Southern Ocean carbon ‘anomaly’ reveals what models can still miss, Pg2

Key Highlights:

• The Southern Ocean, covering 25-30% of the global ocean area, absorbs approximately 40% of human-emitted carbon dioxide.
• Climate models predicted the Southern Ocean would become less of a carbon sink due to stronger westerly winds and increased greenhouse gases.
• New data reveals the Southern Ocean has been absorbing more carbon since the early 2000s, contrary to model predictions.
• A study in Nature Climate Change explains this resilience through a surface process that climate models missed: increased freshwater stratification.
• Since the 1990s, circumpolar deep waters have risen by about 40 meters, increasing carbon dioxide pressure in the subsurface.
• Increased rainfall and meltwater from Antarctica's glaciers have led to a fresher surface layer, strengthening stratification and trapping carbon-rich water.
• The stratified layer began to thin in the 2010s, and rising surface salinity suggests the carbon sink weakening could re-emerge sooner than expected.

Detailed Insights:

• The Southern Ocean's ability to absorb carbon dioxide is due to its cold, fresh surface layers that trap carbon, but shifts in this layering can alter its role as a carbon sink.
• Climate models correctly predicted the strengthening of westerly winds and the rise of deep, carbon dioxide-laden waters, but failed to account for surface freshwater stratification.
• The increased stratification, caused by melting glaciers and rainfall, created a barrier preventing deep, carbon-rich waters from releasing carbon dioxide into the atmosphere.
• This stratification is governed by complex processes at different scales, such as eddies and ice-shelf cavities, making it challenging for models to capture accurately.
• Recent observations indicate that the stratified layer is thinning, potentially leading to a re-emergence of the predicted weakening of the carbon sink as deep waters mix more easily.
• Continuous, year-round observations are crucial to understanding the Southern Ocean's future role in absorbing or releasing carbon and improving the accuracy of climate models.
• The study reinforces the importance of climate models in identifying vulnerabilities and guiding scientific observations, highlighting the interplay between models and real-world data.

Scientific/Technical Concepts Involved:

• Carbon Sink: A natural or artificial reservoir that accumulates and stores carbon-containing chemical compounds for an indefinite period.
• Stratification: The layering of water masses with different properties (temperature, salinity, density), creating barriers to vertical mixing.
• Upwelling: The process where deep, cold, nutrient-rich water rises towards the surface, often bringing dissolved inorganic carbon.
• Meridional Overturning Circulation: A global system of surface and deep currents driven by temperature and salinity differences, playing a key role in heat and carbon distribution.