Carbon is critical to sustain a huge range of Earth’s functions. Not only is it present in all living beings, it is a major component of a number of minerals (e.g. graphite and diamond). This means that it is abundant across the world, in: the atmosphere (air); biosphere (living and dead organism organisms); hydrosphere (oceans, rivers, and lakes); and lithosphere (soil and rocks). These act as storage areas of ‘reservoirs’ of carbon, either in the short-term (a few minutes) or long-term (millions of years). As the Earth is such a dynamic environment, processes such as erosion, evaporation, photosynthesis, respiration, and decomposition constantly move carbon between these reservoirs. Carbon enters, is stored, and leaves the different spheres of the Earth through different methods, and in different quantities:
- Atmosphere; carbon is stored in the atmosphere as both methane (CH4) and carbon dioxide (CO2); greenhouse gases which absorb and retain heat. CO2 is released into the atmosphere through respiration by living organisms, volcanic eruptions, weathering, and human activity. It is removed from the atmosphere by dissolution into water and through photosynthesis by plants. CH4 is released into the atmosphere through animal emissions, decomposition, and burning of fossil fuels.
- Biosphere; all living and deceased organisms contain organic carbon. Organisms gain carbon by either extracting it from CO2 in the atmosphere through photosynthesis, or by consuming other organisms and therefore receiving their carbon. Carbon remains in an organism until it decomposes sufficiently to release carbon to the atmosphere or lithosphere.
- Hydrosphere; the upper layers of oceans hold a vast amount of dissolved organic carbon, and the lower ocean waters are rich in dissolved inorganic carbon. Dissolved organic carbon in the surface layers is rapidly exchanged with the atmosphere because they are constantly in contact with each other. In contrast, the dissolved inorganic carbon is much deeper in the water column, and remains stored for longer periods of time – up to thousands of years. It is the thermohaline circulation, which leads to the large scale mixing of ocean waters, which allows exchange between the upper and lower ocean layers.
- Lithosphere; carbon in the lithosphere is held in soil in the form of both organic and inorganic carbon (often as calcium carbonate). Carbon can leave the soil through soil respiration – which releases CO2, or by erosion – which can carry it into rivers or the ocean, where it then enters the hydrosphere. Within the Earth’s crust a large amount of carbon is stored in limestone and kerogens (the term given to organic matter held within sedimentary rocks). These organics are made of decomposed and highly compressed living matter. Once they become lithified (transformed into rock), some of the kerogens can become crude oil or natural gas – these are a source of fossil fuels. These forms of carbon are highly stable and can remain in the lithosphere for millions of years. If rock is subducted into the Earth’s mantle, it will melt, and the CO2 it contains is released into the atmosphere via subsequent volcanic eruptions. Alternatively, the extraction and burning of fossil fuels by human activity can release carbon into the atmosphere.
All of these reservoirs are very closely linked. The carbon cycle is the term used to describe the ways in which carbon moves between them, and the proportion of carbon stored in each component. See the figure below for an indication of how much carbon is stored in each reservoir, and for some of the processes by which the carbon moves. By measuring the storage and transport of carbon, reservoirs can be classified as either carbon sinks (where more carbon is absorbed than released, so carbon is accumulated and stored) or carbon sources (where more carbon is emitted than stored). This is an important distinction that is often used as a measure of human impact on the natural environment.
The human impact on the Carbon Cycle
The carbon cycle is a natural process, and has been ongoing throughout Earth’s history. Left unperturbed (by natural or human processes) it maintains a stable concentration in the atmosphere, biosphere, hydrosphere, and lithosphere (see the table above). As the reservoirs are linked (either directly or indirectly), a change in any of the carbon reservoirs causes changes in the others. Actions by humans have resulted in the removal of carbon from carbon sinks (such as the oil and coal deposits mentioned above), directly adding it to the atmosphere. This has been most notable since the Industrial Revolution in the 18th and 19th Century.
The two main human impacts on the carbon cycle are:
1. Burning of fossil fuels.
Under natural conditions the release of carbon from fossil fuels occurs slowly, as they are subducted into the mantle, and CO2 is released through volcanic activity. However, humans are heavily reliant on fossil fuels, and extract it from the lithosphere in great quantities. Burning coal, oil, natural gas, and other fossil fuels – for industrial activity and power generation for example, removes the carbon from the fossil fuels and emits it as CO2 into the atmosphere.
2. Land use and land cover change (e.g. deforestation)
Large amounts of carbon are stored in living plants (c.1,000 gigtones). Therefore, land use changes, especially the clearance of forests (which are very densely inhabited by plants, and therefore contain a large amount of carbon), can influence the carbon cycle in two ways. Firstly, the removal of vegetation eliminates plants which would otherwise be capturing carbon from the atmosphere through photosynthesis. Secondly, as dense forests are replaced by crops/pasture land/built environments, there is usually a net decrease in the carbon store, as smaller plants (and worse still, concrete) store far less carbon than large trees. Deforestation also allows much more soil to be eroded, and carbon stored in the soil is rapidly taken into rivers.
Because of the cyclical nature of the carbon cycle, the impacts humans cause can lead to a number of amplifications and feedbacks. Increasing atmospheric CO2 and CH4 (along with other greenhouse gases) causes higher global air temperatures which in turn increases decomposition in soil, thereby releasing more CO2 to the atmosphere. Increases in global temperature also affect ocean temperatures, modifying oceanic ecosystems and having the potential to disrupt the oceanic carbon cycle, limiting the ocean’s ability to absorb and store carbon.