Warmer, saltier water from deep in the Southern Ocean has started rising to the surface, bringing up more carbon dioxide, a recent study shows.
Fresher, cooler surface waters had helped sea ice expand for more than thirty years, but the new satellite data shows this reversed a decade ago, when the surface began getting saltier as Antarctic sea ice coverage dropped.
Human activity is expected to cause fresher surface water that traps warmer and saltier layers below, so the reason for this change is unclear.
The SMC asked experts to comment.
Dr Melissa Bowen, Associate Professor of Oceanography, School of Environment, University of Auckland, comments:
“The increased salinity in the Southern Ocean is puzzling. Clearly changes in the ocean, ice and atmosphere are linked and have been underway for the last decade. These changes have global implications: the ocean around Antarctica is a hub for ocean circulation because the density of water is transformed here and it moves to different depths, taking heat and gases with it. These changes in the Southern Ocean, and our lack of understanding about why they are occurring, set off alarm bells about how well we can anticipate future climate.”
Conflict of interest statement: “No conflicts of interest. I receive research funding from the Antarctic Science Platform.”
Professor Wolfgang Rack, glaciologist and Director of Gateway Antarctica, University of Canterbury, comments:
“Sea ice in the Southern Ocean was regarded for a long time as a climate paradox. Until around 2015. The best observed climate variable by satellite was increasing its size despite global warming and ice volume decreasing elsewhere. Although still unclear why this was the case, one theory claims increased melt of the Antarctic ice sheet freshened the surface water, which can then more easily freeze. Many scientists expected the trend to reverse at some point, but the rate of the current retreat is completely unexpected and mind boggling.
“Although the chain of processes remains unclear, this new study shows a significant increase in surface salinity, exactly starting at the time of sea ice loss. On the observed scale, this is best explained by the upwelling of warm and salty deep water. It prevents sea water from freezing, and once frozen it can melt the ice more easily from below. It is the classic example of a positive feedback process. Less sea ice results in more solar heating of surface water, which results in even less sea ice.
“The findings are based on relatively short observations of satellites and autonomous buoys. Still, the earth south of our capital Wellington is the most under-surveyed region globally. With 85% ocean, of which one quarter is covered once a year by sea ice, the reasons of the dramatic sea ice decline require more and urgent research. Only then we will know how this important piece of information fits in the greater puzzle, and how quickly we will see flow on effects for ecosystems and global climate.”
No conflicts of interest.
Associate Professor Natalie Robinson, Marine Physicist, Earth Sciences New Zealand, Associate Professor in Sea Ice Science and Oceanography, Victoria University of Wellington, and Director of the Antarctic Science Platform, comments:
“The Southern Ocean is a critical player in our global climate. Although it covers only 17% of the globe, it absorbs more than half of the additional heat, and ~2/5 of the excess carbon dioxide, that result from human activities. One of the main Southern Ocean processes that help with this climate stabilisation is the annual cycle of sea ice formation and decay: each winter the surface of the ocean freezes, creating dense, cold brine that sinks to the ocean floor. This flows away from Antarctica into all of the major ocean basins, controlling how heat, oxygen, and nutrients are distributed around the world. This is the ‘heartbeat’ of the global ocean.
“But sea ice is changing. The past 10 years have seen a precipitous decline in Antarctic sea ice, with the Ross Sea leading the change. This is a clear symptom of significant changes in both the atmosphere and ocean of the southern polar latitudes. Shifting currents, changes to stratification, increasing storminess, and warming of both ocean and atmosphere are all playing their part in a complex interplay.
“This study delves into one aspect, linking surface salt content (which determines density in very cold water) to enhanced deep mixing, and an increased ability to draw heat up from below. This makes the surface waters both warmer and saltier, and therefore less likely to freeze.
“We are moving into uncharted climate territory, and Antarctica is responding. What happens in Antarctica has implications for the entire globe, but here in New Zealand we are impacted by changes to Antarctic sea ice more directly than most. Early indications are that a warmer Southern Ocean, exacerbated by retreat of the sea ice edge, contributes to increased storminess for New Zealand. When combined with the steadily warming waters of New Zealand’s Exclusive Economic Zone, we should expect to see more of the intense rainfall events we’ve experienced recently, since warmer air can hold more moisture.
“In order to secure a liveable future for ourselves and our children, it is imperative that we drastically reduce our reliance on fossil fuels. The stable climate that has allowed human civilization to develop and thrive is now threatened by our intense and continued greenhouse gas emissions. This is an issue of intergenerational equity which impacts every aspect of our lives.”
No conflicts of interest.
Professor Craig Stevens, Principal Scientist – Marine Physics, Earth Sciences New Zealand and Department of Physics, University of Auckland, comments:
“This study from the group at Southampton in the UK uses some new approaches to data to look at the implications for sea ice with the changing ocean salinity around Antarctica. These combined effects are hugely important for a range of planetary and ecosystem processes.
