Unexpected decline in the ocean carbon sink under record-high sea surface temperatures in 2023
- Hakan Sener
- Oct 8
- 4 min read
Ocean absorbed 10% less CO₂ than expected in 2023 despite El Niño conditions that typically strengthen the sink—revealing unprecedented extratropical warming effects that overwhelmed tropical patterns.
A new study led by Jens Daniel Müller (ETH Zurich) uses satellite observations and machine learning models to reveal that 2023's record ocean temperatures caused an unexpected 10% decline in oceanic CO₂ uptake compared to baseline projections.
The research demonstrates that while the tropical Pacific behaved as expected during El Niño—absorbing more CO₂—this was completely offset by anomalous CO₂ outgassing in subtropical and subpolar regions, particularly across the Northern Hemisphere. The finding challenges assumptions about ocean carbon sink resilience under extreme warming.
Key Findings: When Natural Feedbacks Fail to Compensate
Unexpected 0.17 PgC decline in carbon uptake
The global non-polar ocean absorbed approximately 0.17±0.12 PgC yr⁻¹ less CO₂ than expected in 2023, representing a ~10% reduction relative to the baseline trend that accounts for rising atmospheric CO₂.
This decline contradicts historical patterns: previous warm years (especially El Niño events in 1997-1998, 2015-2016) showed strengthened ocean carbon sinks. Based on the SST-CO₂ flux relationship from 1990-2022, 2023 should have experienced enhanced uptake of -0.11±0.04 PgC yr⁻¹.
North Atlantic led unprecedented extratropical outgassing
The North Atlantic subtropical permanently stratified region experienced an extraordinary SST anomaly of +0.50±0.05°C—50% higher than any previous record—causing reduced CO₂ uptake by +0.04±0.01 PgC yr⁻¹.
Combined with the North Pacific (+0.10±0.07 PgC yr⁻¹) and Southern Hemisphere extratropics (+0.08±0.11 PgC yr⁻¹), the extratropical regions contributed +0.27±0.13 PgC yr⁻¹ of reduced uptake, completely overwhelming the expected tropical strengthening.
Tropical Pacific performed as expected, but couldn't compensate
The eastern equatorial Pacific behaved according to historical El Niño patterns, experiencing reduced CO₂ outgassing by -0.09±0.06 PgC yr⁻¹ due to suppressed upwelling of CO₂-rich waters during warm conditions.
However, this tropical response—typically the dominant signal during warm years—was completely negated by the widespread extratropical warming, marking 2023 as fundamentally different from previous El Niño years.
Thermal vs. Non-Thermal Processes
Temperature-driven solubility effects dominated initially
Each 1°C warming increases seawater CO₂ fugacity by ~4% through reduced solubility. The +0.21±0.02°C global anomaly would have raised oceanic fCO₂ by ~4 μatm under isochemical conditions, nearly eliminating the ocean-atmosphere CO₂ gradient.
Thermal effects accounted for >95% of the initial CO₂ flux anomaly across most regions, with wind speed changes playing only minor roles except in specific areas like the North Pacific subpolar zone.
Dissolved inorganic carbon depletion provided negative feedback
As warming drove CO₂ outgassing, surface waters became depleted in dissolved inorganic carbon (DIC), creating a compensating effect that partially offset continued thermal forcing.
In the subtropical North Atlantic, surface DIC-TA (total alkalinity) anomalies of -3 μmol kg⁻¹ by year's end produced an fCO₂ effect (~5 μatm) nearly equal but opposite to the thermal forcing from the +0.5°C SST anomaly.
Regional Variations and Seasonal Dynamics
North Atlantic subtropical waters showed evolving responses
Monthly SST anomalies peaked in summer at +1°C, driving strongest CO₂ flux anomalies during that period through thermal effects on fugacity gradients.
The anomalously shallow mixed layer depth suggested increased stratification and reduced mixing of remineralized DIC into surface waters, though the DIC depletion accumulated primarily through direct CO₂ outgassing rather than biological processes.
Eastern equatorial Pacific maintained strong non-thermal dominance
The integrated DIC-TA anomaly over the top 100m reached -2 mol m⁻² by year's end, far exceeding the cumulative CO₂ flux anomaly of ~-0.1 mol m⁻².
This substantial DIC depletion reflects reduced upwelling of remineralized carbon—the classic El Niño response—converting the fCO₂ reduction of ~50 μatm to substantially overpower the thermal forcing of ~30 μatm.
How They Did It
The team analyzed four machine learning-based fCO₂ products trained on Surface Ocean CO₂ Atlas observations through December 2023, combined with satellite SST data and reanalysis wind fields to calculate sea-air CO₂ fluxes.
Two global ocean biogeochemical models forced with reanalysis data helped interpret surface anomalies through subsurface processes. All anomalies were calculated relative to linear trend baselines (1990-2022) to isolate 2023's exceptional behavior from long-term anthropogenic forcing.
Why It Matters for Climate Projections
Resilience mechanisms may have limits: While DIC depletion provided negative feedback that partially compensated thermal forcing in 2023, this study reveals these stabilizing mechanisms can be overwhelmed by sufficiently extreme or widespread warming events.
Extratropical regions increasingly important: Historical focus on tropical Pacific dynamics during El Niño may underestimate the growing role of extratropical warming in modulating the global ocean carbon sink under climate change.
Early warning for future extremes: Marine heatwaves are projected to become more frequent, intense, and longer-lasting under continued warming. If protective feedbacks weaken or DIC depletion becomes more severe, the ocean's capacity to buffer atmospheric CO₂ could decline substantially.
Monitoring gaps need addressing: The study relied on statistical extrapolation beyond training data, highlighting the critical need for sustained, high-quality surface ocean CO₂ observations to track real-time changes in the carbon sink.
A Fragile Climate Buffer Under Stress
This study provides the first comprehensive analysis of how 2023's record ocean temperatures—made virtually impossible without anthropogenic warming—unexpectedly weakened the ocean carbon sink through mechanisms not seen in previous warm years.
The research reveals a critical vulnerability: while the ocean has historically provided negative feedbacks that stabilize CO₂ uptake under warming, these mechanisms failed to prevent a net sink weakening when faced with unprecedented SST patterns dominated by extratropical extremes.
The finding that thermal effects temporarily "won the tug of war" against compensatory processes in key regions raises fundamental questions about ocean carbon sink stability under future climate scenarios. As the North Atlantic continues experiencing exceptional warming and marine heatwaves become more common globally, understanding whether 2023 represents a temporary perturbation or a harbinger of long-term sink degradation becomes critical for climate projections and policy planning.
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