SOUTHERN OCEAN CARBON ANOMALY from Vajirao & Reddy Institute

SOUTHERN OCEAN CARBON ANOMALY

• Scientists have discovered that the Southern Ocean has absorbed more carbon dioxide than expected, contradicting long-standing climate model predictions.
• This unexpected behaviour is referred to as the Southern Ocean carbon anomaly.
• The findings were published in Nature Climate Change in October 2025 by researchers from Germany.

• The Southern Ocean surrounds Antarctica and is one of the least explored oceans on Earth.
• It plays a critical role in regulating global climate.
• The Southern Ocean covers about 25–30 percent of the global ocean area.
• It absorbs nearly 40 percent of the carbon dioxide taken up by the world’s oceans.
• Its ability to absorb heat and carbon makes it a major buffer against global warming.

• The surface of the Southern Ocean is cold and relatively fresh, making it lighter than deeper waters.
• Below the surface lie warmer, saltier, and carbon-rich deep waters.
• This layered structure, known as stratification, traps carbon dioxide below the surface.
• As long as this layering remains stable, the ocean continues to absorb more carbon than it releases.

• For nearly two decades, climate models predicted that the Southern Ocean would become a weaker carbon sink.
• Rising greenhouse gases and ozone depletion were expected to strengthen westerly winds in the Southern Hemisphere.
• Stronger winds were expected to increase upwelling, bringing carbon-rich deep waters to the surface.
• This process was expected to release carbon dioxide into the atmosphere and accelerate climate change.

• Observations since the early 2000s show that the Southern Ocean has been absorbing more carbon, not less.
• Long-term ocean chemistry data revealed that deep waters are indeed rising, as models predicted.
• Circumpolar deep waters have moved upward by about 40 metres since the 1990s.
• Carbon dioxide pressure in subsurface waters has increased by around 10 microatmospheres.
• Despite this, carbon dioxide has not escaped into the atmosphere.

• Scientists found that a thin but persistent layer of freshwater at the surface prevented carbon release.
• The Southern Ocean has become less salty due to:
  • Increased rainfall
  • Melting Antarctic glaciers
  • Transport of sea ice
• Fresher water is lighter and strengthens surface stratification.
• This stratification traps carbon-rich waters 100–200 metres below the surface, preventing gas exchange with the atmosphere.

• Two opposing mechanisms are operating simultaneously:
  • Upwelling, which pushes deep carbon-rich water upward
  • Stratification, which blocks vertical mixing
• Climate models captured the upwelling correctly but underestimated surface stratification.
• Stratification is difficult to model because it depends on complex processes like:
  • Ocean eddies
  • Ice-shelf cavities
  • Small-scale turbulence
• Limited observational data in the Southern Ocean worsened this modelling gap.

• Scientists warn that the current situation may not last long.
• Since the early 2010s, the stratified freshwater layer has started to thin in some regions.
• Surface salinity has begun increasing again in parts of the Southern Ocean.
• Strong winds could soon penetrate the weakened stratified layer.
• This could allow carbon-rich deep waters to mix with surface waters and release carbon dioxide into the atmosphere.

• The deep carbon reservoir is now closer to the surface than before.
• If stratification weakens further, carbon dioxide could be released suddenly and in large amounts.
• This could cause a rapid weakening of the Southern Ocean carbon sink, as models had originally predicted.

• The findings do not invalidate climate models.
• Instead, they show that models:
  • Correctly identify long-term vulnerabilities
  • May miss short-term or regional exceptions
• Observations complement models by revealing unexpected real-world behaviour.
• Together, models and observations improve understanding of Earth’s climate system.

• Scientists stress the need for continuous, year-round monitoring of the Southern Ocean.
• The region is one of the harshest environments on Earth, making data collection difficult.
• Improved observations are essential to predict whether the ocean will absorb or release carbon in the future.

• The Southern Ocean carbon anomaly highlights the complex and dynamic nature of Earth’s climate system.
• Temporary processes like surface freshening can delay long-term climate impacts.
• However, these buffers may weaken, leading to sudden changes.
• Understanding this balance is crucial for accurate climate prediction and global climate policy.