The Importance of the Carbon Cycle
Carbon is an element found in all living substances as well as in many inorganic materials. Both diamond and coal are nearly pure carbon, but with different structures. Carbon is a key element for life, composing almost half of the dry mass of the earth’s plants (that is, the mass when all water is removed). The carbon cycle is the exchange of carbon among three reservoirs or storage places: the land, the oceans, and the atmosphere. The amount of carbon in these reser-voirs is so large that it is expressed in gigatons (Gt): 109 metric tons (1 metric ton equals 1,000 kilograms or 2,200 pounds).
The atmosphere is the smallest pool of actively cycling carbon, that is, carbon that stays in a reservoir less than a thousand years or so. The land and its plants and animals, which scientists call the terrestrial biosphere, is the next largest reservoir of carbon. The oceans are the earth’s largest active carbon reservoir by far (Figure 1). The carbon budget is the balance of carbon among the three reservoirs.
The carbon cycle is vitally important to life on earth. Through photosynthesis and respiration (see p. 3), it is the way the earth produces food and other renewable resources. Through decomposition, it serves as the earth’s waste disposal system. In addition, the carbon cycle is important because carbon-containing gases in the atmosphere affect the earth’s climate (see “The Greenhouse Effect,” p.2). Increased carbon dioxide (CO2) in the atmosphere has been responsible for more than half of the climate warming observed in recent decades.
The processes by which carbon moves through the earth’s reservoirs take place on very different time scales. The short-term carbon cycle includes processes that transfer carbon from one reservoir to another in a matter of years, including photosynthesis, plant, and animal respiration,
and the movement of CO2 across the air-sea interface. Other processes, such as the transformation of carbon into limestone and its subsequent release as the rock is weathered, occur very slow-ly (thousands to millions of years). Scientists call these slower transfers of carbon among reservoirs the long-term carbon cycle.
The carbon cycle is inextricably linked to other chemical cycles, including those of nitrogen, phosphorus, and sulfur, as well as to the global hydrological cycle. Please see the Global Change Instruction Program module Global Biogeochemical Cycles and the Physical Climate System or other works for information on these important topics.
Actively cycling carbon in its three reservoirs affects human life every day. Carbon in the atmosphere serves as food for plants (in the pres-ence of sunlight); carbon in the soil serves as energy for the growth of miniature animals called microbes; carbon in plants feeds humans and
Figure 1. Magnitudes of the reservoirs of actively cycling CO2 in gigatons of carbon. Of the ocean pool, roughly 1,000 Gt are in contact with the atmosphere in any given decade. The terrestrial biosphere consists of soil (ca. 1,600 Gt) and vegetation (ca. 600 Gt). Data from Schimel et al. (1995).(after Christensen, 1991).
Photosynthesis and respiration
In the short-term carbon cycle, photosynthesis and respiration are the primary processes that involve the atmosphere. Photosynthesis is the process by which green plants make their food. In this process, plants combine CO2 and water using light energy to make carbon-containing compounds (sugars and starches, called carbo-hydrates). Oxygen is produced during the reac-tion and released to the atmosphere. On land, plants use CO2 from the atmosphere (Figure 4). In the oceans, phytoplankton use CO2 dissolved in seawater; much of this dissolved CO2 also origi-nally came from the atmosphere. Photosynthesis also releases oxygen to the atmosphere.
Thus, thanks to the activity of photosynthetic organisms, all other forms of life on the earth have oxygen to breathe and food to eat—since even carnivores feed on animals that eat plants.
Respiration is the chemical process by which carbon-containing compounds are broken down within cells. It is essentially the opposite of photo-synthesis: oxygen and carbohydrates react to pro-duce CO2 and water, releasing energy during the process. Living organisms use the energy released by respiration to power everything they do. For example, reading this page requires energy, and that energy is supplied by respiration.
|The Carbon Cycle|
|Concentration of Carbon Dioxide||260||Departure of Temperature|
|in the Atmosphere (ppm)||220||+2.5||from Current Level (|
160 120 80 40 0 Thousands of Years Before Present
Figure 3. Variations in global temperature (bottom line) and atmo-spheric CO2 concentration (top line) over the last 160,000 years. The strong pattern of higher temperatures when atmospheric CO2 levels are high and cooler temperatures when concentrations are lower has been used to suggest that future increases in atmo-spheric CO2 concentration could lead to warmer temperatures. From Barnola et al. (1987), p. 410.
