Which of the following is an example of a natural carbon sink?

• A. Oceans
• B. Fossil fuel combustion
• C. Cement production
• D. Industrial processes

Answer: (A)

Carbon sinks are parts of the Earth that remove more carbon dioxide from the atmosphere than they release, helping to regulate the climate. The oceans fit this role as a natural sink because they continually absorb CO2 from the air through both physical and biological processes. Physically, CO2 dissolves in seawater and is transported to deeper layers; chemically, it forms bicarbonate and carbonate ions, effectively storing carbon for long periods. Biologically, phytoplankton photosynthesize, converting CO2 to organic matter, and when organisms die, their remains sink, transferring carbon to deep ocean waters in a process known as the biological carbon pump. These combined mechanisms remove substantial CO2 from the atmosphere on various timescales, making oceans the natural carbon sink among the given options. In contrast, fossil fuel combustion, cement production, and industrial processes release CO2 rather than absorb it, so they are sources, not sinks.