This is illustrated by the above image, from a study led by Violaine Pellichero (2018), showing water-mass transformation within the Southern Ocean mixed-layer under sea-ice. Schematic cross-section illustrating the main water-masses in the Southern Ocean (Antarctic Intermediate and Mode Waters in red, Circumpolar Deep Waters in gray, and Dense Shelf Waters and Antarctic Bottom Waters in blue) and their interaction with ice and the surface. The water-masses are denoted by their neutral density values and the arrows corresponding to each water-masses indicate subduction (downward) or upwelling (upwards). The violet arrows illustrate the effect of northward sea-ice extent and freshwater transport. The green line is the mixed-layer.
A study led by Alessandro Silvano (2025) finds that, over the years, surface waters have become more salty.
Less sea ice also comes with loss of albedo (water is less reflective than ice, feedback #1), loss of the latent heat buffer (as sea ice disappears, heat can no longer be consumed by the process of melting, and the heat will instead go into increasing the temperature, feedback #14) and loss of emissivity (water is less efficient than ice in emitting in the far-infrared region of the spectrum, feedback #23), while warmer water result in more water vapor and less low-level clouds that reflect sunlight back into space (feedback #25).
• Rising surface salinity and declining sea ice: A new Southern Ocean state revealed by satellites - by Alessandro Silvano et al. (2025)
https://www.pnas.org/doi/full/10.1073/pnas.2500440122
discussed on facebook at:
https://www.facebook.com/groups/arcticnews/posts/10162876582119679
• Abrupt Antarctic Ocean Regime Shift: Reversed SMOC - Southern Meridional Overturning Circulation - video by Paul Beckwith
By combining satellite observations with data from underwater robots, researchers built a 15-year picture of changes in ocean salinity, temperature and sea ice, as illustrated by the above image. Around 2015, surface salinity in the Southern Ocean began rising sharply – just as sea ice extent started to crash.
When surface waters become saltier, they sink more readily, stirring the ocean’s layers and allowing heat from the deep to rise. This upward heat flux can melt sea ice from below, even during winter, making it harder for ice to reform. This vertical circulation also draws up more salt from deeper layers, reinforcing the cycle.
In addition to heat rising up from the deep, there is the danger that increasing amounts of both heat and carbon dioxide (CO₂), previously stored in the deep ocean by sinking circumpolar waters, will instead remain at the surface and cause both atmospheric temperatures and CO₂ concentrations to rise.
Saltier water, less sea ice
The higher the water's salt content, the lower its melting point. Seawater typically has a salinity of about 3.5% (35 grams of salt per liter of water). Sea ice starts melting when the temperature rises to about -2°C (28.4°F). By contrast, freshwater remains frozen as long as the temperature remains below 0°C (32°F).
More evaporation typically makes the sea surface more salty, while more precipitation, melting of sea ice and run-off from rivers and glaciers typically make the ocean surface fresher. As the recent study shows, the Southern Ocean surface is becoming more salty, which contributes to higher sea surface temperatures and in more melting of the sea ice. It's a self-amplifying feedback, in that saltier water at the ocean surface draws up more heat from the deep ocean, making it harder for sea ice to regrow. Increasing amounts of heat and CO₂ that were previously stored in the deep ocean by sinking circumpolar waters, threaten to instead remain at the surface and cause both atmospheric temperatures and CO₂ concentrations to rise.
In the video below, Paul Beckwith discusses the recent study.
The video below by @JustHaveaThink also discusses the recent study.
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[ Saltier water, less sea ice ] |
What is causing the Southern Ocean surface to become more salty? Higher temperatures come with feedbacks, such as stronger evaporation resulting in both a lot more water vapor and a lot more heat getting transferred from the surface to the atmosphere.
Much of the water vapor will return to the surface in the form of precipitation such as rain and snow, but part of this precipitation will fall over Antarctica. Increased snowfall over Antarctica can be attributed to rising air temperatures and stronger evaporation, changes in atmospheric circulation and the effects of ozone depletion.
Furthermore, 7% more water vapor will remain in the atmosphere for every degree Celsius rise in temperature. Since water vapor is a potent greenhouse gas, this will further increase temperatures, making it a self-amplifying feedback that can significantly contribute to further acceleration of the temperature rise.
Accumulating feedbacks
Furthermore, 7% more water vapor will remain in the atmosphere for every degree Celsius rise in temperature. Since water vapor is a potent greenhouse gas, this will further increase temperatures, making it a self-amplifying feedback that can significantly contribute to further acceleration of the temperature rise.
Accumulating feedbacks
Warmer oceans result in stronger stratification (feedback #29), further contributing to make it harder for heat to reach the deeper parts of oceans. As a result, a larger proportion of the heat that was previously entering oceans will instead remain in the atmosphere or accumulate at the ocean surface, and slowing down of the Atlantic Meriodinal Overturning Circulation (AMOC) further contributes to this.
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[ from earlier post ] |
Less sea ice also comes with loss of albedo (water is less reflective than ice, feedback #1), loss of the latent heat buffer (as sea ice disappears, heat can no longer be consumed by the process of melting, and the heat will instead go into increasing the temperature, feedback #14) and loss of emissivity (water is less efficient than ice in emitting in the far-infrared region of the spectrum, feedback #23), while warmer water result in more water vapor and less low-level clouds that reflect sunlight back into space (feedback #25).
The image below, from an earlier post, illustrates that higher temperatures come with feedbacks and the impact of one feedback can amplify the impact of other feedbacks.
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[ image from: 10°C or 18°F warmer by 2021? ] |
The above image depicts some of the dangers of feedbacks for the Arctic. Many feedbacks also apply to the Antarctic, but the bottom part of the image on the right may be particularly applicable to the Southern Hemisphere, which has more ocean surface and Antarctica constitutes a huge land mass on and around the South Pole.
Covering more than 70% of Earth’s surface, our global ocean has absorbed about 90% of the warming that has occurred in recent decades due to increasing greenhouse gases, and the top few meters of the ocean store as much heat as Earth's entire atmosphere, as described by a NASA post.
Even a small change could therefore result in a huge rise in the global air temperature.
Even a small change could therefore result in a huge rise in the global air temperature.
Climate Emergency Declaration
The situation is dire and the precautionary principle calls for rapid, comprehensive and effective action to reduce the damage and to improve the situation, as described in this 2022 post, where needed in combination with a Climate Emergency Declaration, as discussed at this group.
Links
• The southern ocean meridional overturning in the sea-ice sector is driven by freshwater fluxes - by Violaine Pellichero (2018)
The situation is dire and the precautionary principle calls for rapid, comprehensive and effective action to reduce the damage and to improve the situation, as described in this 2022 post, where needed in combination with a Climate Emergency Declaration, as discussed at this group.
Links
• The southern ocean meridional overturning in the sea-ice sector is driven by freshwater fluxes - by Violaine Pellichero (2018)
• Rising surface salinity and declining sea ice: A new Southern Ocean state revealed by satellites - by Alessandro Silvano et al. (2025)
https://www.pnas.org/doi/full/10.1073/pnas.2500440122
discussed on facebook at:
https://www.facebook.com/groups/arcticnews/posts/10162876582119679
• Abrupt Antarctic Ocean Regime Shift: Reversed SMOC - Southern Meridional Overturning Circulation - video by Paul Beckwith
https://www.youtube.com/watch?v=12Ch-NIYxvQ
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