Which statement best describes natural carbon sinks and anthropogenic carbon sources in the carbon cycle?

• A. Natural sinks remove carbon from the atmosphere (oceans and forests); natural and anthropogenic sources release carbon; human activities shift the balance by increasing sources and reducing sinks.
• B. Natural sinks release carbon to the atmosphere; anthropogenic sources remove it.
• C. Natural sinks are limited to oceans; anthropogenic sources include fossil fuel combustion only.
• D. Natural sinks never change; anthropogenic sources are the sole drivers of the current carbon balance.

Answer: (A)

Natural carbon sinks remove CO2 from the atmosphere, mainly through the oceans and terrestrial ecosystems like forests and soils. Carbon sources are processes that add CO2 to the atmosphere, arising from natural cycles and, importantly, human activities such as burning fossil fuels, cement production, and land-use changes. The statement that best fits how the carbon cycle works is that natural sinks pull carbon out of the air, while both natural and anthropogenic sources release it; human activities tilt the balance by increasing sources and, in many cases, reducing the effectiveness of sinks, which helps explain the rise in atmospheric CO2.

Saying sinks release carbon would be the opposite of what they do, and claiming sinks are limited to oceans ignores the significant role of land-based sinks. Additionally, stating that sinks never change ignores the dynamic nature of the carbon cycle, where sink capacity can shift with climate, land management, and other factors, while human emissions are a major driver of the current imbalance.