SOUTHERN OCEAN CARBON ‘ANOMALY’

NEWS

• A new study (October, Nature Climate Change) reports that the Southern Ocean has absorbed more carbon dioxide since the early 2000s, contradicting long-standing climate model projections.
• This unexpected behaviour highlights gaps between model predictions and real-world observations.

HIGHLIGHTS

Significance of the Southern Ocean

• Covers 25–30% of global ocean area.
• Accounts for ~40% of oceanic absorption of human-emitted CO₂.
• Acts as a major climate buffer due to cold surface waters and strong circulation.

What Climate Models Predicted

• Rising greenhouse gases → stronger westerly winds.
• Stronger winds → enhanced upwelling of carbon-rich deep waters.
• Result → release of CO₂ to the atmosphere and weakening of the oceanic carbon sink.

What Observations Revealed

• Deep waters are rising (by ~40 m since the 1990s), consistent with models.
• Subsurface CO₂ pressure increased (~10 microatmospheres).
• Yet, atmospheric CO₂ release did not occur; instead, carbon uptake increased.

The Missing Link: Surface Freshwater Stratification

• Increased rainfall and Antarctic ice melt freshened surface waters.
• Fresher water is lighter → stronger stratification.
• This created a “freshwater lid”, trapping carbon-rich waters 100–200 m below the surface.
• Models underestimated small-scale processes (eddies, ice-shelf cavities) and data scarcity.

Why This Is a Temporary Reprieve

• Since the mid-2010s, surface salinity is rising again in parts of the Southern Ocean.
• Stratified layer is becoming shallower and weaker.
• Strong winds may soon mix deep carbon-rich waters to the surface.
• The model-predicted weakening of the carbon sink may re-emerge abruptly.

Key Takeaways

• Models are not wrong but incomplete; they reveal vulnerabilities.
• Observations explain exceptions and short-term deviations.
• Highlights the need for continuous, year-round ocean monitoring.
• Prelims Angle: Role of oceans as carbon sinks; upwelling vs stratification.