Carbon and the greenhouse effect
The carbon cycle plays a key role in regulating the Earth’s climate by controlling the carbon dioxide (CO2) concentration in the atmosphere. CO2 is important because it contributes to the greenhouse effect, a process by which the atmosphere traps some of the Sun’s thermal radiation, which warms the Earth.
Around 30% of the solar energy that reaches the top of the Earth’s atmosphere is reflected back to space, mainly by clouds and small particles in the atmosphere called ‘aerosols’. The remaining 60% is absorbed primarily by the Earth’s surface and to a lesser extent by the atmosphere. To balance the incoming energy the Earth needs to radiate the same amount of energy back out into space. Because the Earth is much colder than the sun, the Earth radiates at much longer wavelengths compared to the incoming solar energy (primarily in the infrared part of the spectrum). Much of this thermal radiation is absorbed by water vapour (H2O), CO2, ozone, methane (CH4) and nitrous oxide (N2O) and reradiated back to Earth, warming the surface and the lower atmosphere. Atmospheric constituents that absorb longwave radiation and hence contribute to the greenhouse effect are known as greenhouse gases (GHGs). Water vapour is the most important greenhouse gas; however, human activities have only a small direct influence on its amount in the atmosphere because of its rapid turnover
In the past 420,000 years, pre-industrial CO2 concentrations in the atmosphere oscillated between 180-280 parts per million by volume (ppmv). Recent studies have revealed that post-industrial atmospheric CO2 concentrations are now 110 ppmv higher, and have risen at a rate of between 10 and possibly 100 times faster than at any other time in this period (e.g. Falkowski et al., 2000, Le Quéré et al., 2013). CO2 increase plays a significant role in enhancing the greenhouse effect and it is believed to be the main cause of anthropogenic climate change. With CO2 emissions set to rise further, major efforts are being made at the international scale to stabilize its atmospheric concentration (e.g. United Nations Framework Convention on Climate Change (UNFCCC), 1992)
Slowing the increase of atmospheric CO2 presents a major challenge to society and requires a thorough understanding of the carbon cycle. In fact, the rate of change in CO2 that stays in the atmosphere depends not only on human activities, but also on biogeochemical and climatological processes and their interactions with the carbon cycle.
Overview of the global carbon cycle
In order to understand the global carbon cycle some terms need to be defined first. The following definitions are taken from the Intergovernmental Panel on Climate Change (IPCC, 2003).
- **Carbon stock – The quantity of carbon in a ‘pool’ (i.e. a reservoir containing carbon) at a specified time. At the global scale, this is usually expressed in Petagram (Pg) or Gigatonne (Gt) of carbon (C); 1 PgC = 1 GtC = 1×1015gC. At the regional scale Teragram (Tg) is used; 1 TgC = 1×1012 gC
- **Carbon flux – The transfer of carbon from one carbon pool to another in units of measurement of mass per unit of area and time (e.g. tonnes C ha-1 yr-1 )
- **Source – Any process or activity which releases a greenhouse gas, an aerosol or a precursor of a greenhouse gas into the atmosphere.
- **Sink – Any process, activity or mechanism which removes a greenhouse gas, an aerosol, or a precursor of a greenhouse gas from the atmosphere.
- **Carbon budget – The balance of the exchanges of carbon between carbon pools. The examination of the budget of a pool will provide information whether it is acting as a source or a sink
- **Carbon sequestration – The process of increasing the carbon content of a carbon pool other than the atmosphere.
The main components of the carbon cycle include the atmosphere, land, ocean and the Earth’s crust. The Earth’s crust is the largest carbon pool (~ 100,000,000 PgC), followed by the oceans (~38,000 PgC), the terrestrial biosphere (~ 2,000 PgC) and the atmosphere (~ 780 PgC). Atmospheric CO2 continuously exchanges with ocean and terrestrial ecosystems and the rate of the exchanges determines the overall atmospheric CO2 concentration.
The main carbon fluxes are described below.
Carbon cycling in the ocean
Atmospheric CO2 exchanges with oceanic CO2 at the surface, through the process of diffusion. This exchange amounts to ~ 90 PgC yr-1 in each direction. Once in a dissolved form, CO2 forms a carbonic acid (H2CO3) that reacts with carbonate anions (CO3 2- ) and water to form bicarbonate. While the formation of carbonate allows oceans to uptake a larger amount of CO2 than it would be possible if dissolved CO2 remained in that form, this capacity is limited by the supply of mineral cations (positively charged molecules) from the slow weathering of coastal sediment. The concentration of total dissolved inorganic carbon (DIC) increased markedly in the ocean interior (from 725 PgC to 37,275 PgC) as a result of the combination of two processes: the ‘solubility pump’ and the ‘biological pump’.
The ‘solubility pump’ is a process where DIC is transported from the upper ocean to its interior. Sequestration of atmospheric CO2 in the ocean interior depends on the ocean circulation and mixing where cold dense waters, especially in the North Atlantic and Southern Ocean, sink to the deep ocean dissolving and capturing carbon, until decades to several hundreds of years later the water masses re-surface again. The ‘biological pump’ is a process that contributes to the absorption of atmospheric CO2 in the ocean where microscopic organisms called phytoplankton assimilates CO2 through photosynthesis. The carbon is fixed in their biomass of which 25% sinks to the deep ocean, where it oxidizes and adds to the dissolved carbon concentration (Falkowski et al., 2000).
Carbon cycling on land
Atmospheric CO2 is removed through photosynthesis of plants and stored in organic matter. The gross primary production (GPP) (total amount of organic carbon produced by photosynthesis) is estimated to be ~ 120 PgC yr-1 . Because plants can live for hundreds of years, carbon is temporarily locked away and it is estimated that ~ 610 PgC is stored in plants structures at any given time. About half of the GPP is released back into the atmosphere through plant respiration. Virtually all of the remainder (59 PgC yr-1 ) joins the litter pool and is eventually returned to the atmosphere through soil decomposition. Some detritus has a short turnover time (< 10 years), while some is converted into modified soil carbon which has decadal to centennial turnover time and it is estimated that ~ 1500 PgC is stored in the organic matter of the world’s soils at any given time.
Today the global carbon cycle includes two important fluxes that stem from human activities. The most important is the combustion of fossil fuels (coal, oil and natural gas). These materials are the remains of organisms that once lived in oceans and lakes and their transformation into fossil fuels took place over millions of years. Since the start of the industrial revolution these fuels have been extracted from the Earth’s crust and combusted at increasing rates as a primary source of energy. The combustion of fossil fuels release large amounts of CO2 in the atmosphere (~ 8 PgC yr-1 ) which alters the natural carbon cycle (Le Quéré et al., 2013).
Another human activity responsible for a relatively new and rapid flux of carbon is land cover change, largely in the form of deforestation. As a result of human population increase and expansion of human settlements, large areas of native forests have been cleared and converted into agricultural and urban areas. Because terrestrial carbon storage occurs primarily in forests, the land cover changes have resulted into a net flux of carbon to the atmosphere. Although forests are regrowing in some places, where they can act as a sink of carbon, the net effect of land cover change at the global scale represents a flux of about 1 PgC yr-1 (Le Quéré et al., 2013).
References and further reading
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