Context:
- The Southern Ocean, which plays a disproportionately large role in regulating Earth’s climate, has shown an unexpected increase in carbon dioxide absorption since the early 2000s.
- This contradicts long-standing climate model predictions that expected the region to become a weaker carbon sink due to stronger westerly winds and enhanced upwelling of carbon-rich deep waters.
- The findings are based on a long-term ocean chemistry analysis published in Nature Climate Change by researchers from Germany.
Key Highlights:
Scientific Background / Carbon Sink Dynamics:
- The Southern Ocean covers 25–30% of global ocean area.
- It absorbs about 40% of the oceanic uptake of human-emitted CO₂.
- Its effectiveness as a carbon sink depends on strong stratification:
- Cold, fresh surface waters form a lid over warm, salty, carbon-rich deep waters.
Model Predictions vs Observations:
- Climate models predicted:
- Strengthening and poleward shift of westerly winds.
- Increased upwelling of deep, carbon-rich waters.
- Enhanced CO₂ release to the atmosphere, weakening the carbon sink.
- Observed reality since early 2000s:
- Southern Ocean has been absorbing more carbon, not less.
- Circumpolar deep waters have risen by ~40 metres since the 1990s.
- Subsurface CO₂ pressure increased by ~10 microatmospheres, matching model physics.
What Models Missed – The ‘Anomaly’:
- Increased freshwater input due to:
- Higher rainfall
- Meltwater from Antarctic glaciers
- Fresher water is less dense, strengthening surface stratification.
- This stratified layer trapped carbon-rich waters 100–200 m below the surface, preventing CO₂ outgassing.
- Models struggled to capture this due to:
- Complex eddy dynamics
- Ice-shelf cavity processes
- Data scarcity in polar regions
Temporary Nature of the Buffer:
- Since the early 2010s, the stratified layer has begun to thin.
- Surface salinity is rising again in parts of the Southern Ocean.
- Stronger winds may soon penetrate deeper, allowing carbon-rich waters to mix upward.
- This could trigger a sudden weakening of the carbon sink, aligning with earlier model projections.
Significance:
- Highlights the limits of climate models in capturing fine-scale ocean processes.
- Demonstrates the importance of continuous, long-term observations.
- Shows that climate systems may exhibit temporary resilience, masking long-term risks.
Relevant Prelims Points:
- Issue: Southern Ocean carbon uptake anomaly.
- Key Facts:
- Absorbs ~40% of oceanic anthropogenic CO₂.
- Covers ~25–30% of global ocean area.
- Processes Involved:
- Upwelling
- Ocean stratification
- Meridional overturning circulation
- Impact: Temporary strengthening of global carbon sink.
- Challenge: Predicting tipping points in climate systems.
Relevant Mains Points:
- Facts & Concepts:
- Carbon sinks are crucial for limiting global warming.
- Oceans regulate climate through heat and carbon absorption.
- Keywords: Carbon Sink, Ocean Stratification, Upwelling, Climate Models, Southern Ocean.
- Static Linkages:
- Climate feedback mechanisms
- Uncertainty in climate projections
- Importance of polar regions in global climate regulation
- Way Forward:
- Expand year-round ocean observations (floats, satellites, polar missions).
- Improve climate models by integrating fine-scale ocean physics.
- Use combined model–observation approaches for policy-relevant climate forecasts.
UPSC Relevance
- GS Paper III: Environment, Climate Change, Oceanography
- GS Paper I: Physical Geography – Oceans and Climate Systems