What is blue carbon? ​

Blue carbon is the carbon captured and stored by the ocean and coastal ecosystems such as mangroves, salt marshes and seagrass meadows. The term was popularised by a 2009 UNEP report, which showed that these coastal habitats sequester carbon much more intensively per hectare than many terrestrial forests, and that much of it stays locked in waterlogged, oxygen-poor sediments for centuries.

Why the other options fail:

• Option B describes forest biomass and agricultural soils. That is terrestrial carbon, often called green carbon, not blue carbon.
• Option C refers to fossil carbon in petroleum and natural gas. These are geological reserves that add to atmospheric CO2 when burnt, so they are the opposite of a sink.
• Option D is atmospheric carbon, the pool that carbon sinks are meant to reduce, not a sink itself.

Core concept: carbon sinks and the carbon cycle. For Mains GS-III (environment, climate change, coastal ecosystems), blue carbon links mangrove conservation to climate mitigation and coastal protection, which is why several coastal states run mangrove restoration programmes.

Trap: Aspirants either read "blue" as the colour of water without checking the definition, or confuse it with green carbon in forests (option B).