Oceans Under Stress: Why Record Heat Weakened Earth’s Carbon Shield

Ring-Billed Gull

13-09-2025

As time passes, news about the now hotter Earth buzzes through the bird village. Those kingfishers residing along the banks of the Red River often report drying riverbeds and skinny fish. As Kingfisher casts his gaze upon the events that have unfolded, he can’t help but feel a sense of unease creeping up within him.

In “GHG Emissions”; Wild Wise Weird [1]

© Lucie Dorne

The ocean is one of humanity’s most important allies in fighting climate change. By absorbing roughly a quarter of human-made carbon dioxide (CO₂) emissions each year, it slows the pace of global warming [2,3]. But new evidence suggests this natural buffer is declining.

In 2023, global sea surface temperatures (SSTs) hit unprecedented highs, fueled by both long-term warming and a strong El Niño. Historically, such warming—especially during El Niño—was expected to enhance the ocean’s carbon uptake by reducing outgassing in the tropical Pacific. Instead, a new study by Müller and colleagues [4] shows the opposite happened: the global non-polar ocean absorbed about 10% less CO₂ than expected.

Using four observation-based CO₂ datasets and ocean biogeochemical models, the researchers found that while tropical oceans did take up more carbon, this was outweighed by anomalous outgassing in the subtropics and subpolar regions, particularly the North Atlantic. Here, record marine heatwaves slashed the ocean’s ability to absorb CO₂, overturning expectations. The net effect was a weakened ocean carbon sink—something not previously observed in years with similarly extreme warmth.

The study highlights a delicate tug-of-war between thermal effects, which reduce CO₂ solubility, and non-thermal processes, such as ocean mixing and nutrient cycling, which can sometimes counterbalance warming. In 2023, the balance tipped toward outgassing, raising concerns about the stability of the ocean sink under future extremes [5,6].

By heating the planet through fossil fuel emissions, humanity disrupts the ocean processes that protect us. Recognizing this feedback cultivates the society’s Nature Quotient (NQ)—the collective capacity to perceive ecological interdependence and act with foresight. Strengthening NQ in science, governance, and society means acknowledging that protecting the oceans and humanity's continuity is inseparable from cutting emissions [7]. Without such awareness, Earth’s greatest carbon shield could falter just when we need it most [8].

References

[1] Vuong QH. (2024). Wild Wise Weird. https://books.google.com/books?id=N10jEQAAQBAJ

[2] Friedlingstein P, et al. (2023). Global carbon budget 2023. Earth System Science Data, 15, 5301-5369. https://doi.org/10.5194/essd-15-5301-2023

[3] Gruber N, et al. (2023). Trends and variability in the ocean carbon sink. Nature Reviews Earth & Environment, 4, 119-134. https://www.nature.com/articles/s43017-022-00381-x

[4] Müller JD, et al. (2025). Unexpected decline in the ocean carbon sink under record-high sea surface temperatures in 2023. Nature Climate Change, 15, 978-985. https://www.nature.com/articles/s41558-025-02380-4

[5] Frölicher TL, Fischer EM, Gruber N. (2018). Marine heatwaves under global warming. Nature, 560, 360-364. https://www.nature.com/articles/s41586-018-0383-9

[6] Burger FA, Terhaar J, Frölicher TL. (2022). Compound marine heatwaves and ocean acidity extremes. Nature Communications, 13, 4722. https://www.nature.com/articles/s41467-022-32120-7

[7] Vuong QH, Nguyen MH. (2025). On Nature Quotient. Pacific Conservation Biology, 31, PC25028. https://doi.org/10.1071/PC25028 

[8] Nguyen MH. (2024). How can satirical fables offer us a vision for sustainability? Visions for Sustainability, 23(11267), 323-328. https://doi.org/10.13135/2384-8677/11267