Options for Mitigating CO2 Releases
In an ideal world, mitigation options would not be necessary because people would realize the potential damage caused by perturbation of the natural carbon cycle and would work to curtail emissions from land use and use of fossil fuels. This not being a perfect world, there are several options for mitigating human-induced car-bon cycle perturbation. The options center on three basic themes: (1) reduce emis-
sions, (2) switch to a renewable energy source, and (3) increase the amount of carbon stored in the ocean and in the terrestrial biosphere.
Obviously, these themes overlap. For example, switching to a renewable energy source, such as solar power, reduces emissions from fossil fuels. Likewise, planting forests to store carbon creates a renewable energy source.
Emissions will be reduced only if new tech-nology becomes available that makes conserva-tion economically sound or if governments force the reduction, through either subsidies, incen-tives, or taxes (disincentives). Keep in mind that any major transformation of the energy, industry, and transportation sectors of the economy is not likely to happen overnight, so emissions will con-tinue to grow in the immediate future.
We are probably all familiar with some of the options for reducing emissions, such as taking public transportation instead of driving a car and keeping buildings cooler in the winter and warmer in the summer. This section will concen-trate on the other types of mitigation options, those involving alternative energy sources and
Understanding Global Change: Earth Science and Human Impacts
carbon storage. One issue that must be remem-bered when assessing any of these options is whether it contributes to or undermines the goal of supporting a growing human population and simultaneously providing the required amounts of food, fodder, fiber, and biomass. An analysis of the socioeconomic costs associated with various mitigation options is presented in IPCC (1997).
Developing Renewable Energy Sources
Development of renewable energy sources would reduce emissions of traditional fuels. However, this transformation would have to be economically feasible, either through technologi-cal developments or via incentives. For example, vegetable oil crops are already used to produce biodiesel, and this fuel can be used in current diesel engines. The cost of biodiesel, however, is slightly higher than the cost of petroleum diesel, and so its use is not common.
Planting biofuel crops not only promotes alter-native energy use, but it also reduces emissions of traditional fuels and temporarily increases carbon stor-age. It is estimated that dedicated energy plants could be grown sustainably on 8–11% of marginal to good cropland in the temperate zone. One draw-back is that biofuel crops compete with food crops for limited resources (e.g., water and nutrients).
Use of recycled wood and paper products and industrial timber and paper industry wastes as biofuels is an interesting option, but one that is not yet feasible.
Increasing Carbon Storage
Where sufficient land is available, planting forests can result in a fairly long-term (50 to hun-dreds of years) storage of carbon. Forestry cur-rently offsets 90% of carbon released in Sweden through fossil-fuel burning. Table 6 shows the percentage of fossil-fuel emissions that is offset by forestry in eight other countries. In addition to serving as a storehouse for carbon, forests, like agricultural crops, can be used as an alternative source of energy. Wood grown in plantations can be used to generate electricity instead of using coal; this substitution can dramatically reduce CO2 emissions to the atmosphere. Figure 19
Carbon stored in trees and forest litter as a percentage of national fossil-fuel emissions
After Brown et al. (1996).
|and Litter||Emissions||Removed by|
*Mt is millions of tons.
depicts the amount of carbon that can be either stored or saved (not released to the atmosphere) by forests in several latitudinal bands. Note that the forests of the tropics have the greatest poten-tial to affect future atmospheric CO2 levels.
In some parts of the ocean, lack of iron (rather than lack of nutrients) appears to limit marine plant growth. It has been suggested that
Figure 19. Average annual rate of carbon uptake and storage per decade through forest management practices by latitudinal region. Note that the forests of the tropics have the greatest potential to reduce future atmospheric CO2 concentrations. From Brown et al. (1996), p. 786.
The Carbon Cycle
increasing oceanic iron concentrations in these areas could speed up the biological pump and thus reduce atmospheric CO2 levels. However, experiments in adding iron to ocean waters (called iron fertilization) have shown that although productivity was greatly increased at first, the final effect was negligible because the additional plant material simply joined the food chain. In other words, iron fertilization led to more sharks rather than to less atmospheric CO2. Most scientists now believe that even excessive iron fertilization of selected areas of the ocean would have a relatively small impact on atmo-spheric CO2 levels.
Several carbon storage options also not only reduce climate change but increase or restore ecosystem health. One such option is to convert marginal or surplus cropland and pasture to for-est or natural (ungrazed) grassland.
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