The carbon cycle describes the continuous movement of carbon atoms between the atmosphere, oceans, land, and living organisms. This process is essential for life, forming organic matter and regulating Earth’s temperature through atmospheric carbon dioxide (CO2). Historically balanced, human activities have profoundly altered this equilibrium, leading to measurable changes across the planet.
The Natural Carbon Cycle
One major natural process is photosynthesis, where plants absorb carbon dioxide from the atmosphere to create organic molecules. This carbon is then transferred as animals consume plants, and both plants and animals release carbon dioxide back into the atmosphere through respiration.
Decomposition also plays a role, as bacteria and fungi break down dead organic matter, releasing carbon dioxide into the atmosphere and carbon into the soil. Oceans act as significant carbon reservoirs, exchanging carbon dioxide with the atmosphere through absorption and release. This exchange is influenced by factors such as water temperature and marine photosynthesis.
Over geological timescales, carbon moves through slower processes, such as the formation of sedimentary rocks and fossil fuels. Plants and animals that lived millions of years ago, after dying, had their carbon trapped and transformed into coal, oil, and natural gas deep beneath the Earth’s surface. Volcanic activity and erosion naturally release some of this stored carbon back into the atmosphere, maintaining a long-term balance.
Major Human Activities Altering the Carbon Cycle
Human activities have introduced substantial disruptions to the natural carbon cycle, primarily by releasing vast quantities of stored carbon into the atmosphere and reducing natural carbon absorption capacities. These disruptions largely stem from industrialization and associated practices over the last few centuries.
The burning of fossil fuels, including coal, oil, and natural gas, represents the largest human source of carbon dioxide emissions. These fuels contain carbon that has been sequestered underground for millions of years, and their combustion for energy in electricity generation, transportation, and industry rapidly releases this ancient carbon into the atmosphere. For instance, annual emissions from burning fossil fuels reached an estimated 37.4 billion tons of carbon dioxide in 2024. This rapid release contrasts sharply with the slow geological processes that originally stored the carbon, overwhelming Earth’s natural systems. Since the mid-20th century, annual emissions from fossil fuels have consistently increased each decade.
Deforestation and other land use changes also significantly alter the carbon cycle. Forests absorb CO2 from the atmosphere, acting as carbon sinks. When cleared for agriculture, urbanization, or logging, the carbon stored in trees and soil is released into the atmosphere. This adds carbon and diminishes the planet’s ability to remove atmospheric carbon. Land use change, primarily deforestation, accounts for approximately 12–20% of global greenhouse gas emissions.
Industrial processes, particularly cement production, contribute to carbon emissions. The manufacturing of cement involves heating limestone (calcium carbonate), which chemically breaks down and releases significant amounts of carbon dioxide as a byproduct. This process emission accounts for about 60% of the cement industry’s total emissions. Overall, global cement manufacturing produced 1.6 billion metric tons of CO2 in 2022, representing about 8% of the world’s total CO2 emissions.
Agricultural practices also influence the carbon cycle. Livestock farming, particularly through enteric fermentation, produces methane, a potent greenhouse gas. Certain cultivation methods, such as those involving specific fertilizers, can also release nitrous oxide. While not CO2, these gases are potent greenhouse gases that impact the carbon cycle’s equilibrium.
Consequences of Human-Induced Carbon Changes
Human-driven alterations to the carbon cycle have led to significant, observable outcomes across the planet, notably an increase in atmospheric carbon dioxide concentrations. Before the Industrial Revolution, atmospheric CO2 levels were around 280 parts per million (ppm), but by 2024, the global average reached approximately 422.7 to 427 ppm. This represents a roughly 50% increase in atmospheric CO2 content in less than 200 years, a rate significantly faster than any natural increases seen in the last 800,000 years.
This excess atmospheric carbon dioxide intensifies the natural greenhouse effect. This process, where certain gases in Earth’s atmosphere trap heat from the Sun, keeps the planet warm enough to support life. With increased CO2 concentrations, more heat is trapped, leading to a rise in global temperatures, commonly referred to as global warming.
Oceans absorb a substantial portion of excess atmospheric carbon dioxide, leading to a decrease in seawater pH, known as ocean acidification. Since the 1700s, ocean acidity has increased by about 25%, a change greater than any other time in the last 2 million years. This increased acidity challenges marine organisms, particularly those building shells or skeletons from calcium carbonate, such as corals and oysters. This impairs their health, growth, and reproduction.
The altered carbon cycle also contributes to broader impacts on ecosystems and global climate. Rising temperatures contribute to changes in weather patterns, including more frequent and intense heat waves, wildfires, and extreme storms. Melting glaciers and ice sheets, coupled with the thermal expansion of warming ocean water, contribute to rising sea levels, threatening coastal areas with erosion and increased flooding. These environmental shifts affect biodiversity, as many species struggle to adapt to rapidly changing conditions, leading to shifts in geographic ranges, population declines, and increased extinction risks for some species.