Carbon Cycle
Carbon accounts for approximately 50% of the dry mass (water removed) of plants and animals and is present in all aspects of the planet [11]. It is in the land, the atmosphere and the oceans. Over time it is cycled in and among all of these components. This carbon cycle has a large impact on Earth. Globally, the carbon cycle influences the Earth's climate by regulating the amount of CO 2 in the atmosphere. Land-based ecosystems store as much carbon as the atmosphere, so plants and soils play an important role in regulating climate. The carbon cycle is also a factor in keeping ecological systems in balance. The atmosphere itself contains nearly 800 billion gigatonnes of carbon (GtC), which is more carbon than all of the Earth's living vegetation contains [12].
Biological organisms play an important role in the carbon cycle through the processes of photosynthesis and respiration. Almost all forms of life depend upon the production of sugars from solar energy and CO 2 through photosynthesis and the metabolism of the sugars to produce energy through respiration.
Photosynthesis is a process by which green plants absorb solar energy to remove CO 2 from the atmosphere and combine the CO 2 with water in the presence of chlorophyll to produce carbohydrates (sugars) and oxygen O 2. This is a complex series of chemical reactions that happens within the plant cells. This process is the original source of all important fuels including oil, coal, wood, and natural gas. You can also look at this process as the source of all our foods. . .how is that for importance! For my purposes in this unit, I will give a bit of background on this and not focus on these reactions, but if you are interested in an excellent explanation and in-depth analysis of this please refer to Connie Wood's unit from the same 2007 National Seminar on Renewable Energy.
Photosynthesis is carried out in organelles in plant cells called chloroplasts. They contain a pigment called chlorophyll. Chlorophyll appears green since it absorbs the blue and red wavelengths of light and reflects the green wavelengths. This pigment is responsible for trapping the sunlight for the reaction to occur. In a simplified explanation of photosynthesis, CO 2 and water are the raw materials used to form glucose and oxygen. The balanced reaction can be written as: 6 CO 2 + 12 H 2O —> C 6H 1 2O 6 + 6 H 2O +6 O 2. Looking at this reaction, you see that water is both a reactant and product. This is because there are two steps involved in photosynthesis; the light and dark reactions. In the light reactions, light energy is converted to chemical energy and water is split into hydrogen and oxygen atoms. In the dark reactions, carbohydrates are formed from the carbon dioxide and hydrogen atoms. They both occur in the chloroplasts, and please do not confuse the idea that dark reactions occur at night. These reactions both occur in the daylight; they are simply called dark reactions since they do not require light energy. Structurally, the chloroplast contains stacks of membranes called grana which store the chlorophyll; they are surrounded by what is called the stroma. The light reactions occur in the grana, while the dark reactions occur in the stroma.
Respiration can be seen as the opposite of photosynthesis. In respiration, plants start with glucose which is broken down in the presence of oxygen and releases both water and carbon dioxide along with all the energy that was stored in the bonds. The reaction for respiration can be written as: C 6H 1 2O 6 + 6 H 2O + 6 O 2 —> 6 CO 2 + 12 H 2O. Respiration occurs in a structure called mitochondria. Respiration takes the biologically fixed carbon back to the atmosphere.
The amount of carbon taken up by photosynthesis and released back into the atmosphere by respiration each year is 1,000 times greater than that which moves through the geological cycle [12]. This major exchange of carbon through biological processes can be seen if you look at the oscillations in atmospheric carbon dioxide concentrations. Over the course of a year, the biological fluxes of carbon are ten times greater than the amount of carbon introduced to the atmosphere by fossil fuel burning, although currently we are seeing the effects of this input increase dramatically.
In magnitude of the global carbon cycle, human activities contribute a relatively small amount of carbon, primarily as CO 2. Fossil fuel combustion adds less than 5% to the total amount of carbon released from the oceans and land surface to the atmosphere each year. CO 2, alone, is responsible for over half of the change in Earth's radiation balance. CO 2 concentration in the atmosphere currently is approximately 380 ppm [13]. Carbon dioxide released from fossil fuel combustion mixes readily into the global carbon pool. This increased concentration in the atmosphere is increasing so rapidly, since we have released in a couple of hundred years what took millions of years to accumulate geologically. Where fossil fuels are burned makes relatively little difference to the concentration in the immediate atmosphere, emissions in any region of the Earth affect the concentration of CO 2 everywhere else in the atmosphere. Concentrations of CO 2 are slightly higher in the northern hemisphere compared to the southern hemisphere, by several ppm, because most of the emissions of CO 2 from human activities happen there.
The oceans, vegetation, and soils exchange carbon with the atmosphere constantly. Carbon from fossil fuels is not exchanged with the atmosphere but transferred one way from geologic storage. Some of the CO 2 currently in the atmosphere may become fossil fuels someday, after being captured by photosynthetic organisms, and buried and subjected to heat and pressure to form, but the process takes millions of years.
