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Title:
Inconsistency in the Ocean Surface Carbon Mass Balance in IPCC AR6

In the IPCC Sixth Assessment Report (AR6, WG1, Chapter 5, especially Fig. 5.12), a substantial increase in the gross flux of CO₂ between the atmosphere and the ocean is presented:
From approximately 54 GtC/year in the pre-industrial period (circa 1850) to over 80 GtC/year by 2020 — an increase of about 47 % (IPCC, 2021, Ch. 5, Fig. 5.12, Table 5.1).
At the same time, the inventory of dissolved inorganic carbon (DIC) in the upper ocean (commonly defined as the top 1000 m) is estimated at about 900 GtC (IPCC, 2021, Sec. 5.2.2.1; Takahashi et al., 2009), and is assumed to remain approximately stable.
This leads to a fundamental inconsistency in the carbon mass balance:
At a gross flux of 80 GtC/year and an oceanic reservoir of 900 GtC, the implied residence time τ ≈ 11 years.
A 47 % increase in the gross flux requires — assuming constant τ — a proportional increase in the size of the exchange pool:

Cnew​=τ⋅Fnew​≈16,6yr⋅80GtC/yr=1333GtC

However, the observed increase in surface-ocean DIC is only about 15–20 % (Lauvset et al., 2022; GLODAPv2.2022).
This discrepancy is not addressed in the IPCC report. A possible shortening of the residence time (e.g. due to increased wind-driven mixing or altered stratification) is not quantified. While ocean chemistry (carbonate buffering) explains why DIC does not scale linearly with atmospheric pCO₂, it does not explain how a 47 % increase in gross carbon flux occurs without a corresponding increase in the surface reservoir.
Thus, several key questions arise:

1. How can the increase in gross air–sea CO₂ flux (+47 %) be reconciled with the relatively modest DIC increase in the surface ocean (+20 %)?
2. Is a shorter residence time for carbon in the surface layer implicitly assumed?
3. Is part of the enhanced CO₂ exchange attributed to direct coupling with deeper layers, beyond the typical surface reservoir (0–1000 m)?
4. Why is this core mass-balance relationship τ=C/Fτ = C/Fτ=C/F not made explicit in the IPCC carbon cycle chapters?

Given the central role of the ocean as a carbon sink, a clear, transparent, and physically consistent mass balance of the surface carbon pool is essential for evaluating both historical and future carbon cycle feedbacks.

 References

• IPCC AR6 (2021): Climate Change 2021: The Physical Science Basis.
• Working Group I Contribution, Chapter 5, Fig. 5.12, Table 5.1
• https://www.ipcc.ch/report/ar6/wg1/
• GLODAPv2.2022: Lauvset, S.K. et al. (2022), An updated global interior ocean biogeochemical data product, Earth Syst. Sci. Data, 14, 5543–5572.
• https://doi.org/10.5194/essd-14-5543-2022
• SOCATv2024: Bakker, D.C.E. et al. (2024), Surface Ocean CO₂ Atlas (SOCAT) Release 2024.
• https://www.socat.info
• Takahashi et al. (2009): Climatological mean and decadal change in surface ocean pCO₂, Deep-Sea Research II, 56(8), 554–577.
• https://doi.org/10.1016/j.dsr2.2008.12.009