German researchers say critical Antarctic ocean warming could be avoided if global temperatures are kept within the Paris Climate Agreement target.
Failure to limit average global temperatures to 2°C above pre-industrial levels by 2100 will likely lead to a ‘regime shift’ in the Weddell Sea, and potentially a tipping point where the 430,000km2 Filchner Ronne Ice Shelf melts rapidly.
That outcome, which would also drive global sea level rise, occurred in 2 of 4 simulations recently run by researchers from the Alfred Wegener Institute for Polar and Marine Research (AWI).
Aligned to four ‘Shared Socioeconomic Pathway’ scenarios mapped out by the Intergovernmental Panel on Climate Change, each of these models demonstrates ocean temperatures in the Weddell Sea and a deep sea cavity called the ‘Filchner Trough’ will continue to increase to the end of the century.
Only the model consistent with the 2°C target showed weaker warming signals over the next 75 years. This ‘SSP1’ projection is the most optimistic, where carbon emissions peak near 2040 and decline to the end of the century.
Other models, consistent with more greenhouse gas emissions and higher end-of-century temperatures of at least 2.5 degrees, would have a more severe warming impact on the Weddell Sea and the Filchner Trough.
“Out of the four climate scenarios for the future, two scenarios lead to a clear regime shift in the Filchner Trough, where the dense shelf water is mostly replaced by modified Warm Deep Water,” Vanessa Teske, a doctoral researcher at AWI, told Cosmos.
Teske is the lead author of the study published today in the journal Communications Earth & Environment.
Warming water in upper levels of the ocean prevents the sinking of colder, nutrient-rich water inherent in marine systems. Oceanographers have consistently warned that increased ocean warming may lead to slowdowns in major ocean circulation processes.
Such regime shifts may escalate and pass tipping points, which may take hundreds or thousands of years to reverse.
“While a regime shift only describes the process of large or small changes in a system like the increased warm water transport onto the continental shelf, changing the state of the Filchner Trough from cold to warm. A tipping point is reached when this process is not easily reversible,” Teske says.
“When multiple such points in the Antarctic region are crossed, increased basal melting, ice shelf instabilities and rising sea level are but a few of the consequences.
“The resulting changes will not just affect the local systems, but also the global ocean and all that depend on it. If we are able to stay below the 2°C climate target, we have a good chance that at least one of these tipping points will not be crossed anytime soon.”
Not the first warning on circulation
The AWI research comes as scientists look with increasing concern at the state of sea ice formation in Antarctica.
Sea ice forms around the continent during the southern hemisphere’s winter and melts as oceans warm, usually seeing a minimum around February and March.
The last two years have been at levels not previously seen. While the record low winter maximum was recorded in 1986, 2022 marked an unprecedented summer melting, and narrowly avoided setting a new winter low point.
It was surpassed last year with both a record lower summer and winter extent, with an area of ice the size of the Northern Territory effectively failing to form in 2023, compared to the 2022 maximum.
And 2024 looks to be a similar story. Monitoring from the US National Snow and Ice Data Center already shows this year’s summer minimum is around the same low point.
Will Hobbs, a physical oceanographer and sea ice expert at the University of Tasmania, says it’s likely a regime shift is occurring across the Southern Ocean. A recent study published in the Journal of Climate highlights “multiple lines of evidence” that warming oceans in driving these shifts.
“We explored whether a significant change in sea-ice variability has led to more extreme sea-ice events over the last decade or so, and whether that’s because of a changed response to the atmosphere or the ocean,” Hobbs says.
“We found that the variability of Antarctic sea ice, and how long the anomalies last, has increased significantly since 2006.
“Perhaps the most striking change for scientists is that while the atmosphere has always been the main driver of Antarctic sea-ice variability, the recent extreme fluctuations over the last decade can’t be explained by the atmosphere alone.”
Hobbs describes the future of frozen oceans as “increasingly uncertain”. The data studied with colleagues from several Australian research institutions doesn’t point to the cause of this likely regime change, but at least draws out statistical evidence that a shift is underway.
For Teske at the AWI, her simulations show that sea ice will continue to decline in line with rising global temperatures.
“Winter sea ice extent largely remains constant over the course of the century, only the thickness… reduced during the simulations,” she says.
“A strong sea ice decline than the simulations suggest might move the regime shift forward in time.”
Scientists have recently reported slowing ocean circulation patterns around the globe, including in the North Atlantic and Southern Ocean.