How much carbon can be stored in each pool, especially the atmosphere, is a key factor in determining how severe global warming will be. Oceans, soils and vegetation are considered sinks for carbon since each takes up carbon from the atmosphere. We must also realize that each is a source of carbon for the atmosphere, because of the constant exchange between them and the atmosphere. Whether these storage pools are net sinks or net sources of carbon in the future depends on the balance of mechanisms that drive their behavior and how they change.
Over 90 GtC of carbon is exchanged each year between the atmosphere and the oceans, and close to 60 GtC is exchanged between the lands surface and the atmosphere annually. Human activities, fossil fuel combustion and land-use changes (such as building roads for transportation), contribute just less than 9 GtC to the atmosphere each year [14]. If the human contribution of CO 2 is removed from the equation, the average net flux, amount of CO 2 released to the atmosphere verses the amount taken up by the oceans, soils, and vegetation, is close to zero. This is reflected by the relatively stable concentration of CO 2 in the atmosphere, between 260-280 ppm, for the past 10,000 years prior to 1750 [15].
Currently the atmospheric concentration of CO 2 is almost 100 ppm higher than it was before 1750 because human activities are adding carbon to the atmosphere faster than the oceans, vegetation, and soils can remove it. This is occurring since the uptake processes are much slower than our current burning of fossil fuels. For example, about 45% of the CO 2 released from fossil fuel combustion during the 1990s has remained in the atmosphere, while the remainder has been taken up by the oceans, vegetation, and soils. CO 2 is also nonreactive in the atmosphere and has a relatively long residence time, although eventually most of it will return to the ocean and land sinks. To give you an idea of the persistence in the atmosphere, consider the following. About 50% of a single release of CO 2 will be removed within 30 years, a further 30% will be removed in a few centuries, and the remaining 20% may persist in the atmosphere for thousands of years [15].
The oceans accumulate more carbon than they emit to the atmosphere each year, acting as a net sink of about 1.7 GtC per year [16]. The oceans also have a huge capacity to store carbon compared to the land surface. Ultimately, the oceans could store more than 90% of all the carbon released to the atmosphere by anthropogenic sources, but the process takes thousands of years. Concern right now is on how the CO 2 is accumulating in the oceans and what impact that will have on ocean chemistry and marine life [17].
Carbon dioxide enters the oceans by dissolving into seawater at the ocean surface, at a rate controlled by the difference in CO 2 concentration between the atmosphere and the sea surface. Just like everything in nature, things move from a high concentration to a low concentration. The problem comes when the relatively small volume of surface waters become concentrated with CO 2. Mixing between surface waters and the deeper portions of the ocean is very slow, with some mixing taking hundreds of years [14].
As CO 2 is added to the surface of the oceans from the atmosphere, it increases the acidity of the sea surface waters, and impacts organisms. Corals and calcifying phytoplankton and zooplankton, are susceptible to increased acidity as their ability to make shells is inhibited or possibly reversed, dissolving their shells. Sea surface pH has dropped 0.1 pH units since the beginning of the Industrial Revolution [18].
Phytoplanktons are the most important biomass producers in the ocean. They are also a driving force for removing carbon dioxide from the atmosphere and transferring the carbon to other trophic levels. Declining populations will set off a domino effect with fewer krill and young fish leading to fewer numbers of seabirds and even death for seals and sea lions. Lowering of the phytoplankton populations will reduce the carbon dioxide absorbing capacity of the oceans.
Unfortunately, we cannot significantly increase the capacity of the oceans to sequester carbon. Proposed techniques for increasing ocean sequestration of carbon are in the experimental stages and have unknown long-term environmental consequences. Deep ocean injections of CO 2 are proposed, but this would move the problem from the shallow surface waters to the deeper waters where organisms are even more impacted by changes in their environment. Iron fertilization is another proposed idea in which iron added to surface waters would stimulate the growth of phytoplankton [19]. When these organisms and the organisms that eat them reach the end of their lifecycles and die, their bodies and the carbon inside them will leave the surface waters and fall to the deeper waters allowing more capacity for carbon sequestration to the surface layer.
Most models of the carbon cycle indicate that the land surface, vegetation and soils accumulate more carbon per year than they emit to the atmosphere, acting as a sink for CO 2. Land use is the largest uncertainty of any component in the overall carbon cycle. Even though deforestation releases more carbon than is captured by regrowth in some regions, net regrowth in other regions uptakes sufficient carbon so the land surface acts as a global net sink of approximately 1GtC per year. Approximately 50% of the terrestrial carbon sink stems (no pun intended!) from regrowth of forests on abandoned farmland. Woody encroachment, the increase of woody biomass occurring on grazing lands, is thought to be another large sink, possibly 20% [15].
The role of forests as potential carbon sinks is a priority for scientific research at present. The thought is that land use change in eastern deciduous forests has had a significant impact on the carbon cycle. They may have played a major role in the past carbon dioxide intake due to the regrowth following the major cutting at the turn of the previous century [20]. We will explore this concept on the local level through this unit.
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