The carbon cycle describes the continuous movement of carbon through Earth’s atmosphere, oceans, land, and living organisms. This natural process regulates the planet’s climate and supports life. For millennia, a natural balance maintained stable atmospheric carbon dioxide levels. Human activities have significantly altered this equilibrium, leading to global consequences.
The Earth’s Intricate Carbon Cycle
Carbon is stored in major reservoirs: the atmosphere, oceans, living organisms (biosphere), and rocks and soils (lithosphere). Most Earth’s carbon resides in rocks and sediments, with significant amounts also in the ocean, atmosphere, and living organisms. These reservoirs exchange carbon through chemical, physical, geological, and biological processes.
Carbon moves through fast and slow cycles. The fast carbon cycle involves quick exchanges, often completed within years, such as carbon moving between the atmosphere and biosphere. Plants absorb atmospheric carbon dioxide through photosynthesis, converting it into sugars for growth. Animals and microbes release carbon dioxide back into the atmosphere via respiration and decomposition.
The ocean is a large carbon reservoir, holding about 50 times more carbon than the atmosphere. Atmospheric carbon dioxide dissolves into surface waters, continually exchanging between the ocean and atmosphere. Marine organisms also contribute through their life processes and decomposition. Historically, these natural processes were largely balanced, with carbon absorption equaling emission.
The slow carbon cycle operates over millions of years. Geological processes, like fossil fuel formation from ancient organic matter, trap carbon deep within Earth’s crust. Volcanic activity and rock weathering naturally release this stored carbon back into the atmosphere. These slow and fast cycles maintained a dynamic equilibrium, keeping atmospheric carbon levels stable before widespread human influence.
Key Human Activities Altering Carbon Pathways
Human activities disrupt the natural carbon cycle by releasing excess carbon or reducing Earth’s absorption capacity. Burning fossil fuels is the largest contributor to increased atmospheric carbon dioxide. When coal, oil, and natural gas are combusted for energy, transportation, and industrial processes, they release large quantities of stored carbon. Fossil fuel combustion accounted for about 65% of human-caused carbon dioxide emissions in 2020.
Land use changes, especially deforestation, also alter carbon pathways. Forests are substantial carbon sinks, absorbing carbon dioxide through photosynthesis. Clearing forests for agriculture, logging, or urban development releases stored carbon from trees and soil back into the atmosphere through burning or decomposition. Land use changes are responsible for about 15% of global carbon dioxide emissions.
Industrial processes also contribute to carbon emissions. Cement production, for instance, releases carbon dioxide as a byproduct of chemical reactions. Urbanization impacts local carbon cycles by replacing natural vegetation with built-up areas, reducing carbon sequestration capacity.
Agricultural practices add to greenhouse gas emissions. Tilling, for example, disturbs soil, releasing stored carbon. Livestock farming produces methane, a potent greenhouse gas, through digestive processes. Nitrogen-based fertilizers release nitrous oxide, another powerful warming gas.
Consequences of an Imbalanced Carbon Cycle
Increased atmospheric carbon dioxide from human activities traps heat, causing global warming and rising temperatures. Concentrations have risen substantially since the industrial era, from 280 ppm in the late 1700s to 422.7 ppm in 2024. This 50% increase over pre-industrial levels occurred about 100 times faster than natural increases. This rapid change alters weather patterns and contributes to more frequent extreme weather events.
Ocean acidification is another consequence of increased atmospheric carbon dioxide. The ocean absorbs a significant portion of human-released carbon dioxide, estimated at 29% of global emissions since the preindustrial era. When carbon dioxide dissolves in seawater, it forms carbonic acid, lowering the ocean’s pH and making it more acidic. Ocean acidity has increased by about 25% since the 1700s, a 0.1 unit drop in pH.
This change in ocean chemistry negatively affects marine life, especially organisms building shells or skeletons from calcium carbonate. Corals, shellfish, and plankton struggle to form and maintain structures in acidic waters due to reduced carbonate ions. Increased acidity forces marine organisms to expend more energy, impacting their health and survival. These impacts ripple through marine food webs, disrupting ecosystems.