The ocean carbon sink, the process by which oceans absorb carbon dioxide (CO₂) from the atmosphere, plays a crucial role in slowing climate change. Oceans currently take up about a quarter of human-made CO₂ emissions each year, reducing the pace of global warming. Without this buffer, CO₂ would accumulate more quickly, worsening extreme weather, sea-level rise, and biodiversity loss. But the ocean carbon sink declined unexpectedly in 2023, even as sea surface temperatures (SSTs) reached record highs, raising new concerns about the resilience of Earth’s natural carbon absorbers.

What Is the Ocean Carbon Sink and Why Does It Matter?

• Oceans absorb CO₂ directly from the atmosphere.
• They usually act as a stabilizer, slowing down the effects of climate change.
• Warmer water holds less CO₂, so rising SSTs weaken this ability.
• Without the sink, atmospheric CO₂ growth would accelerate rapidly.

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Ocean Carbon Sink Declined Unexpectedly in 2023

In 2023, the non-polar ocean (between 50°S and 65°N, which covers approximately 90% of ice-free waters) absorbed about 10% less CO₂ than expected.

• Uptake was -1.84 PgC yr⁻¹, but anomalies showed a weakening of +0.17 ± 0.12 PgC yr⁻¹ relative to past trends.
• Globally, including polar regions, the sink weakened by +0.34 ± 0.25 PgC yr⁻¹, with uptake at -1.99 PgC yr⁻¹.
• The Global Carbon Budget (GCB) 2024 confirmed an ocean sink of 2.9 ± 0.4 GtC yr⁻¹ in 2023, a slight increase from 2022 but weaker than projected.

Source: Nature

Source: Nature

Regional Variations in 2023

Regional CO₂ flux changes in 2023. Source: Nature

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Why Did the Ocean Carbon Sink Decline Take Researchers by Surprise?

Typically, high SST years linked to El Niño events strengthen the sink. For example, in 2015, reduced CO₂ outgassing in the tropical Pacific boosted uptake. Based on past data (1990–2022), the 2023 anomaly of +0.21°C should have led to a greater absorption of -0.11 ± 0.04 PgC yr⁻¹.

• Other models, including spatial SST patterns and regression with El Niño indices, also predicted more substantial uptake.
• Instead, uptake weakened by +0.27 ± 0.13 PgC yr⁻¹, showing the system behaved differently.
• The key driver: excess outgassing from Northern Hemisphere oceans outweighed tropical gains.

Observed vs predicted CO₂ sink response: 2023 weakened unexpectedly compared to statistical models. Source: Nature

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What Caused the Decline?

The decline came mainly from subtropical and subpolar Northern Hemisphere waters, where record SSTs reduced CO₂ solubility.

• A 1°C rise in SST raises fCO₂ by ~4%, favoring CO₂ release back to the air.
• This narrowed the ocean-air gradient by increasing fCO₂ by about four µatm due to the 2023 anomaly (+0.2°C).
• Hotspots:
  • North Atlantic subtropical biome (+0.50°C anomaly, +0.04 PgC yr⁻¹ weakening)
  • North Pacific regions show similar patterns.
• Over 95% of anomalies were thermal effects, not wind or circulation changes.
• Some negative feedback originated from less dissolved inorganic carbon (DIC), which helped to counter outgassing in specific locations.

Thermal effects accounted for more than 95% of anomalies, with hotspots in the subtropical North Atlantic and North Pacific. Source: Nature

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What Does This Mean for 2024 and Beyond?

Preliminary GCB data show a slight recovery in 2024: the sink reached ~3.0 GtC yr⁻¹, despite neutral El Niño conditions and continued high SSTs (+0.19°C).

• SOCAT v2025 confirms variability, but no collapse, unlike the land carbon sink, which nearly failed in 2024.
• If natural feedbacks are overpowered by more frequent marine heatwaves, the ocean’s ability to act as a carbon buffer may be diminished.
• The fact that the ocean carbon sink declined unexpectedly in 2023 shows just how vulnerable it is to climate-driven changes.

Long-term variability of the ocean carbon sink. Source: Nature.

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Frequently Asked Questions

Q: Did the ocean sink in 2023 represent a complete disaster?

A: It eventually weakened by about 10%, but the oceans continued to absorb almost 2 GtC worldwide. However, the fact that the ocean carbon sink declined unexpectedly in 2023 shows the system’s growing fragility.

Q: How did this affect atmospheric CO₂?

A: Growth accelerated. 2023 saw +2.8 ppm, and 2024 levels reached 422.5 ppm.

Q: Why didn’t warm oceans absorb more carbon as in previous El Niño events?

A: Outgassing in extratropical latitudes was stronger than tropical uptake, diverging from past years.

Q: What actions are needed?

A: Expand ocean observations and cut emissions to keep the sink resilient.

All the data are directly sourced from Nature.

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