands of years. Parts of the North American continent are still rising as they recover from the weight of thick ice sheets, which formed during the last ice age.
Ten thousand years ago, one of the thickest areas of ice in North America was just south of Hudson Bay, Canada. There, a series of ancient shorelines (strandlines) record the isostatic rebound. A total of 185 ancient strandlines rise like a giant staircase, each one representing a period in time when its rock and soil were at sea level. Collectively they record 995 feet of uplift starting about 8,000 years ago. Initially, the continent rebounded at about 33 to 39 feet each 100 years. Today that rate is only 4.3 feet per 100 years.
Most of the northern hemisphere’s ice sheets have disappeared since the last ice age, and the Greenland Ice sheet remains one of the last vestiges of a cold, ice age Earth. The ice covering Greenland began forming about three million years ago, slowly depressing the continent.
The Changes are Noticeable
Anthropocene climate change is rapidly undoing three million years of mother nature’s work. The heat from global warming eats away at Greenland’s ice cap, melting water at its surface and calving ice into the Atlantic Ocean at the seaward ends of glaciers.
Dissolution of the Greenland ice sheet is a major driver of sea-level rise. There is also work indicating that the large volumes of freshwater accumulating in the North Atlantic are interfering with the North Atlantic circulation system — the Atlantic Meridional Overturning Circulation (AMOC). This system drives the famous Gulf Stream, which carries warm water to the North Atlantic from the tropics, providing Europe with mild winters from heat supplied by the Gulf Stream.
As the ice melts, Greenland rises due to isostatic rebound, and some evidence indicates the Earth’s crust may be warping in other ways. A recent study analyzed satellite data from 2003 to 2018 and detected both horizontal and vertical crustal movement in Greenland. Interestingly, the data indicated Greenland’s ice melt might also affect much larger areas of the Northern Hemisphere.
Greenland rises, and continents move as humankind’s grand experiment in climate change continues warming the only planet we have.
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Sources:
Dynamic ice loss from the Greenland Ice Sheet driven by sustained glacier retreat (By Michalea D. King, Ian M. Howat, Salvatore G. Candela, Myoung J. Noh, Seonsgu Jeong, Brice P. Y. Noël, Michiel R. van den Broeke, Bert Wouters & Adelaide Negrete; Nature — Communications Earth & Environment)
Record Hit for Most Ice to Melt in Antarctica in One Day, Data Suggests (Kashmira Gander — Newsweek)
Ice Sheets (Source: National Science Foundation)
Glacial Rebound, Warped Beaches and the Thickness of the Glaciers in North Dakota (by John P. Bluemle)
How and When Did Greenland Become Covered in Ice? (by Holly Chavez; Oceanside Expeditions)
So Much Ice Has Melted, That the Earth’s Crust Is Shifting in Weird, New Ways (by Dharna Noor; Gizmodo)
The Global Fingerprint of Modern Ice-Mass Loss on 3-D Crustal Motion (by Sophie Coulson, Mila Lubeck, Jerry X. Mitrovica, Evelyn Powell, James L. Davis, Mark J. Hoggard; Geophysical Research Letters)
