Instability in Antarctic ice projected to make sea level rise rapidly

Instability in Antarctic ice projected to make sea level rise rapidly
Part of Thwaites Glacier crumbles into the ocean. It is part of the normal life of a glacier, but the rate of ice flow into the ocean of some Antarctic glaciers has markedly accelerated, raising concerns. Credit: NASA/OIB Jeremy Harbeck

Images of vanishing Arctic ice and mountain glaciers are jarring, but their potential contributions to sea level rise are no match for Antarctica's, even if receding southern ice is less eye-catching. Now, a study says that instability hidden within Antarctic ice is likely to accelerate its flow into the ocean and push sea level up at a more rapid pace than previously expected.

In the last six years, five closely observed Antarctic glaciers have doubled their rate of ice loss, according to the National Science Foundation. At least one, Thwaites Glacier, modeled for the new study, may be in danger of succumbing to this instability, a volatile process that pushes ice into the ocean fast.

How much ice the glacier will shed in coming 50 to 800 years can't exactly be projected due to unpredictable fluctuations in climate and the need for more data. But researchers at the Georgia Institute of Technology, NASA Jet Propulsion Laboratory, and the University of Washington have factored the instability into 500 ice flow simulations for Thwaites with refined calculations.

The scenarios diverged strongly from each other but together pointed to the eventual triggering of the instability, which will be described in the question and answer section below. Even if were to later stop, the instability would keep pushing ice out to sea at an enormously accelerated rate over the coming centuries.

And this is if due to warming oceans does not get worse than it is today. The study went with present-day ice melt rates because the researchers were interested in the instability factor in itself.

Glacier tipping point

"If you trigger this instability, you don't need to continue to force the by cranking up temperatures. It will keep going by itself, and that's the worry," said Alex Robel, who led the study and is an assistant professor in Georgia Tech's School of Earth and Atmospheric Sciences. "Climate variations will still be important after that tipping point because they will determine how fast the ice will move."

"After reaching the tipping point, Thwaites Glacier could lose all of its ice in a period of 150 years. That would make for a of about half a meter (1.64 feet)," said NASA JPL scientist Helene Seroussi, who collaborated on the study. For comparison, current sea level is 20 cm (nearly 8 inches) above pre-global warming levels and is blamed for increased coastal flooding.

Instability in Antarctic ice projected to make sea level rise rapidly
Thwaites Glacier's outer edge. As the glacier flows into the ocean, it becomes sea ice and drives up sea level. Thwaites Glacier ice is flowing particularly fast, and some researchers believe it may have already tipped into instability or be near that point, though this has not yet been established. Credit: NASA/James Yungel

The researchers published their study in the journal the Proceedings of the National Academy of Sciences on Monday, July 8, 2019. The research was funded by the National Science Foundation and NASA.

The study also showed that the instability makes forecasting more uncertain, leading to the broad spread of scenarios. This is particularly relevant to the challenge of engineering against flood dangers.

"You want to engineer critical infrastructure to be resistant against the upper bound of potential sea level scenarios a hundred years from now," Robel said. "It can mean building your and nuclear reactors for the absolute worst-case scenario, which could be two or three feet of sea level rise from Thwaites Glacier alone, so it's a huge difference."

Q&A

Why is Antarctic ice the big driver of sea level rise?

Arctic sea ice is already floating in water. Readers will likely remember that 90% of an iceberg's mass is underwater and that when its ice melts, the volume shrinks, resulting in no change in sea level.

But when ice masses long supported by land, like , melt, the water that ends up in the ocean adds to sea level. Antarctica holds the most land-supported ice, even if much of that land is seabed holding up just part of the ice's mass, while water holds up part of it. Also, Antarctica is an ice leviathan.

"There's almost eight times as much ice in the Antarctic ice sheet as there is in the Greenland ice sheet and 50 times as much as in all the mountain glaciers in the world," Robel said.

Instability in Antarctic ice projected to make sea level rise rapidly
Ice melt at the grounding line contributes to seawater and thus sea levels, but the larger effect is to send more ice above it out into the water, where it also drives up sea level. When sea bottom behind the grounding line, under the ice, slopes downward going inland, it exacerbates the process, which can become unstable, perpetually pushing ice out to sea. Credit: antarcticglaciers.org, Creative Commons non-commercial license

What is that 'instability' underneath the ice?

