Bad news everybody: thanks to global warming, the Earth is losing ice at a rate best described as “staggering” and “literally the worst case we could imagine.” According to a recent paper, the consequences of this mega-melt aren’t just limited to mercury-laced water or the overnight disappearance of Florida – no, it turns out the melting of our planet’s polar ice caps is warping the Earth’s crust itself.
“Think of a wooden board floating on top of a tub of water,” explained Sophie Coulson, lead author of the paper which was published last month in the journal Geophysical Research Letters. “When you push the board down, you would have the water beneath moving down. If you pick it up, you'll see the water moving vertically to fill that space.”
This is the “rebounding” effect, responsible for the seemingly paradoxical fact that in places like Canada and Scotland, sea levels are currently dropping year on year. If the board-in-a-bathtub analogy doesn’t float for you, you can think of it like sitting on a couch: the Earth is the seat, and your butt takes the role of a gigantic ice sheet (sorry, we don’t make the science rules.) When you sit down, the cushion depresses under you, but bunches up all around – when you stand up, on the other hand, your butt imprint springs back up (or “rebounds”) and the surrounding cushion flattens out again.
The same is true for the Earth’s crust: as the Laurentide Ice Sheet, for instance, melted away into the sea, Canada and Alaska were no longer weighed down by the ice – and the southern US was no longer pushed up as a result. That’s why now, the land around Canada’s Hudson Bay is rising by about half an inch every year while Washington, D.C. is set to be largely underwater by 2200.
“Scientists have done a lot of work directly beneath ice sheets and glaciers,” Coulson said. “So they knew that it would define the region where the glaciers are, but they hadn't realized that it was global in scale.”
In fact, as an accompanying feature in Nature explained, the melting ice is causing the Earth to deform “even in spots more than 1,000 kilometers from the ice loss.” What scientists studying the rebounding phenomenon hadn’t counted on – and what Coulson’s team discovered – was that in some places, the crust was moving more horizontally than it was vertically.
“In some parts of Antarctica, for example, the rebounding of the crust is changing the slope of the bedrock under the ice sheet, and that can affect the ice dynamics,” explained Coulson. “On recent timescales, we think of the Earth as an elastic structure, like a rubber band, whereas on timescales of thousands of years, the Earth acts more like a very slow-moving fluid […] Ice age processes take a really, really long time to play out, and therefore we can still see the results of them today.”
The discovery that modern ice loss warps the Earth’s surface like this – far more than previously thought, and in all directions – has implications that aren’t just limited to scary headlines about global warming. A better, more complete understanding of how and why the Earth’s crust moves is “really important for a wide range of Earth science problems,” Coulson explained, and will help inform a huge number of studies in the future.
“For example, to accurately observe tectonic motions and earthquake activity, we need to be able to separate out this motion generated by modern-day ice-mass loss,” she said.