The carbon cycle describes the natural process through which carbon atoms move continuously between the atmosphere, oceans, land, and living organisms. This fundamental biogeochemical cycle involves carbon being absorbed from the atmosphere, incorporated into living systems and geological formations, and then released back into the atmosphere. This continuous movement maintains a natural balance, which is essential for supporting life on Earth. Human activities have significantly altered this balance, leading to substantial influences on the carbon cycle.
Activities Releasing Stored Carbon
Human activities extract and release carbon from long-term geological or biological reservoirs, transferring it into the atmosphere. The burning of fossil fuels, such as coal, oil, and natural gas, for energy generation is a primary driver. These fuels contain carbon stored underground for millions of years. When combusted for electricity, transportation, and industrial processes, vast amounts of carbon dioxide (CO2) are emitted.
Global carbon emissions from fossil fuels reached record levels in 2023, with approximately 37 billion tons of CO2 released from this source alone. Industrial processes also contribute to atmospheric carbon. Cement production, for instance, releases CO2 through both the energy used in manufacturing and a specific chemical reaction.
During cement production, limestone (calcium carbonate) is heated in kilns, causing it to decompose and directly release CO2 as a byproduct. This process is known as calcination. Process emissions from this chemical reaction accounted for about 60% of the cement industry’s total emissions in 2020-2021.
Activities Reducing Carbon Sequestration
Human actions also diminish the Earth’s natural capacity to absorb and store atmospheric carbon, or they release carbon previously held within ecosystems. Deforestation, the clearing of forests for agriculture, logging, or urban development, significantly impacts the carbon cycle. Trees and other plants naturally absorb CO2 from the atmosphere through photosynthesis, storing carbon in their biomass and the surrounding soil.
When forests are cleared, especially through burning, the stored carbon is released back into the atmosphere as CO2. This reduces the planet’s ability to absorb future CO2, as the carbon sinks are eliminated. In 2023, tropical forest loss alone, totaling 3.7 million hectares, contributed approximately 6% of estimated global CO2 emissions.
Beyond deforestation, other land-use changes also reduce carbon sequestration. Converting natural landscapes, such as wetlands and grasslands, into developed areas or agricultural land can lead to the release of soil organic carbon. Wetlands, for example, store significant amounts of carbon in their soils and biomass, and their conversion can result in substantial carbon emissions.
Agricultural Practices and Carbon
Modern agricultural practices influence the carbon cycle through various emissions, including greenhouse gases beyond CO2. Livestock, particularly ruminants like cattle, produce significant amounts of methane (CH4) through a digestive process called enteric fermentation. This accounts for a substantial portion of anthropogenic methane emissions.
Rice cultivation, especially in flooded paddies, is another source of methane. The anaerobic decomposition of organic material in these submerged fields generates methane, which then escapes into the atmosphere. This process makes rice paddies a notable contributor to global methane emissions from agriculture.
The application of nitrogen-based fertilizers also affects the carbon cycle by leading to emissions of nitrous oxide (N2O). Microbes in the soil convert nitrogen from these fertilizers into N2O through processes like nitrification and denitrification. Nitrous oxide is a powerful greenhouse gas, and its emissions from agricultural soils, largely driven by fertilizer use, have increased significantly. Certain tillage practices can release carbon stored in soils, while sustainable practices like no-till farming and cover cropping can help sequester it.
Waste Management and Carbon Emissions
Human waste management practices also contribute to greenhouse gas emissions, influencing the carbon cycle. A significant source of these emissions comes from landfills, where organic waste decomposes in the absence of oxygen. This anaerobic decomposition produces landfill gas, composed of roughly 50% methane and 50% carbon dioxide. Methane is a potent greenhouse gas, considerably more effective at trapping heat than CO2 over a 100-year period.
Landfills are recognized as a notable source of methane emissions. The amount of gas produced depends on factors such as the type of waste, moisture content, and temperature within the landfill. Even after closure, a landfill can continue to generate gases for many decades.
Waste incineration, or burning waste, also releases CO2 and other pollutants into the atmosphere. While modern incinerators may capture some energy, the combustion process directly emits CO2, including biogenic carbon from organic materials and fossil carbon from plastics. Each tonne of municipal solid waste incinerated can release between 0.7 and 1.7 tonnes of CO2.