The tipping point for Antarctica is closer than we thought.

Three new studies raise the specter that the great ice platforms of the Antarctic continent are more unstable and susceptible to collapse than previously thought.

Antarctica holds ninety percent of the world’s store of freshwater and would raise sea levels 230 feet if it all melts.

East Antarctica’s ice is on the ground with no marine extension. That is not true for the majority of Antarctica’s glaciers, which have enormous marine extensions which hold back the land ice from flowing into the ocean. 

Examples are West Antarctica and the peninsula that are vulnerable to warm ocean water eating away at the underbelly of the marine glacier leading to collapse.

Surface melting occurring on the bitter cold icy surface of the Antarctic peninsula can cause hydrofracturing which collapses ice shelves in the Arctic and Antarctica. According to CIRES-based researchers who have observed the peninsulas Larsen B collapse found that the ice shelf bent “under the weight of ponding meltwater on top.”

But, a new study found that the massive ice shelves have been thinning for three hundred years — the study published in the journal Scientific Reports.

Grist provides a short summary:

Antarctic ice sheets have been melting rapidly for hundreds of years, much longer than scientists previously thought, according to a study out Thursday. The findings suggest that estimates for global sea-level rise need to be reworked and that we’re even closer to the day that fish start chasing each other through New York City’s subway tunnels.

The scientists behind the new study in Scientific Reports were able to reconstruct a 6,250-year record of how fast Antarctic glaciers slipped into the sea. They did this by drilling the bottom of the Southern Ocean between Antarctica and Tierra del Fuego and analyzing the layers of mud they pulled up.

The story this mud tells between 4300 B.C. and 300 A.D. is uneventful. But around 1400, the skeletons of diatoms — ubiquitous, jewel-like sea creatures often used for dating ocean sediments — suggest that the weather became warmer. More oxygen isotopes that come from fresh (as opposed to saltwater) started showing up, meaning the glaciers were melting. Then around 1706, the ice began to melt even faster than before.

So natural climate change had cued up the massive Antarctic ice shelves to collapse before human-caused climate change turned up the heat. A random shift in wind patterns has been melting the ice caps for the last 300 years, the scientists wrote, “potentially predisposing them to collapse under intensified anthropogenic warming.”

Fabienne Lang writes in Interesting Engineering

The years following the year 1400 saw an increasing amount in glacial meltwater discharge, reaching their peak in 1706.

Another period when the discharge was particularly high was after 1912.

Ultimately, the findings of the research pointed out that the ice shelves in the region have been thinning for around 300 years. This, in turn, may predispose them to collapse more easily when global warming occurs, as it is happening now.

snip

The study authors believe that part of the reason for the thinning ice shelves is due to the Southern Annular Mode (SAM) — a climate driver that can influence rainfall and temperature rise in Australia.

The SAM then led to strong westerly winds, atmospheric warming, and ice shelf melting in the eastern Antarctic peninsula. At the same time, it also directed warmer water to the Weddell Gyre, which could have led to ice shelves melting from underwater.

Look at this Crabeater seal drool after a juicy meal of krill. Krill changes the colour of their saliva and poop to pink-red. https://t.co/WoimVh4kaA

— Gateway Antarctica (@GatewayAntarct1) October 20, 2019

The second worrying study was published in the journal Science Advances.

From the University of Colorado:

Upside-down "rivers" of warm ocean water are eroding the fractured edges of thick, floating Antarctic ice shelves from below, helping to create conditions that lead to ice-shelf breakup and sea-level rise, according to a new study.

snip

The scientists' new work focuses on two factors conspiring to weaken ice shelves. First, flowing ice often stretches and cracks along its edges or "shear margins," especially when it's flowing quickly, Alley said. "In MODIS and other satellite images, you see all these crevasses."

As those craggy features flow toward the ocean and become part of floating ice shelves, they're vulnerable to erosion from below, by warm plumes of ocean water, the team reported.

Here's the zoom-in to the new crack developing on Pine Island Glacier's ice shelf. pic.twitter.com/Xq2giSxlxp

— Bert Wouters (@bert_polar) October 28, 2019

Warm and fresh water is more buoyant than cold and salty water, so it has a tendency to "find" high spots in floating ice, sometimes forming a type of "upside-down river" that can grow miles wide and tens of miles long. Alley and her colleagues first mapped those rivers or "basal channels" a few years ago, spotting them as wrinkles or sags in otherwise smooth ice surfaces.

Now, they've put it all together, showing that large basal channels are more likely to form at the shear margins—the weakest parts—of fast-flowing ice shelves. While the ice is still on land, large troughs form in the shear margins, becoming thin spots when the ice flows onto the ocean. Warm ocean water finds those thin spots along the base of the ice shelf, further eroding and weakening margins, making ice shelves more vulnerable to retreat and collapse.

Ice shelf Getz smaller: #Sentinel1 movie showing #Antarctic iceberg B-47 spinning away https://t.co/SB1zFUgkoepic.twitter.com/euG4hL9O3x

— Mark Drinkwater (@kryosat) October 17, 2019

From the journal Nature Geoscience abstract.

Recent major melting events in West Antarctica have raised concerns about a potential hydrofracturing and ice shelf instability. These events often share common forcings of surface melt-like anomalous radiative fluxes, turbulent heat fluxes and föhn winds. Using an atmospheric river detection algorithm developed for Antarctica together with surface melt datasets, we produced a climatology of atmospheric river-related surface melting around Antarctica and show that atmospheric rivers are associated with a large percentage of these surface melt events. Despite their rarity (around 12 events per year in West Antarctica), atmospheric rivers are associated with around 40% of the total summer meltwater generated across the Ross Ice Shelf to nearly 100% in the higher elevation Marie Byrd Land and 40–80% of the total winter meltwater generated on the Wilkins, Bach, George IV and Larsen B and C ice shelves. These events were all related to high-pressure blocking ridges that directed anomalous poleward moisture transport towards the continent. Major melt events in the West Antarctic Ice Sheet only occur about a couple times per decade, but a 1–2 °C warming and continued increase in atmospheric river activity could increase the melt frequency with consequences for ice shelf stability.

This is a VERY cool paper. Atmospheric rivers are shooting firehoses of moisture and heat into West Antarctica. https://t.co/qqtvdI8cen

— Peter Neff (@peter_neff) October 30, 2019