“One of the main themes of the paper is the evolution of the so-called Maud Rise polynya. This is a remarkable phenomenon that occurs in the Atlantic sector of the Southern Ocean where a patch of open water appears even in winter. The study really hammers home the point that the ocean is made up of layers that mix or don’t mix depending on external conditions.
“So, while it’s far away from Aotearoa New Zealand, this polynya still has a global impact as it enables water to rapidly mix from the surface down to the seabed and “ventilates” the oceans. The polynyas in our sector operate differently but still create that ventilation.
“One of the exciting things about the paper is the inclusion of satellite detected estimates of surface salinity and how this gets combined with robotic subsurface data. We’ve still so much to learn and will need all the data we can gather if we are going to improve our understanding of future changes to the oceans around Antarctica and how they will affect the rest of the planet.”
Conflict of interest statement: “I’m funded by the MBIE Antarctic Science Platform and the Marsden Fund and am on the council of the NZ Association of Scientists. I was also a co-author with the lead author of this study on a major 2023 review of ocean changes around Antarctica – it had around 50 authors, so everyone in the community.”
Associate Professor Inga Smith, sea ice physicist in the Department of Physics, University of Otago and Co-Director of He Kaupapa Hononga: Otago’s Climate Change Research Network, comments:
“The extremely important short paper “Rising surface salinity and declining sea ice” contains shocking results for those of us researching Antarctic sea ice: the surface ocean waters around Antarctica are getting saltier as well as warmer. In a warming world, fresher water from melting of land-based ice sheets and floating ice shelves would be expected to dominate at the ocean surface.
“In 2023, Antarctic sea ice in the winter did not reach out as far from the coast as it had previously, with an extent of only around 17 million square kilometres, compared to more like 18.5 million square kilometres normally. Summer sea ice extent had also been dropping since around 2015/2016 after years of relative stability, but the winter absence of sea ice in 2023 raised new levels of concern.
“The role of warmer waters in the decline of Antarctic sea ice had already been documented by Ariaan Purich and Edward Doddridge, using data from autonomous ocean instruments called “Argo floats”. This new paper by Alessandro Silvano and colleagues combines salinity data from Argo floats with satellite-derived ocean salinity data to show that the waters around Antarctica were more salty as well as warmer when sea ice extent changed.
“In the Southern Ocean near Antarctica, the density of sea water depends more on its salinity than its temperature, with saltier waters usually remaining at depth. The paper by Silvano and his colleagues does not identify the processes leading to the appearance of this saltier and warmer water near the surface; the authors will no doubt explore this in a longer paper sometime soon.
“The authors point out how critical it is for satellite measurements and direct ocean monitoring to be funded and supported so that scientists can work out what is happening in the Southern Ocean near Antarctica. No matter which country is involved, monitoring is often hard to get funding for from government science agencies who prefer more immediately impactful results. This paper shows the value of long-term, international and national investments in monitoring programmes. Ahead of the upcoming coordinated international projects Antarctica InSync (2027-2030) and International Polar Year (2032-2033), the value of such international cooperation will only become more important.”
Conflict of interest statement: “I was not involved with the research. I have not published research papers with the lead author yet (we are working on a collaborative proposal), but I have published research papers with three of the co-authors.”
Dr Ken Hughes, Senior Lecturer in Coastal Processes, University of Waikato, comments:
“Antarctic sea ice extent was stable, perhaps even increasing slightly over time, back when I first began research in 2012. We assumed back then that the ocean and big ice shelves buffered the Antarctic system in some way so that the warming climate was not wreaking havoc on the ice extent in the same way it was for the Arctic.
“Fast forward a decade. The security of Antarctic sea ice is no longer taken for granted. The science has shifted to identifying the causes of the decline in the past years.
“The most difficult question is will the decline continue? Although this new study finds clear trends over the 13 year period from 2011 to 2023, textbooks tend to recommend a minimum of 30 years to confirm a climate trend. But a best guess now is much better than waiting until 2040 to check the correct answer.”
No conflicts of interest.
Professor James Renwick, Professor of Physical Geography, Victoria University of Wellington, comments:
“The new paper provides more evidence of a major change in the way the southern oceans are working, following publications of changes in upper-ocean heat content and analyses of how the southern oceans respond to changes in overlying westerly winds. The research all suggests that climate change is really starting to be expressed throughout the southern oceans.
“The implications are very worrying. Antarctic sea ice extent will likely continue to trend downwards from here (with ups and downs from year to year). That will accelerate the melting of ice shelves and land-based ice, increasing the rate of sea level rise and pushing us closer to the irreversible loss of the West Antarctic ice sheet. It will also reduce the reflectivity of the planet, bringing more warming.”
Conflict of interest statement: James Renwick is a climate scientist based at Victoria University of Wellington-Te Herenga Waka, an IPCC author and a Principal Investigator in the Antarctic Science Platform. He has received New Zealand government funding for climate change research from the Marsden Fund and MBIE Endeavour fund.