“The findings are concerning as they provide very strong evidence that [the] West Antarctic Ice Sheet becomes unstable and collapses when global temperatures warm by more than 1.5C above preindustrial and if that warming is sustained,” Tim Naish, a co-author of the new study, said in an email.
“The world is on track to pass the 1.5C climate target in the next five years,” added Naish, a paleoclimatologist at Victoria University of Wellington in New Zealand. “We are dangerously close” to triggering collapse of the Antarctic Ice Sheet, which would raise today’s global sea levels by as much as 4 meters over upcoming centuries.
The West Antarctic Ice Sheet has collapsed multiple times during past warm periods in Earth’s history. Some records indicate the ice sheet collapsed sometime in the past 1 million years, but scientists lacked evidence to better pinpoint the time. Now, octopuses would be their clock.
Turquet’s octopuses may not be the most charismatic animals on the continent, but the small seafloor-dwelling creatures have inhabited the region for millions of years. Individuals only live for a handful of years, but their DNA is a time capsule of the lives of their ancestors.
Like performing a 23andMe test on a human, the team analyzed genetic material from nearly 100 octopuses from the Southern Ocean preserved in museums and for research. Some samples were decades old and degraded, but new genetic sequencing technology allowed the team to analyze the genomic data in higher resolution than ever before.
The findings were surprising. Octopuses located thousands of miles apart around the continent — in the Weddell, Amundsen and Ross seas — were genetically similar. Today, the ice sheet physically separates those seas. Given that Antarctic octopuses don’t travel much, it’s unlikely they trekked around the continent. So, how did the species branch out to such distances?
Using the same types of models to test past human migration patterns, the team simulated different scenarios to figure out the octopuses’ historical voyages. They ran hundreds of thousands of variations under scenarios in which the ice sheet was completely intact, partially collapsed and completely collapsed.
One path stood out: The octopuses traveled in direct routes between the seas after the western ice sheet fully melted, opening up gaps in the rocks between the regions.
The octopuses “found pathways [in the sea] for them to migrate” over generations, said Sally Lau, lead author of the study and a biologist at James Cook University. “When they migrate, they interbreed with each other, and [genes flow] from one population to the other population.”
This exchange of genetic exchange, according to the study’s models, occurred during the most recent warm period, about 129,000 to 116,000 years ago, known as the Last Interglacial. During the Last Interglacial, the global average temperature was 0.5 to 1.5 degrees Celsius warmer than preindustrial levels. Global sea levels were 5 to 10 meters higher than today, and Naish said meltwater from Antarctica was probably a large contribution.
Scientists worry the Last Interglacial period could be an indicator of what’s to come for today’s Earth. Today’s global temperatures are already about 1.2 degrees Celsius above the preindustrial average.
“We are certain that if we keep the amount of warming at the level we are experiencing right now, sea level will ultimately reach that level” seen during the Last Interglacial, said Tina van de Flierdt, a paleoclimate researcher at Imperial College London who was not involved in the study. “What we do not know is how fast this will happen.”
Sea levels are already projected to increase 30 centimeters by the end of this century, but van de Flierdt said the rise could be as high as 1 or 2 meters if the West Antarctic Ice Sheet collapses at 1.5 or 2 degrees of warming — as the study suggests.
At our current temperature, parts of the West Antarctic Ice Sheet may have already hit a melting tipping point. Ice in the Amundsen Sea, home to the Thwaites “doomsday” glacier, is already showing an irreversible vulnerability to melting. One saving grace is that the section of the ice sheet near the Ross Sea may be less susceptible to melting for now, said van de Flierdt.
“It is melting in both areas, the Ross Sea and the Amundsen Sea, that would constitute a full collapse” of the West Antarctic Ice Sheet, she said.
Van de Flierdt said the study was “fascinating” and was surprised at the authors’ confident results in narrowing down the time period to 125,000 years ago. The results are also timely, as van de Flierdt and her colleagues are currently in Antarctica drilling through the western ice sheet to find direct geological evidence revealing when the ice sheet collapsed — a similar objective to the new study, though a different approach.
The international project, named SWAIS2C, will drill to find old sediment deposited under the ice. Ice cores record the atmosphere’s composition and temperature at that time, giving clues to what was happening on the planet.
“The geological past provides a window into a future world that might be 1, 2, 3 or even 4 degrees warmer,” said van de Flierdt, co-chief scientist of SWAIS2C. SWAIS2C is designed to study the vulnerability of the West Antarctic ice sheet in the event of up to 2 degrees Celsius of warming.
“There is no such thing as a perfect geological analogue for the experiment we are conducting with our planet right now,” she said. “But the geological past is our best bet to learn about how the planet might react to unprecedented greenhouse gas emissions.”
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