The line between where the ice sheet rests on the seafloor and where it extends over water is called the grounding line. In spots where the bedrock underneath the ice behind the grounding line slopes down, deepening as it moves inland, the instability can kick in.

On deeper beds, ice moves faster because water is giving it a little more lift. Also, warmer ocean water hollows out the bottom of the ice, adding a little more water to the ocean. More importantly, the ice above the hollow loses land contact and flows faster out to sea.

"Once ice is past the grounding line and just over water, it's contributing to sea level because buoyancy is holding it up more than it was," Robel said. "Ice flows out into the floating ice shelf and melts or breaks off as icebergs."

"The process becomes self-perpetuating," Seroussi said, describing why it is called "instability."

How did the researchers integrate instability into sea level forecasting?

The researchers borrowed math from statistical physics that calculate what random variables do to predictability in a physical system, like ice flow, acted upon by outside forces, like temperature changes. They applied the math to simulations of possible future fates of marine like Thwaites Glacier.

They made an added surprising discovery. Normally, when climate conditions fluctuate strongly, Antarctic ice evens out the effects. Ice flow may increase but gradually, not wildly, but the instability produced the opposite effect in the simulations.

Credit: Georgia Institute of Technology, PNAS (2019). doi/10.1073/pnas.1904822116

"The system didn't damp out the fluctuations, it actually amplified them. It increased the chances of rapid ice loss," Robel said.

How rapid is 'rapid' sea level rise and when will we feel it?

The study's time scale was centuries, as is common for sea level studies. In the simulations, Thwaites Glacier colossal ice loss kicked in after 600 years, but it could come sooner.

"It could happen in the next 200 to 600 years. It depends on the bedrock topography under the ice, and we don't know it in great detail yet," Seroussi said.

So far, Antarctica and Greenland have lost a small fraction of their ice, but already, shoreline infrastructures face challenges from increased tidal flooding and storm surges. Sea level is expected to rise by up to two feet by the end of this century.

For about 2,000 years until the late 1800s, sea level held steady, then it began climbing, according to the Smithsonian Institution. The annual rate of sea level rise has roughly doubled since 1990.


Explore further

Near-term ocean warming around Antarctica affects long-term rate of sea level rise

More information: Alexander A. Robel el al., "Marine ice sheet instability amplifies and skews uncertainty in projections of future sea-level rise," PNAS (2019). www.pnas.org/cgi/doi/10.1073/pnas.1904822116
Citation: Instability in Antarctic ice projected to make sea level rise rapidly (2019, July 8) retrieved 24 July 2019 from https://phys.org/news/2019-07-instability-antarctic-ice-sea-rapidly.html
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User comments

Jul 08, 2019
did antarctic ice do the hoopty during the 4000 year long Holocene Thermal Maximum?

Jul 08, 2019
Oh, look there, the liar deniers have another drum to beat on.

A little quick search shows that the HTM wasn't worldwide, didn't affect the tropics, and didn't last 4000 years over most of the world.

Gee, they're lying again. How about that. Whooda thunk.

Jul 08, 2019
The Planet has been warming since the last ICE Age. Sea level rise has leveled off significantly. If the situation is so dire, shouldn't we wipe China and India off the map? No one ever answers this question. The answer is no. We will adapt like we always have. So tired of the hysteria.

Jul 08, 2019
Ummm, if you're proposing killing a few billion people, everyone is going to think you're a psychotic.

Jul 08, 2019
To eliminate the biggest threat to our planet? Thought this stuff was critical.....shouldn't some humans survive? I really don't want to do that at all, I'm just making a point. It's not a big deal, we will all be fine We will go into another Ice Age in the geological near future.

Jul 08, 2019
Ban Plutonium!

Jul 08, 2019
You said no one would answer it.

I answered it.

Now what?

The sound of shifting goalposts is audible.

