The Ocean as Carbon Sink: How Long Does This Last?

# The Ocean as Carbon Sink: How Long Does This Last?

The ocean has absorbed roughly 25-30% of all CO2 emitted by human activity since industrialization. That number is genuinely remarkable. Without it, atmospheric CO2 concentrations would be roughly 50 ppm higher than they currently are — we'd already be at a warming trajectory well above 2°C above preindustrial. The ocean is the reason the climate crisis isn't worse than it already is.

The mechanism works at two timescales. The **solubility pump** is faster: CO2 dissolves directly into seawater at the surface, particularly in cold, high-latitude waters where gas solubility is higher. Cold water absorbs more CO2 than warm water — which is why warming surface temperatures are a problem not just because of the heat itself but because they reduce the ocean's absorption efficiency.

The **biological pump** is slower and larger. Phytoplankton — microscopic marine plants — absorb CO2 through photosynthesis just like land plants. When they die, a fraction of their biomass (and the carbon it contains) sinks to deeper water and eventually to the seafloor, where it can remain sequestered for centuries to millennia. This is how carbon that was dissolved in surface waters gets exported to the deep ocean.

Here's the fundamental problem: the ocean's carbon uptake capacity is not infinite, and it's not constant. It depends on ocean circulation (bringing low-carbon deep water to the surface to absorb more CO2), water temperature, and biological productivity. All three are changing.

Some ocean regions have already transitioned from net CO2 sinks to net CO2 sources in particular seasons. Parts of the Southern Ocean — one of the most important carbon-absorbing regions — have shown decreasing uptake efficiency in observations. If ocean carbon absorption weakens significantly, the same emissions trajectory produces higher atmospheric concentrations, which produces more warming, which further reduces ocean absorption. This is one of the feedback loops that makes the carbon cycle non-linear.

The uncertainty in this projection is real, but the direction is not: a warming ocean is, other things equal, a less effective carbon sink.

The Ocean as Carbon Sink: How Long Does This Last?

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