The natural carbon cycle involves a continuous exchange of carbon among major reservoirs: the atmosphere, the land (including plants and soils), and the ocean. This balanced system maintained stable atmospheric carbon dioxide concentrations for thousands of years. Since the Industrial Revolution, human activity has rapidly accelerated the transfer of carbon from long-term storage reservoirs into the active cycle. This fundamentally disrupts the speed and equilibrium of the global process, resulting in a measurable increase of carbon in the atmosphere and ocean.
Primary Release of Stored Carbon
The largest human impact on the carbon cycle comes from extracting and burning geological carbon reserves. Fossil fuels, such as coal, oil, and natural gas, represent carbon sequestered over hundreds of millions of years from ancient biomass. Burning these fuels for energy rapidly converts this stored carbon into carbon dioxide gas, releasing it into the atmosphere on a timescale the natural cycle cannot accommodate.
Approximately 90% of human-caused carbon emissions originate from the combustion of these geological stores. This process bypasses the slow geological part of the carbon cycle, inputting vast amounts of carbon directly into the fast atmospheric cycle. Industrial processes also contribute significantly, particularly cement manufacturing, where the chemical conversion of limestone releases additional carbon dioxide as a byproduct.
Reducing the Terrestrial Carbon Sink
Beyond direct emissions, human land-use practices impair the Earth’s natural ability to absorb atmospheric carbon dioxide. Terrestrial ecosystems, primarily forests and soils, function as major carbon sinks by taking carbon out of the atmosphere through photosynthesis and storing it in biomass and organic matter. Land-use change, such as deforestation and the expansion of agriculture, diminishes this absorption capacity.
When forests are cleared or burned, the carbon stored in the trees is released back into the atmosphere. Converting natural land to cropland often involves tilling, which disturbs the soil structure and accelerates the decomposition of organic matter, venting soil carbon into the air. Overall, human influence is estimated to have depleted natural land carbon stocks by about 24% globally. This combination of removing existing carbon stores and reducing the remaining sink capacity creates a double burden on the atmospheric carbon balance.
The Atmospheric Imbalance
The combined effect of releasing geological carbon and reducing natural land absorption capacity is the measurable rise in atmospheric carbon dioxide concentration. Carbon dioxide acts as a greenhouse gas, meaning it absorbs and re-emits heat radiated from the Earth’s surface. This process, known as the greenhouse effect, is a natural phenomenon that keeps the planet habitable.
Adding excess carbon dioxide to the atmosphere amplifies this natural process, leading to an enhanced greenhouse effect. This traps additional heat energy near the surface, causing global surface temperatures to increase. Before the Industrial Revolution, the concentration of carbon dioxide in the atmosphere was approximately 277 parts per million (ppm); recent measurements exceed 420 ppm. The annual rate of increase has been measured at a speed 100 times faster than natural increases, linking human activities to accelerated warming.
Ocean Absorption and Acidification
The ocean acts as another significant reservoir, absorbing roughly 30% of the excess atmospheric carbon dioxide emissions. This absorption occurs as carbon dioxide dissolves directly into the surface water. While this process slows the rate of atmospheric warming, it initiates a chemical reaction that alters ocean chemistry.
When carbon dioxide dissolves in seawater, it reacts with water molecules to form carbonic acid. This weak acid releases hydrogen ions, which lowers the water’s pH level, a process termed ocean acidification. Since the pre-industrial era, the average pH of ocean surface waters has fallen by about 0.1 units, corresponding to a 26% increase in acidity. This change reduces the availability of carbonate ions, which are necessary building blocks for marine organisms like corals, oysters, and certain plankton to create their calcium carbonate shells and skeletons.