Jul 08, 2019
maybe cause the the ice depth is increasing , oops, i forgot , its a ' simulation' lol

https://climate.n...nowfall/

Jul 08, 2019
More AGW Cult bullshite.
It is obscene what idiots the AGW Cult takes their Chicken Shites, for.
The only place in the Antarctic where the Cult can find globull warming is exactly where there is intense geothermal activity.

Gobble up Chicken Shites.

Jul 08, 2019
What about all the volcanoes under Antarctica. Maybe that's why the ice is melting????? Climate change the world should have ended like what 10 times now. You cant tell me there are not volcano in Antarctica. When there are volcanoes all over the world but none in Antarctica. Lol make laugh

Jul 08, 2019
the latest expectation is 1-meter sea level increase by the end of the century, not 2 feet, this is already old a past science.

Jul 08, 2019
"pipes aren't inheritable" is a line from a movie that has no bearing or connection to this article.
Same as most of the comments.
I will not be silly and ask what you are all so afraid of, I know very well what fear does to thinking.
Living in fear 100% of the time is what cowards do, and you deniers wallow in your fear.

Jul 09, 2019
Liar... Liar... Pants on fire...

The Planet has been warming since the last ICE Age.
- Maddy11

The Earth reached it's peak interglacial temperature 10,000 years ago and has been slowly cooling since then - until the current warming started at the start of the industrial era and it's emissions of CO2.

So why did you feel a need to lie about it Maddy11?

Are you mentally ill? Or is someone paying you to lie here?

Sea level rise has leveled off significantly. If the situation is so dire, shouldn't we wipe China and India off the map? No one ever answers this question. The answer is no. We will adapt like we always have. So tired of the hysteria.

Jul 09, 2019
You be a stupid boy...

What about all the volcanoes under Antarctica. Maybe that's why the ice is melting????? Climate change the world should have ended like what 10 times now.
- David6363

Can you explain how these magical volcano's are causing the Arctic polar ice cap to melt, causing the global atmosphere to heat, and causing the vast ice sheets in the antarctic to retreat when is no way such vast quantities of heat can be generated by geologically minute events like a volcano.

On the scale of the Antarctic, a volcano is so tiny in comparison, isn't even visible .

You are a clueless retard.

Jul 09, 2019
Liar... Liar... Pants on fire.

It's not a big deal, we will all be fine We will go into another Ice Age in the geological near future.
- Maddy11

We are already well past the heating required to stave off the next ice age.

So why do you feel a need to lie about it?

Are you mentally ill? Who is posting you to tell lies here?

Jul 09, 2019

We are told that:

Warm Circumpolar Deep Water flows in under the sea ice, under the icebergs, under the ice shelf and melts the ice sheet at the grounding line. Then as cold surface water it flows out of the sub-ice shelf cavity where it forms sea ice.

I am reminded of the ads for three bladed safety razors where we are told:

With a 3 bladed razor, each blade serves a purpose:

First blade: hooks the hair follicle, pulling it up
Second blade: cuts the more exposed hair
Third blade: backup, ensuring nothing is missed.

Corporate Marketing managers or climate scientists seem to be quite sure that the general population are gullible rubes who will believe anything.

Steve Case - Milwaukee, WI

Jul 09, 2019
the vast ice sheets in the antarctic to retreat

how does that happen at sub zero temps year round ?

Jul 09, 2019
Can you explain how ..HAWW...HAWW...HEEE....
On the scale of the Antarctic, a volcano is so tiny in comparison, isn't even visible .

You are a clueless retard.

Well, in order to explain, first you need to replace your shit for brains, with actual grey matter.
On the scale of the planet, a volcano is even tinier, yet when they erupt they can cool the planet for decades.
You are a braying jackass.

https://qz.com/10...w-study/

Jul 09, 2019
Denial is so sweet when you know you won't live long enough to know if you're right or wrong. Please consider that humanity is vulnerable--and we have a responsibility to make it past our own time. Common sense says that the increased number of "animals" on earth must have an effect. My guess is that it is not all positive. So what if there was a past ice age. Do you honestly think the next will be unaffected by our actions?

Jul 09, 2019
The AGW Cult's Chicken Shites believe CO2 will be the end of the earth, yet they continue to burn fossil fuels as if there is no tomorrow. So, they bray at the heretics because it gives them that sweet sense of accomplishment.

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