Hot Seas, Weak Sponge: How Overheated Seas Weakened the Planet's Largest Carbon Sink
Record sea surface temperatures in 2023 weakened Earth’s largest carbon sponge.
Breaking the Ice:
A recent report challenges long-held assumptions about the ocean’s ability to buffer climate change. The ocean, which typically absorbs about one-quarter of human-made carbon dioxide emissions, faltered in 2023.
Global sea surface temperatures soared to unprecedented levels, fueled both by warming and a strong El Niño. Historically, such conditions were expected to increase the ocean’s carbon uptake, especially in the tropical Pacific where El Niño reduces carbon outgassing. Instead, the scientists found a roughly 10 percent decline in CO₂ absorption compared to expectations. The culprit was anomalous outgassing from subtropical and subpolar waters—particularly in the North Atlantic and North Pacific—where hotter seas reduced carbon solubility. While the tropical Pacific did, as expected, absorb more CO₂, this gain was overwhelmed by losses in the northern oceans.
Quick Melt:
The implications of a weakened ocean carbon sink ripple far beyond scientific curiosity. If the ocean takes in less carbon, more lingers in the atmosphere, accelerating warming. In 2023, the decline in uptake amounted to roughly the annual emissions of a mid-sized industrial nation.
The study raises questions about the ocean’s reliability as a climate ally. While negative feedbacks—like shifts in dissolved carbon levels—helped partially offset warming-driven outgassing, these stabilizers may not hold under long-term warming or more extreme temperature spikes.
Weakened carbon sinks would force humanity to rely even more heavily on emission cuts and land-based carbon storage. If oceans begin to “breathe out” CO₂ instead of absorbing it, climate models projecting manageable warming scenarios could prove too optimistic. Targeted monitoring of regional hot spots like the North Atlantic may help anticipate shifts in ocean behavior.
The Thaw:
How Did Seawater Warming Weaken the Ocean’s Carbon Sponge? AccumulationZone Explains.
Cold water holds more carbon dioxide than warm water—like how a chilled soda keeps its fizz, while a warm one goes flat. Each degree Celsius of warming makes seawater roughly 4 percent less able to hold CO₂. So when global sea surface temperatures rose by more than 0.2 °C in 2023, the oceans’ sponge-like ability to soak up carbon was already under stress.
Normally, natural processes compensate for this thermal effect. Currents stir carbon-rich waters away from the surface, and marine life helps draw CO₂ down into the deep ocean. In past El Niño years, these “non-thermal” processes actually tipped the balance toward stronger carbon uptake, even when surface waters were warm. But in 2023, the balance shifted. While the tropical Pacific behaved as expected—absorbing more CO₂—the subtropical and subpolar oceans in the Northern Hemisphere exhaled it back into the air.
The North Atlantic, in particular, stood out. Sea surface anomalies there were more than half a degree above normal—far higher than anything recorded in the past three decades. Warmer, more stratified waters trapped carbon near the surface, preventing it from mixing downward. The result: the ocean released more carbon than anticipated, overriding the gains in the tropics.
Final Thoughts
The ocean’s carbon sink is not a single, monolithic system. It is a tug-of-war between physics (temperature and mixing), chemistry (the balance of dissolved carbon and alkalinity), and climate variability (El Niño and other large-scale patterns). In 2023, that tug-of-war ended differently than anyone expected. And while the ocean’s resilience held for now—feedbacks kept the sink from collapsing completely—the study warns that such resilience might not endure in a world of ever-hotter seas.