If all the ice covering West Antarctica were to melt, global sea levels would rise 4-5 meters higher, causing major flooding and problems for communities around the world. It's an alarming scenario, but it's happened before. Just ask the octopus.
According to a new study published Thursday in the journal Science, Turke octopuses, a species of cephalopod that lives in the Southern Ocean, were able to move around the melting West Antarctic ice sheet as early as 125,000 years ago. This time is key because it is also the last time Earth's temperature matched today's exceptional heat. This may indicate that another ice-free period in the region may be imminent, signaling the impending collapse of the West Antarctic Ice Sheet.
"The findings are worrying because they provide very strong evidence that the West Antarctic Ice Sheet is becoming unstable and collapsing when global temperatures rise more than 1.5 degrees Celsius above pre-industrial levels and if this warming continues," said co-author Tim Naish. new research.
"The world is on track to meet the climate target of 1.5 degrees Celsius in the next five years" due to warming from greenhouse gas emissions, added Naish, a paleoclimatologist at Victoria University of Wellington in New Zealand.
The West Antarctic Ice Sheet has collapsed several times during past warm periods in Earth's history. These warm periods were caused by normal oscillations in Earth's orbit, which changed the amount of sunlight reaching the planet over tens of thousands of years, unlike today's human-induced global warming.
Some records indicate that the ice sheet collapsed naturally sometime in the last 1 million years, but scientists lacked enough evidence to put a better time. Octopuses would now be their watch.
Turke octopi may not be the most charismatic animals on the continent, but the tiny creatures that live on the sea floor have inhabited the region for millions of years. Individuals live only a few years, but their DNA is a time capsule of their ancestors.
Similar to the 23andMe test on humans, the team analyzed the genetic material of nearly 100 Southern Ocean octopuses that are stored in museums and for research. Some of the samples were decades old and degrading, but new genetic sequencing technology allowed the team to analyze genomic data at a higher resolution than ever before.
The conclusions were unexpected. Octopuses thousands of miles apart across the continent—in the Weddell, Amundsen, and Ross seas—were genetically similar. Today, an ice sheet physically separates these seas. Given that Antarctic octopuses do not travel much, it is unlikely that they have made treks around the continent. So how did the species branch out over such distances?
Using the same types of models used to test past human migration patterns, the team simulated different scenarios to figure out the historical journeys of octopuses. They ran hundreds of thousands of variations on scenarios in which the ice sheet was completely intact, partially destroyed, and completely destroyed.
One way stood out: Octopuses traveled the routes between the seas after the western ice sheet completely melted, opening gaps in the rocks between the regions.
Octopuses have been "finding ways [in the sea] to migrate" for generations, said Sally Lau, the study's lead author and a biologist at James Cook University. "When they migrate, they interbreed with each other and [genes] flow from one population to another."
This genetic exchange, the study models, occurred during the last warm period, about 129,000 to 116,000 years ago, known as the last interglacial. During the last interglacial, the average global temperature was 0.5 to 1.5 degrees Celsius warmer than pre-industrial levels. Global sea levels were 5 to 10 meters higher than they are today, and Naish said meltwater from Antarctica was likely a major contributor.
Scientists are concerned that the last interglacial period may be an indicator of what lies ahead for modern Earth. Today's global temperature is already about 1.2 degrees Celsius above the pre-industrial average. According to Naish, without any foreseeable technological solution to removing carbon from our atmosphere, we are "dangerously close to triggering the collapse of the Antarctic ice sheet," which will continue to raise global sea levels for centuries.
"We're confident that if we keep the amount of warming we're experiencing now, sea levels will eventually reach the level" seen during the last interglacial period, said Tina van de Flierdt, a palaeoclimate researcher at Imperial College London. who did not participate in the study. "We don't know how quickly that will happen."
Sea levels are already predicted to rise by 30 centimeters by the end of this century, but van de Flierdt said the rise could reach 1 or 2 meters if the West Antarctic Ice Sheet collapses under 1.5 or 2 degrees of warming.
At current temperatures, parts of the West Antarctic Ice Sheet may have already reached a melting tipping point. Ice in the Amundsen Sea, home to the "doomsday" Thwaites Glacier, is already showing irreversible vulnerability to melting. One saving grace is that the area of the ice sheet near the Ross Sea may be less susceptible to melting, van de Flierdt said.
"Melting in both areas, the Ross Sea and the Amundsen Sea, would mean a complete collapse" of the West Antarctic Ice Sheet, she said.
Van de Flierdt said the study was "exciting" and was surprised by the authors' confident results in narrowing the time period down to 125,000 years ago.
An international project called SWAIS2C will drill to find old sediment deposited beneath the ice. Chemical signatures in rocks can provide clues to what was happening in the environment at the time, helping scientists reconstruct Earth's response to past warm periods.
"The geological past provides a window into a future world that could be 1, 2, 3 or even 4 degrees warmer," said van de Flierdt, one of SWAIS2C's principal scientists. SWAIS2C is designed to study the vulnerability of the West Antarctic Ice Sheet to 2 degrees Celsius of warming.
"There is no perfect geological counterpart for the experiment we're doing with our planet right now," she said. "But the geological past is our best bet for learning about how the planet might respond to unprecedented greenhouse gas emissions."
