Carbon, a fundamental element for all life on Earth, continuously moves through various parts of our planet in what is known as the carbon cycle, involving a constant exchange between the atmosphere, oceans, land, and living organisms. Carbon sources release carbon into the atmosphere, while carbon sinks absorb and store it. The balance between these sources and sinks dictates the concentration of carbon dioxide in the atmosphere.
Natural Carbon Sources
Natural processes release carbon into the atmosphere. All living organisms, including plants, animals, and microbes, release carbon dioxide through respiration, breaking down organic compounds for energy.
When plants and animals die, their organic matter undergoes decomposition, primarily by microorganisms in soils and oceans. This breakdown releases stored carbon back into the atmosphere as carbon dioxide or into the soil as organic carbon.
The surface waters of the ocean also exchange carbon dioxide with the atmosphere. Warmer ocean waters tend to release dissolved carbon dioxide into the air, while colder waters absorb it. This exchange is influenced by temperature and carbon dioxide concentration.
Volcanic eruptions, though a minor contributor compared to other natural sources, also release carbon dioxide from the Earth’s interior into the atmosphere. While significant during large events, their overall annual contribution is much smaller than other natural fluxes.
Natural Carbon Sinks
Natural systems absorb and store carbon, functioning as significant carbon sinks. Forests and other land vegetation draw carbon dioxide from the atmosphere primarily through photosynthesis. Plants use sunlight to convert carbon dioxide and water into sugars for growth, storing carbon within their leaves, stems, roots, and wood. This stored carbon can remain in biomass for decades or even centuries.
Oceans represent the largest active carbon sink on Earth, holding approximately 50 times more carbon than the atmosphere. Carbon dioxide dissolves directly into cold surface waters, a process known as the “solubility pump.” As these waters become denser, they sink to deeper ocean layers, sequestering carbon away from the atmosphere for extended periods.
A separate mechanism, the “biological pump,” also transfers carbon to the deep ocean. Microscopic marine plants called phytoplankton absorb dissolved carbon dioxide during photosynthesis in sunlit surface waters. When these organisms die or are consumed, their organic matter sinks as “marine snow” to deeper waters. This process exports carbon, which can remain sequestered for hundreds to thousands of years.
Soils serve as another substantial carbon reservoir, storing carbon as organic matter derived from decaying plant material and microorganisms. This organic carbon is a mixture of plant and animal residues. Healthy soils with high organic carbon content enhance their ability to store carbon, contributing to the global carbon balance.
Human Impact on the Carbon Balance
Human activities have significantly altered the natural balance of the carbon cycle by introducing new sources and degrading existing sinks. The burning of fossil fuels, such as coal, oil, and natural gas, for energy, transportation, and industry, is the largest contributor to increased atmospheric carbon dioxide. This releases carbon sequestered underground for millions of years into the active carbon cycle at an accelerated rate.
Industrial processes, particularly cement production, also release substantial amounts of carbon dioxide. The chemical reactions involved in producing cement clinker directly emit carbon dioxide into the atmosphere. These emissions add billions of tons of carbon dioxide to the atmosphere annually.
Human land-use changes further weaken the planet’s ability to absorb carbon. Deforestation, the clearing of forests for agriculture or development, removes a major carbon sink. When trees are cut down, the carbon they stored is released back into the atmosphere. Degrading land-use practices can also lead to the loss of carbon stored in soils, diminishing natural sequestration capacity.
Increased atmospheric carbon dioxide also affects ocean sinks. As the ocean absorbs more carbon dioxide, it leads to ocean acidification. This process lowers the pH of seawater, which can harm marine organisms, particularly those that form shells or skeletons, and impact the effectiveness of the ocean’s biological pump.
Technological Carbon Capture
To address the imbalance created by human activities, technologies are being developed to remove carbon dioxide from the atmosphere or capture it at emission sources. These engineered solutions aim to supplement natural carbon sinks and reduce atmospheric carbon levels.
One approach is Direct Air Capture (DAC), which pulls carbon dioxide directly from the ambient air. DAC systems use chemical solutions or solid sorbents that bind with carbon dioxide molecules. Once captured, the concentrated carbon dioxide can be separated and then stored or utilized.
Another strategy is Carbon Capture and Storage (CCS), which captures carbon dioxide emissions directly from large industrial sources, such as power plants or factories, before they enter the atmosphere. The captured carbon dioxide is then compressed and transported for permanent storage deep underground in geological formations. This prevents significant amounts of carbon from reaching the atmosphere.