BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.4. Biogeochemical cycles: water and nutrient cycles

10.4.2. The carbon cycle

The primary reservoir of carbon is in rocks, which are estimated to contain roughly 75 quadrillion tonnes. Another 5 trillion tonnes is locked up in fossil fuels—coal, oil, natural gas, and peat. Approximately 150 billion tonnes is found in the upper layer of ocean floor sediments. Under normal conditions, these reserves are unavailable to living organisms. Far more important to them is the active carbon pool, illustrated in Fig. 10.12.

Class="center">

Fig. 10.12. Global carbon reserves and current carbon cycle. Numbers inside the boxes represent billion tonnes in storage, and numbers next to the arrows represent billion tonnes per year for fluxes. (Climate change 1994, Radiative forcing of climate change (1995) IPCC/CUP.)

The main source of carbon for living organisms is carbon dioxide, which is present in the atmosphere and dissolved in surface waters. Through Photosynthesis, green plants, Algae, and cyanobacteria convert this inorganic compound into CARBOHYDRATES, which then form the carbon Skeleton of all other organic molecules. The photosynthetic assimilation of Carbon dioxide is counterbalanced by its release through Respiration, helping to maintain natural equilibrium. However, not all fixed carbon dioxide returns to the atmosphere via respiration. In anaerobic environments, such as marshes or the dimly lit bottoms of standing Water bodies, the Mineralization of organic matter proceeds very slowly, leading to its accumulation as silt or peat. Under certain conditions and over long periods of time, these deposits can form fossil fuel reserves.

In the oceans, the main mechanisms for absorbing atmospheric carbon dioxide are photosynthesis, carried out primarily by phytoplankton, and dissolution in surface waters. A significant portion of this bound carbon dioxide is rapidly returned—either directly from solution or through respiration. However, much like in terrestrial ecosystems, a fraction of the carbon is sequestered for long periods, such as when cold surface waters sink to the depths or when it is incorporated into the carbonate structures (shells, corals, etc.) built by marine organisms, which eventually transform into rocks such as limestone.

The rate of carbon transfer between its reservoir and active pools can fluctuate from year to year depending on climatic variations. This balance is also influenced by human activities, particularly land-use changes (such as deforestation or afforestation), the burning of fossil fuels, and cement production. Available data indicate that human activity has been the primary driver of the significant increase in atmospheric carbon dioxide since the Industrial Revolution (Section 10.8.1).

The accelerating mobilization of carbon from reserves such as fossil fuels and carbonates (via cement production), along with the potential impact of this acceleration on global climate and ecosystems, is a subject of intense ecological research and debate today (Section 10.8.1). The prevailing consensus is that maintaining current rates of carbon dioxide emissions poses severe risks to the entire planet. Governments are making efforts to reduce industrial carbon dioxide emissions and overall fossil fuel consumption by expanding The Use of alternative Energy Sources, such as solar and wind power.