Syllabus: Climate Change
Background and Context
- The Southern Ocean plays a disproportionate role in regulating Earth’s climate system.
- It covers 25–30% of global ocean area and absorbs about 40% of oceanic anthropogenic CO₂ uptake.
- Its cold, relatively fresh surface waters cap warmer, carbon-rich deep waters, enhancing carbon absorption.
- Even minor changes in this vertical layering can shift the ocean from a carbon sink to a carbon source.
Model Predictions and Expected Trends
- Climate models predicted a weakening Southern Ocean carbon sink under global warming.
- Rising greenhouse gases were expected to strengthen and shift southern westerly winds poleward.
- Stronger winds were projected to intensify upwelling of carbon-rich deep waters.
- Increased surface exposure of deep waters was expected to release CO₂ into the atmosphere.
Observed Carbon ‘Anomaly’
- Contrary to projections, the Southern Ocean has absorbed more carbon since the early 2000s.
- Long-term ocean chemistry data revealed continued strengthening of the carbon sink.
- A study by German research institutions, published in Nature Climate Change (October), explained this resilience.
- Observations confirmed that circumpolar deep waters rose by about 40 metres since the 1990s.
- Subsurface CO₂ pressure increased by around 10 microatmospheres, matching model expectations.
What Climate Models Missed
- Models overlooked a freshwater-driven surface stratification process.
- Increased rainfall and Antarctic glacier melt freshened surface waters.
- Fresher water, being lighter, intensified vertical stratification.
- This stratification trapped carbon-rich waters 100–200 metres below the surface, preventing outgassing.
- Two competing processes emerged: upwelling versus stratification, with stratification temporarily dominating.
- Complex small-scale processes like eddies and ice-shelf cavity dynamics remain poorly captured.
- Limited long-term observational data further constrained accurate model representation.
Why the Resilience May Be Temporary
- Since the early 2010s, the stratified surface layer has begun thinning.
- Rising surface salinity in some regions indicates weakening of the freshwater ‘lid’.
- Stronger winds may soon penetrate deeper, mixing surface and deep carbon-rich waters.
- Once disrupted, re-establishing stratification becomes increasingly difficult.
Implications for Climate Science
- The predicted weakening of the carbon sink may re-emerge sooner than expected.
- Models remain essential, highlighting vulnerabilities and guiding observational focus.
- Continuous, year-round ocean monitoring is critical to refine projections.
- Southern Ocean behaviour will strongly influence future atmospheric carbon levels and global climate stability.