Question

How do oceans act as carbon reserviors, and what might happen if ocean absorption of co2 decreases

Answer 1

How the oceans store carbon (the main processes)
- Physical/solubility pump: CO2 from the atmosphere dissolves into surface seawater (Henry’s law). Some of that carbon stays near the surface and exchanges rapidly with the air (months–years); some is carried to the deep ocean by cooling and sinking (thermohaline circulation) or by mixing. Deep-ocean storage can last decades to millennia.
- Biological pump: Phytoplankton use CO2 for photosynthesis; some organic matter is eaten and recycled in surface waters, but a fraction sinks as particles ("marine snow") or in fecal pellets to the deep ocean or seafloor sediments, where it is sequestered for long times.
- Carbonate (CaCO3) processes: Many organisms build shells from carbonate. Formation, sinking, and burial of CaCO3 and its dissolution affect alkalinity and how much CO2 the ocean can hold on long timescales.
- Scale and role: The ocean holds many times more carbon than the atmosphere (roughly 10s of times more as dissolved inorganic carbon). Since the industrial era the ocean has absorbed roughly a quarter to a third of human CO2 emissions, slowing atmospheric concentration rise and climate warming.

What could happen if ocean CO2 uptake decreases
- Faster rise in atmospheric CO2 and faster warming. If the ocean takes up less CO2, a larger fraction of emissions stays in the atmosphere, increasing greenhouse forcing, accelerating temperature rise, and amplifying climate impacts (extreme weather, sea-level rise, etc.).
- Positive feedbacks that worsen the problem. Warming and increased stratification both lower solubility and reduce vertical mixing and nutrient supply, weakening the solubility and biological pumps and causing still less uptake — a self-reinforcing cycle.
- Changing ocean chemistry and ecosystems. If uptake fell, ocean acidification would slow locally (less CO2 entering the water), which sounds beneficial for calcifying organisms; but the stronger atmospheric CO2 rise and associated warming, deoxygenation, and altered circulation would harm fisheries, coral reefs, and marine food webs in other ways.
- Reduced long-term sequestration. Less carbon exported to the deep ocean and sediments means less long-term removal from the fast carbon cycle, so more carbon persists in the atmosphere for decades–centuries.
- Broader Earth-system consequences. Higher atmospheric CO2 and temperatures make permafrost thaw, soil carbon losses, and fire frequency more likely, releasing more carbon and further weakening the planet’s ability to re-absorb emissions.

Bottom line
The ocean is a major, slow-acting carbon sink that has moderated human-caused climate change. If its ability to absorb CO2 declines, atmospheric CO2 and warming would increase more rapidly, with wide-ranging climate and ecosystem consequences and several reinforcing feedbacks that make the problem harder to reverse. Limiting emissions (and protecting ocean health) is the most direct way to avoid those cascading impacts.