The Ocean Stores and Releases CarbonA carbon store holds carbon for a period of time, while a carbon flux transfers carbon between stores.The ocean acts as a carbon sink when it absorbs more carbon dioxide than it releases and as a source when release exceeds absorption.Carbon dioxide diffuses across the air-sea boundary according to the difference between atmospheric and surface-water concentrations.Cold water dissolves more carbon dioxide than warm water, so high-latitude oceans are important absorption zones.Deep circulation carries dissolved carbon away from the surface and can store it for centuries.Diagram showing carbon transfers between the atmosphere, ocean, living organisms and sedimentsNoteThe ocean is not a permanent one-way sink.Season, temperature, currents and biological activity determine whether a region absorbs or releases carbon dioxide.Physical and Biological Pumps Move CarbonThe solubility pump begins when carbon dioxide dissolves in surface water and is transported into the ocean interior.The biological pump begins when phytoplankton use dissolved carbon dioxide in photosynthesis.Carbon enters food webs, and a small fraction sinks as dead organisms, waste or mineral particles.Decomposition returns much of this carbon to seawater, while some reaches deep water or sediments.Upwelling can return carbon-rich deep water to the surface, where carbon dioxide may be released to the atmosphere.ExampleThe Southern Ocean absorbs large amounts of atmospheric carbon dioxide because cold water and circulation carry carbon into the ocean.Coastal upwelling zones can release carbon-rich deep water while also supporting highly productive ecosystems.Warming Can Weaken Ocean Carbon StorageWarmer surface water holds less dissolved carbon dioxide, which weakens the solubility pump.Stronger stratification can reduce mixing between warm surface water and colder deep water.Changes in circulation can alter how quickly absorbed carbon is moved away from the surface.Changes in nutrients and plankton productivity can strengthen or weaken the biological pump.Common MistakeA smaller sink does not require the ocean to become a net source.It can mean that the ocean continues absorbing carbon dioxide but removes a smaller share of emissions.Absorbed Carbon Dioxide Lowers Ocean pHDissolved carbon dioxide reacts with seawater and forms carbonic acid.Carbonic acid releases hydrogen ions, so the concentration of hydrogen ions rises and pH falls.Some hydrogen ions combine with carbonate ions and convert them into bicarbonate.This reaction leaves fewer carbonate ions available for organisms that build calcium carbonate shells or skeletons.Diagram tracing carbon dioxide absorption, carbonic acid formation and declining carbonate availabilityCommon MistakeOcean acidification means a long-term fall in ocean pH.Seawater remains alkaline on average even though it becomes less alkaline.Coral Reefs Lose Building MaterialReef-building corals use carbonate ions to form calcium carbonate skeletons.Lower carbonate availability can slow calcification and make reef structures harder to build and maintain.Weaker reef growth can reduce habitat complexity, alter food webs and lower fishery productivity.Reef degradation also reduces coastal protection and threatens tourism income in reef-dependent places.Healthy branching Acropora prolifera coral near Lee Stocking Island in the BahamasComparison of shell formation under lower and higher carbon dioxide conditionsNoteOcean warming and acidification are different processes caused by rising atmospheric carbon dioxide.Heat stress can cause coral bleaching, while acidification reduces the carbonate available for skeletal growth.Responses Work at Different ScalesRapid cuts in carbon dioxide emissions address the main global cause of ocean acidification.Protecting seagrass, mangroves and wetlands can support coastal carbon storage and habitat resilience.Reducing pollution, overfishing and physical reef damage lowers local stress but cannot stop the global chemistry change alone.Monitoring pH, carbonate chemistry and coral growth helps identify vulnerable ecosystems and evaluate responses.Active recallWhen does an ocean region act as a carbon sink?How do the solubility and biological pumps move carbon into the ocean?Why does absorbed carbon dioxide reduce carbonate-ion availability?How does lower carbonate availability affect coral reefs?
