A carbon reservoir is a natural or artificial storage area that accumulates and stores carbon-containing chemical compounds. Carbon moves into and out of these reservoirs over timescales ranging from days to millions of years. This storage influences the balance of carbon on Earth, impacting various natural systems.
The Major Natural Carbon Reservoirs
The atmosphere is a dynamic carbon reservoir, primarily holding carbon as carbon dioxide (CO2) and methane (CH4). Though it contains about 800 gigatons of carbon, its content directly influences Earth’s climate. Atmospheric carbon gases allow solar radiation to pass through but trap heat radiating from the Earth’s surface.
The terrestrial biosphere includes all living and dead organic matter on land, such as plants, animals, microorganisms, and carbon in soils. Forests are important, as trees absorb carbon dioxide during photosynthesis, storing it in their wood, leaves, and roots. Soil organic matter, from decomposing material, is a substantial carbon sink, holding more carbon than the atmosphere and all vegetation combined.
Oceans are the largest active carbon reservoir, holding about 50 times more carbon than the atmosphere. Carbon exists in various forms, including dissolved inorganic carbon, organic carbon, and carbon within marine organisms and sediments. Surface waters exchange carbon dioxide rapidly with the atmosphere, while the deep ocean stores carbon for hundreds to thousands of years through slow circulation.
The lithosphere, Earth’s crust and upper mantle, holds the largest amount of carbon. Most is locked in rocks and sediments like limestone and dolomite. Fossil fuels such as coal, oil, and natural gas are also part of this reservoir, formed from ancient organic matter over millions of years. Its natural exchange with other reservoirs occurs over geological timescales, making it a slow component of the carbon cycle.
Carbon Flux and the Global Carbon Cycle
Carbon continuously moves between major reservoirs through carbon flux. This constant movement forms the global carbon cycle, maintaining a natural balance over long periods. Different processes facilitate carbon transfer, influencing its distribution across Earth’s systems.
Photosynthesis is a primary mechanism driving carbon flux from the atmosphere into the terrestrial biosphere. Green plants and other photosynthetic organisms absorb atmospheric carbon dioxide, converting it into organic compounds like sugars and cellulose. This process sequesters carbon in plant biomass, supporting growth within ecosystems.
Respiration and decomposition release carbon back into the atmosphere. Living organisms, including plants, animals, and microbes, respire, breaking down organic compounds and releasing carbon dioxide. When plants and animals die, decomposers like bacteria and fungi break down their organic matter, returning carbon dioxide and methane to the atmosphere and soil.
Oceans exchange carbon with the atmosphere through gas diffusion at the surface. Carbon dioxide from the atmosphere dissolves into surface waters, and can also be released from the ocean back into the atmosphere. This exchange is influenced by water temperature, salinity, and carbon dioxide concentration in both the atmosphere and ocean.
Geological processes contribute to the long-term carbon cycle at slower rates. Volcanic eruptions release carbon dioxide stored within the Earth’s crust and mantle into the atmosphere. Over millions of years, rock weathering also contributes to carbon movement, as dissolved carbon is transported by rivers to the oceans, where it can be incorporated into sediments and form new rocks.
Human Disruption of Carbon Reservoirs
Human activities have altered the natural balance of carbon reservoirs, accelerating carbon transfer, particularly into the atmosphere. These disruptions are primarily responsible for the rapid increase in atmospheric carbon dioxide concentrations observed over the past two centuries.
Fossil fuel combustion is the largest human-caused disruption, releasing carbon stored in the lithosphere for millions of years. Burning coal, oil, and natural gas for energy, transportation, and industry converts solid or liquid carbon into gaseous carbon dioxide. This rapidly transfers carbon from a slow-cycling geological reservoir directly into the fast-cycling atmospheric reservoir.
Land-use change, especially deforestation, alters carbon reservoirs. Forests act as carbon sinks, absorbing atmospheric carbon dioxide. When forests are cleared for agriculture, development, or timber, carbon stored in trees and soil is released into the atmosphere through burning or decomposition. This reduces the terrestrial biosphere’s capacity to absorb carbon, exacerbating atmospheric carbon increases.
Agricultural practices contribute to carbon release from soils. Tilling and plowing expose soil organic matter to oxygen, accelerating its decomposition by microbes and releasing stored carbon dioxide into the atmosphere. Farming methods involving livestock also produce methane, a potent greenhouse gas, further impacting the atmospheric carbon balance.
Industrial processes, beyond energy production, also contribute to carbon emissions. For example, cement production involves heating limestone (calcium carbonate). This chemical reaction releases carbon dioxide as a byproduct, directly transferring carbon from the lithosphere into the atmosphere. Such activities create an imbalance in the global carbon cycle, impacting Earth’s systems.
Consequences of Altered Reservoirs
The increased concentration of carbon in the atmospheric reservoir directly intensifies the greenhouse effect. Carbon dioxide and other greenhouse gases trap heat radiating from Earth’s surface, preventing its escape into space. This enhanced heat retention leads to a measurable increase in global average temperatures, commonly referred to as global warming.
Rising global temperatures contribute to environmental shifts, including more frequent heatwaves, changes in precipitation patterns, and melting glaciers and ice sheets. These climatic alterations can disrupt ecosystems, impact agricultural productivity, and contribute to rising sea levels, posing challenges to human societies and natural habitats.
The oceanic reservoir absorbs excess carbon dioxide from the atmosphere, impacting marine environments. As carbon dioxide dissolves in seawater, it reacts to form carbonic acid, reducing ocean pH. This phenomenon is known as ocean acidification.
Ocean acidification threatens marine ecosystems, particularly organisms that build shells or skeletons from calcium carbonate. Corals, oysters, mussels, and various plankton species struggle to form and maintain their protective structures in more acidic waters. This can disrupt marine food webs, degrade coral reefs, and diminish biodiversity in the oceans.