Biosphere - Biosphere - The carbon cycle: Life is built on the conversion of carbon dioxide into the carbon-based organic compounds of living organisms. Dead plant material enters the soil in two ways -- it falls on the surface as litter, and it is contributed below the surface from roots. Carbon can be stored in a variety of reservoirs, including plants and animals, which is why they are considered carbon life forms. These plants utilize CO2 gas dissolved in seawater and turn it into organic matter, and just like land plants, these phytoplankton also respire, returning CO2 to the surface waters. Our goal is to find an expression for the concentration of H2CO3, so we begin by rearranging the equation on the left above (1) to the following form: Then we rearrange the right side of (1) so that it becomes: Next, we substitute (3) into (2), to give us our desired equation expressing H2CO3 in terms of HCO3- and CO32-. The most straight-forward way to represent the function of the marine biota in our model would be to represent the marine biota in the form of two reservoirs, one for each of the two surface water reservoirs. This flow is defined as a constant in the model, although in reality, it will vary according to the timing of large volcanic eruptions. When plants are young, and growing rapidly, but with not much biomass to maintain, this ratio is even higher; in older, larger plants, this ratio is lower since more carbon needs to go towards maintenance. In defining this flow in our model, we need to split it into the carbon added to the warm surface waters of the ocean and the carbon added to the cold surface waters. This is not because the volume of flow is different in these two processes, but rather because the concentration of carbon in the deep waters of the ocean is greater than that in the shallow surface waters, due in part to the operation of the biologic pump mentioned above. The photosynthesizing plankton require nutrients in addition to CO2 in order to thrive; specifically, they require nitrogen and phosphorus. The flow units are grams of carbon per year, which can then be converted to Gigatons of carbon per year, using the relationship that 1Gt = 1 x 1015g. Fossil Fuel Burning. The formulation here gives an increase in photosynthesis by a factor of 1.4 if the temperature increases 10°C. Adopting this pre-industrial case as our steady state has another advantage in that we know the history of CO2 emissions from human activities pretty well and we know the present state of the carbon cycle pretty well and we even know the rate of change of various parts of the carbon cycle. The difference goes into plant growth and operation. The other form of human alteration of the global carbon cycle is through forest cutting and burning and the disruption of soils. (relavant for timescales of a few hundred years) Processes of Flow in the Terrestrial Realm. Through respiration, plants (and animals) release water and carbon dioxide. It is important to realize that the total alkalinity is really determined by the other ions in solution -- the ones mentioned above. At the same time, many planktonic organisms extract dissolved carbonate ions from seawater and turn them into CaCO3 shells. The surface waters of the worlds oceans are home to a great number of organisms that have as the basis for their food web photosynthesizing phytoplankton. This coefficient, which we will consider to be a constant, is: How has this coefficient been determined? Field experimental photos … Temperature is another important consideration in many life processes, and photosynthesis is no exception. The Khs parameter, the so-called half-saturation value, is the atmospheric CO2 concentration that corresponds to a rate of photosynthesis (Fp) that is half the ultimate saturation value (see Figure 7.04). The magnitude of this flow is small -- about 0.6 Gt C/yr -- relative to the total amount transferred by sinking from the surface waters -- 10 Gt C/yr. This upwelling water also brings with it nutrients such as nitrogen and phosphorus, making these waters highly productive. For the majority of plants, this upper limit is not likely to come into play given the kinds of temperature changes we might expect in the space of a couple hundred years, so we can safely ignore it here (if our model does lead to temperature changes of greater than 10-20°C in a hundred years, we would presume that there is some problem with the model as this is unrealistic behavior). The water cycle. In our model, we will lump these processes together and call them litter fall, keeping in mind that this is only half of the real story (later, we can enhance our model by breaking the soil up into several different boxes, and these two flows will no longer be lumped together). Where the carbon is located — in the atmosphere or on Earth — is constantly in flux. The carbon cycle . It turns out that in terms of net carbon transport, the flow from the warm part of the ocean to the colder part is greater than the return, so we will model this process as a unidirectional flow from warm to cold. The photosynthetic uptake flow is really controlled by the availability of nutrients and the water temperature, but since we are not keeping track of nutrients or the temperature of the water, we will take a far simpler approach. The actual magnitude of this flow is a bit uncertain, although it does appear to be quite small. The formulation here gives an increase in soil respiration by a factor of 2.0 if the temperature increases 10°C. Humus is much less palatable compound, as far as microbes are concerned, and is not decomposed very quickly. This energy is then used to split a water molecule into hydrogen and oxygen; in the process, the plants gain chemical energy that is used in a companion process that converts carbon dioxide into carbohydrates represented by CH2O in the above equation. Upwelling occurs in areas of the oceans where winds and surface currents diverge, moving the surface waters away from a region; in response, deep waters rise up to fill the "void". This makes this flow a first-order kinetic process, like many of the other flows in this model. For instance, methane, the main component of natural gas, has a chemical formula of CH4; petroleum can be represented by the formula of CH2. This is sometimes called the Q10 for the process, and a Q10 of 1.4 is in line with a variety of observations from experiments. The answer is no -- otherwise, how could organisms grow? However, as a general rule, if plants do have sufficient water, they will increase their rate of photosynthesis if there is more CO2 in the air surrounding them -- an effect known as CO2-fertilization. In the oceans, warm surface waters move up to the polar regions and are added to the cold surface waters there, while some of the cold surface water returns back to the equatorial regions, making a large-scale cycle that is important for the transfer of heat energy, as we discussed in our earlier work with energy balance climate models. Thus, the processes of living organisms represent a kind of energy flow system; energy flows in and energy of another form flows out. This increases the complexity of the algebra, giving us a quadratic equation whose solution ends up as: Regardless of whether we consider the more digestible form (10) or the more precise form of expressing HCO3- and CO32- (11), we are finally set, because if we look at the equation for the partial pressure of CO2. Much of the organic material added to the litter (the accumulated material at the surface of the soil) or within the root zone each year is almost completely consumed by microbes; thus there is a reservoir of carbon with a very fast turnover time -- on the order of 1 to 3 years in many cases. In our model of the carbon cycle, we will use an expression for soil respiration that takes these observations into account: The temperature sensitivity part of the equation is a linear function like that used in defining photosynthesis. For our purposes, we will assume that the global collection of plants has a ratio of around 2 to 1. For instance, if the interior of a leaf was filled with lots of CO2, then even if the stomata were wide open, little new CO2 could enter the leaf -- it would be limited by how quickly the plant was consuming CO2 in photosynthesis. By controlling the concentration of CO2 gas dissolved in the surface waters, the planktonic organisms exert a strong influence on the concentration of CO2 in the atmosphere. Pmax is a parameter with units of GtC/yr that is used to force the equation for Fp to give the proper value corresponding to the starting conditions of the model. The magnitude of the flow is thus a function of the volume of water flowing and the average concentration of carbon in the cold surface waters, which is itself a function of the total amount of carbon stored in this reservoir, assuming that the size of the reservoir is not changing appreciably over the few hundred years that this model is intended to be used for. This has been done by a variety of people, and the result is an equation that looks like this: Where DT is the temperature difference, in °C, between the start and any time, t, during the model simulation; 280 ppm (parts per million) is the beginning concentration of CO2 in the atmosphere, representing the pre-industrial world.
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