麻豆淫院

Satellite observations have documented a pronounced decline in Antarctic sea ice extent since 2014, with especially sharp losses in recent years. Whether Antarctica's declining sea ice can recover hinges not only on how much carbon dioxide we emit, but also on how stratified the Southern Ocean is, according to new research in Geophysical Research Letters.

Sirui Li, of the Ocean University of China, and colleagues found that the initial thickness and stability of ocean layers (how strongly lighter, colder surface water lies atop warmer, saltier and denser water) is a key control on whether sea ice loss can be reversed under various future carbon dioxide emission scenarios. If the ocean stratification is strong, surface (above 300 m) and deeper (below 700 m) layers remain more isolated, retaining heat near the surface, which can inhibit ice formation or even melt ice from below.

The scientists ran climate model experiments using the Coupled Model Intercomparison Project Phase 6, approximating future carbon dioxide stabilization and reduction pathways, starting from different initial states of ocean stratification around Antarctica. That is, they compared what happens if the Southern Ocean is already strongly stratified versus less so if carbon dioxide emissions begin to plateau or decline in the future. The goal was to test under what conditions Antarctic sea ice decline is reversible鈥攐r whether the system may cross thresholds beyond which recovery becomes difficult or very slow.

They found that the stronger the initial stratification during elevated atmospheric carbon dioxide levels, the more heat was stored in the surface open ocean layer, rather than being mixed downward, leading to accelerated sea ice melting. This melting enhanced ocean stratification further, leading to additional sea ice loss and creating a positive feedback loop, even when carbon dioxide emissions are reduced.

In the model, by 2129, despite the restoration of carbon dioxide concentrations to pre-Industrial levels, the sea ice area remained 1.4 million km² below its pre-Industrial state, declining a further 0.2 million km² by 2189. This suggests that changes in Antarctic sea ice lag behind carbon dioxide forcing, and the ocean continues to absorb greenhouse gas-induced heat for a considerable time.

Conversely, weaker initial ocean stratification promoted sea ice recovery under multiple emissions reduction scenarios. This is because the researchers found heat was stored at deeper depths or mixed downwards, meaning the surface cooled and promoted ice formation, or at the very least discouraged melt.

Added to this complexity is the fact that observations show stratification is changing: sea ice transport, meltwater, precipitation and wind patterns are altering salinity and temperature profiles, which can impact stratification. Thus, the timing is crucial. If stratification remains weaker long enough while atmospheric carbon dioxide levels are brought under control, there is hope. But if the ocean's layering strengthens too far first, the system may cross into a regime where loss accelerates, and recovery becomes challenging.

The fate of Antarctic sea ice is not just about ice: it influences global climate, ocean circulation, ecosystems, and sea level. For example, less ice means more open water in summer, which absorbs more solar energy and further warms the ocean (ice鈥恆lbedo feedback). Warmer surface waters can accelerate the melting of ice shelves from below, increasing the discharge of land ice and promoting sea level rise. Sea ice also supports specialized ecosystems that depend on its presence, thus posing risks to the organisms that call it home.

Therefore, to improve predictions, the researchers call for better observations of stratification involving more high-resolution measurements of subsurface temperature and salinity around Antarctica, plus better monitoring of meltwater fluxes, winds, and ocean鈥恆tmosphere interactions. Also, they encourage continued attempts to refine models to better resolve vertical mixing processes. They suggest policymakers and climate modelers alike should treat stratification not as a side issue, but as a central factor in the future of Antarctic sea ice.

Ultimately, the team says the reversibility of sea ice loss is possible, but likely only under scenarios where significant carbon dioxide reductions occur sooner rather than later, and where stratification buffers warmer surface and cooler deep waters.