Which of the following best describes a carbon sink?

• A. A forest absorbing carbon during growth.
• B. Fossil fuel combustion releasing CO2.
• C. Atmospheric CO2 being constant.
• D. Ocean releasing CO2 to atmosphere.

Answer: (A)

A carbon sink is something that removes more carbon dioxide from the atmosphere than it releases. A forest growing fits this idea because trees use photosynthesis to convert CO2 into organic matter, storing carbon in wood, roots, and soil for many years. That uptake lowers atmospheric CO2, which is what makes forests effective carbon sinks.

The other scenarios describe processes that add CO2 to the atmosphere or do not actively remove it. Burning fossil fuels releases CO2, making it a carbon source rather than a sink. If atmospheric CO2 were constant, that would imply a balance between sources and sinks, not an active sink itself. And when the ocean releases CO2 to the atmosphere, it is adding CO2 to the air, not absorbing it, so it acts as a source in that moment.