For the past 50 years, scientists have been trying to decipher if the West Antarctic Ice Sheet has ever collapsed, and if so, when. An octopus has beaten them to it.
Researchers, including those from James Cook University, the University of Western Australia and the Western Australian Museum, have used DNA from the Turquet’s octopus (or Pareledone turqueti) to determine that this ice sheet collapsed during the Earth’s last interglacial period, about 125,000 years ago. The ice sheet is currently 1.9 million square kilometres, roughly the size of Mexico, and is estimated to have been a very similar size during the last interglacial period.
An interglacial period refers to the stretch of time between ice ages, similar to what the Earth is experiencing now. During the last interglacial period, the planet was between 0.5 and 1.5 degrees Celsius warmer than pre-industrial levels.
If the sheet collapsed at those temperatures, then it certainly has the potential to do so within the 1.5 to 2 degrees warming range stipulated in the United Nations Paris Agreement. Scientists estimate that could raise the average global sea level by five metres.
Dr Sally Lau, a postdoctoral research fellow in evolutionary genetics at James Cook University (JCU) and lead author of the study, says the Turquet’s octopus – which grows to roughly 15 centimetres in mantle length – was selected because this species lived around the entire Antarctic continent, so its DNA could be compared with that of different octopus populations.
The scientists hypothesised that if the West Antarctic Ice Sheet had collapsed in the last interglacial period, then octopuses in the Weddell Sea, Amundsen Sea and Ross Sea could mate and reproduce. Comparing the DNA from individuals in these different areas could allow the team to study gene flow between octopus populations.
Not only did the team need to demonstrate that different octopus groups had mated, they needed to know the octopuses could not have simply travelled around the ice sheet. Turquet’s octopuses couldn’t have easily made that journey, because unlike many ocean creatures, they brood their young, rather than releasing millions of larvae. So tiny octopuses hatch and begin to crawl.
“They really can only get around by crawling … along the sea floor. They don’t disperse far,” explains evolutionary biologist Dr Nerida Wilson, who was also on the research team. “That means that animals stay in the same area and breed with each other.”
Lau, who also works for the research program Securing Antarctica’s Environmental Future, constructed models to test different levels of connectivity between octopus groups, and the direction of that connectivity in the past. As these populations mated and reproduced, DNA would flow between areas.
Wilson, who is an adjunct senior research fellow at the University of Western Australia and a research associate with the Western Australian Museum, says studies would normally select a specific gene to test for mixing between groups, but this project assessed the entire genome, or genetic material, of its specimens.
“Their [the octopus] genome is like a history book,” she says.
To analyse this history book of information, Lau says, researchers took a tiny bit of tissue, “maybe the size of a grain of rice”, and used a technique called target capture sequencing to retrieve the DNA. This relatively new technique can determine how DNA samples from the different octopus groups are similar or different, and so understand how much genetic information they have in common.
The research team was able to decode similarities between the octopus genomes, and from this, calculate that the West Antarctic Ice Sheet fell in the last interglacial period.
The model that best fitted the DNA analysis was “the scenario of [the shelf’s] complete collapse”, Lau says.
When the following glacial period restored the ice sheet, the study found, these octopuses could no longer interbreed, but the signature of their shared time remained in their DNA.
As with all research, the team needed a vast number of data points over a large timescale to get the most accurate results. The scientists utilised DNA from specimens over a 30-year time span from a variety of museums and collections or sourced directly from Antarctica.
This study has highlighted the value of these specimen collections, and how they can “be used for things that you’d never think of in the beginning”, Wilson says.
The question of whether and when the West Antarctic Ice Sheet has collapsed in the past has been a “burning question” for geoscientists, says Lau.
When the ice sheet fell 125,000 years ago, temperatures were similar to those today, but the sea level was between 5.5 and nine metres higher. The octopus DNA also confirmed another instance of collapse in a previous interglacial period, in the mid-Pliocene, about three million to 3.6 million years ago.
“Every interglacial period is different,” Lau says. But knowing what has happened in the past, under what conditions, can help us better predict the future of our climate. Even with differences in sea level, this study is evidence of the Antarctic’s vulnerability, especially if we exceed the Paris Agreement targets to limit global warming.
“It’s a bit sobering,” says Wilson. “It has some implications for where we’re heading, which is not necessarily good news.”
Understanding the events of previous eras can also help to ensure models have the most up-to-date data. So, if the West Antarctic Ice Sheet were to collapse within the limits of the Paris Agreement targets, what would actually happen?
It is estimated the sheet has been losing ice at a rate of 150 billion tonnes a year. It’s predicted that if it were to suffer a complete collapse, the average global sea level would rise by three to five metres.
This sea level change would mean major Australian cities including Melbourne, Sydney and Perth would all be subject to flooding. Countries such as the Philippines and the Netherlands would be much worse off, with much of their capital cities and populated areas becoming almost completely submerged.
The interdisciplinary team consisted of biologists, geneticists and physical scientists from all over the world collaborating to solve one of Antarctica’s most fraught questions. Their research demonstrates how much the scientific world still has to learn, and how the natural world can provide answers.
The team, Wilson says, is “really excited about applying this approach to other questions and other organisms to this question as well”.
This article was first published in the print edition of The Saturday Paper on February 10, 2024 as "Our octopus history teachers".
This month marks 10 years since the first edition of The Saturday Paper. The paper is as audacious now as it was then: a rejection of conventional wisdom about what makes the news and who will read it.
To celebrate those 10 years - and the issue-defining journalism produced in them - we are offering all new subscribers a two-year digital subscription for the price of one. That's $298 worth of journalism for $109.
Get more of the best journalism in the country - and celebrate the success of a newspaper built on optimism.
Select your digital subscription