Our dynamic planet has an apparent paradox: the more ice melts from landlocked glaciers, the lower the sea level gets in nearby areas. How does this happen? Through the physics of isostatic rebound, when the surface of the planet acts as an elastic sheet dimpling and rebounding under changing loads.
Rocks seem so very solid from our puny human perspective. Things are rock hard, rock solid, and are reliable as the rock itself. But from a geological perspective, rock is an elastic sheet that encompasses our planet in a thin, flexible membrane that responds to every disturbance.
Nowhere is this more evident than with isostatic rebound, a process of geological buoyancy by which the earth's crust, having sunk beneath the weight of glaciers from a preceding ice age, bounces up as ice sheets melt and the water runs back into the sea. While this melting ice is filling the oceans, the land can rebound so quickly that it rises even faster than the climbing sea level. The result is an apparent paradox: where continental glaciers are melting and exposing the land, the local sea levels are dropping.
The Thwaites ice shelf in Antartica as surveyed in October 2013 by Operation IceBridge. Image credit: James Yungel/NASA
During each ice age, massive glaciers crawl across the land. These vast ice sheets contain an enormous quantity of water. And water is very, very heavy.
The crust and mantle deform under the weight of ice sheets. Image modified from NASA
During the last ice age 15,000 to 20,000 years ago, Canada and the United States were groaning under the weight of the Laurentide and Cordilleran ice sheets while Scandinavia struggled under the Fennoscandian ice sheet. The Earth's lithosphere, the rigid crust and uppermost mantle, buckled under the weight of up to 3 kilometers of ice. Like an iceberg floating in water with a vast root hidden under the waves, the crust sank into the mantle until hitting a buoyant balance between the weight of ice and rock over hot mantle. Kept under load for thousands of years, the lithosphere flowed and deformed to reach equilibrium under the new normal.
When the world shook off the ice age, the ice sheets melted quickly. The land was bare in a geologic heartbeat, lifting the weight far, far faster than it built up millennia before. The elastic crust rebounded nearly instantaneously, bouncing back like a balloon's surface freed from an aggressive squeeze. But the more viscous mantle was slower to reach equilibrium in the new isostatic regime, driving slow uplift as the mantle flowed under the dented land. The rebound is ongoing today, with the land recovering at centimetres per year. With the rebound rates akin to the speed at which fingernails grow, it will take another 10,000 years before the land recovers from the last ice age.
Global rebound rates as the world adjusts from the last ice age. Image credit: A. Paulson/S. Zhong/J. Wahr
The same story is happening everywhere that was covered in ice: the lithosphere buckled under the massive weight of ice sheets, and has been slowly recovering in the millennia since they were exposed. From the Antarctic still shedding weight to Canada's Hudson Bay racing upwards at nearly 2 centimeters per year, the surface of our planet is literally reshaping beneath our feet. For people in the far north and south of our planet, every time they trim their nails they can reflect on how much higher their home has bounced since the last manicure.
As the lithosphere rebounds, it carries the entire landscape with it. Sea cliffs and rivers are stranded far above their formation location, and strandlines of past beaches are laid out in beautiful, delicate features tracing sea levels long gone. Even the tilt of the land changes: drainage patterns struggling to adjust to keep water flowing downhill.
A stranded river cuts a new waterfall as the land rebounds above the sea in Alaska. Image credit: Jim & Laura Massie
The arrival and release of weight impacts the stress of the entire region, potentially triggering earthquakes and volcanoes. Before fracking and injection wells made a mess of the continental interior, the biggest causes of intraplate earthquakes far from plate tectonic boundaries were attributed to the shifting stresses of isostatic rebound. These impacts can be far-reaching in both space and time: despite being ice-free, the infamous 1811 New Madrid earthquake in the American south may have been induced by intraplate stresses induced from the last ice age.
The same thing is happening for volcanoes. A key trigger of eruptions is changing in the subsurface pressure and stress adjustments in the magma chamber. As the lithosphere flexes and recovers, this redistribution can be enough to fuel a surge in volcanic activity. Right now, the released pressure in Iceland could be fuelling a surge in volcanism, magma chambers long kept confined expanding and pushing out into surface eruptions from the flight-disrupting Eyjafjallajökull to the ongoing slow, steady trickle of Bárðarbunga.
But most fascinatingly of all, isostatic rebound is the secret process behind how locations can have sea levels changing at odds with the rest of the planet. While we all know about global sea levels rising and falling, geologists also track local sea levels, the relative change in sea level at particular locations.
During an ice age, water once free to flood the oceans is tied up in continental ice sheets. This drops global sea levels, exposing seafloor as the new coastline. Yet the land with these new ice sheets is under load, dropping down relative to its former height. Relatively speaking, despite the global sea levels falling, the local sea level can actually rise.
Right now, we're distinctly not in an ice age. The land-bound glaciers are melting, and sea levels are rising from both the influx of released water and thermal expansion. And yet, for the places suddenly relieved of their frozen load, the land itself is rebounding higher above the waves, maybe even faster than the grasping clutch of the sea. Determining just how quickly each process is occurring is a jumbled mess of scrambling to monitor rapidly changing data to calibrate our models, but for now, parts of Iceland, Greenland, and Canada are climbing faster than their sea levels. From the perspective of beach-side homes, the relative sea level is staying stagnant or even dropping while the rest of the world contends with higher storm surges and floods.
Strandlines mark the relative sea level change from isostatic rebound in Bathurst Inlet, Nunavut. Image credit: Mike Beauregard
Isostatic rebound is just one example of how the surface of our planet is a dynamic, changeable place where the materials behave far differently in aggregate than we perceive them from our daily perspective.