Natural cycles

Lerman et al. (1975) considered several cases in which mankind s activities perturbed the natural cycle. If we assume that all mined P is supplied to the land as fertilizer and that all of this P is incorporated into land biota, the mass of the land biota will increase by 20%. This amount is small relative to the P stored in the land reservoir. Since P incorporated into land biota must first decompose and be returned to the land reservoir before being transported further, there is essentially no change in the other reservoirs. Thus, although such inputs would significantly alter the freshwater-terrestrial ecosystem locally where the P release is concentrated, the global cycle would be essentially unaffected. 

Sulfur in the Earth s Crust, Its Origin and Natural Cycle (K. H. Wedepohl)... 

The physical basis for the Earth s productive natural cycles must not be systematically deteriorated. 

Other than longer-term supply issues the main driver for moving away from fossil resources is pollution. Since pre-industrial times the level of atmospheric CO2 has risen from 280 ppm to 360 ppm, and whilst some observers believe this may be a natural cycle in the Earth s history, most believe it is a direct consequence of burning fossil fuels. This additional CO2 is now thought to be the main cause of global warming via the greenhouse effect (see Box 6.1). 

The natural cycle of carbon involves compounds of the atmosphere, hydrosphere, lithosphere, and biosphere. A certain difference in the 13C isotope content exists between the samples, depending on their origin. To estimate the deviation from the average value of 13C isotope contents 8(%o) scale is used. The deviation may be calculated by Equation 5.14 ... 

The use of wood in long-life products, such as buildings, ensures that this sequestered carbon is held in a materials pool for a longer time. If the use of renewables is encouraged, then more carbon is stored in this manner. Eventually, of course, such systems will establish equilibrium with the environment, as the materials flow into the pool equals the materials flow out into the environment. The use of wood in this way intervenes in a natural cycle, so that wood use and ultimate disposal replaces the natural cycle of wood decay in the forest (Figure 1.5). 

Timber can be viewed as a classic renewable material. Trees absorb carbon dioxide and utilize water and sunlight to produce a material that can be used in construction, to produce paper or to provide chemical feedstocks, with the production of oxygen as a byproduct. Furthermore, at the end of a product life cycle, the material constituents can be combusted, or composted to return the chemical constituents to the grand cycles . In essence, timber use represents a classic example of a cyclic materials flow, mimicking the flows of materials through natural cycles. Provided that we manage our forests well and do not harvest beyond the capacity of the planet to provide timber, we have at our disposal an inexhaustible resource available in perpetuity. 

Griffith, E.J. Modem mankind s influence on the natural cycles of phosphorus. In Phosphorus and the Environment Ciha Poundation, New Series, 57, 1978. 

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

Our consumption of food and the burning of biomass (such as wood) are part of the natural cycles of the earth (and because they are consumed food and fuel do not need to be durable). So too, in a more extended way, is our use of natural materials to build the world. Materials such as cotton and wool, wood and metal are liable to decay or corrode, even if they are part of artefacts. When we stop actively protecting these materials by keeping them dry or cleaning, natural processes take hold and the materials are returned to natural cycles. 

To work compatibly with natural cycles and living systems through the soil, plants and animals in the entire production system. 

If the quotient o>/a>0 is irrational, the path across the toroidal surface will return to a different point on the completion of each cycle. Eventually the trajectory will pass over every point on the surface of the torus without ever forming a closed loop. This is quasi-periodicity , and an example is shown in Fig. 13.11. The corresponding concentration histories do not necessarily give complex waveforms, as can be seen from the figure. However, the period of the oscillations is neither simply that of the natural cycle nor just that of the forcing term, but involves both. 

The role of methylation and demethylation in the natural cycle of arsenic has been discussed (272, 273), as well as environmental transformations of various alkylmetals (274). 

The implementation of an artificial photosynthetic cycle which could recycle C02 [4, 5], thus complementing the natural cycle, could also make an effective contribution to reducing C02 emissions. 

The emission of C02 from anthropogenic activities (the combustion of C-based fossil fuels, deforestation, combustion of woods) amounts to approximately 7.5 Gtc per year, or about 3.5% of the total amount cycled in the natural cycle. However, as the natural systems are unable to use such C02, this leads to its accumulation into the atmosphere. The assumption that an increase of the concentration of C02 in the atmosphere would have boosted both the photosynthesis and the dissolution into the oceans has not been proven to be true. In fact, the solubility of C02 is governed by complex equilibria, while photosynthetic fixation is limited by several factors so that, under the increase of the atmospheric concentration from 280 ppm of the preindustrial era to the present-day 380 ppm, there has not been any sensible improvement of the uptake. Therefore, under natural conditions the uptake of C02 has reached an equilibrium state, and the further increase in atmospheric concentrations may more likely cause climate changes through the greenhouse effect and destabilization of the thermal structure of the atmosphere, than improve the elimination of C02 from the atmosphere. 

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