Environmental Problems, Their Causes, and Sustainability 1

Figure 4-29 Naturalcapital: simplifiedmodel of the globalcarbon cycle that helpskeep you alive. The leftportion shows themovement of carbonthrough marine systems,and the right portionshows its movementthrough terrestrialecosystems. Carbonreservoirs are shown asboxes; processes thatchange one form ofcarbon to another areshown in unboxed print.(From Cecie Starr,Biology: Conceptsand Applications,4th ed., Brooks/Cole[Wadsworth] © 2000)Diffusion betweenatmosphere and oceanCarbon dioxidedissolved inocean waterMarine food websProducers, consumers,decomposers, detritivoresCombustion of fossil fuelsMarine sediments, includingformations with fossil fuelsAerobic respiration in the cells of oxygen-using producers,consumers, and decomposers breaks down glucoseand other complex organic compounds and convertsthe carbon back to CO 2 , which is released into the troposphereand in water for reuse by producers.Carbon dioxide is removed from the troposphereby terrestrial and aquatic producers, which use photosynthesisto convert it into complex carbohydrates suchas glucose (C 6 H 12 O 6 ).This linkage between photosynthesis in producersand aerobic respiration in producers, consumers, anddecomposers circulates carbon in the biosphere and isa major part of the global carbon cycle. Oxygen andhydrogen, the other elements in carbohydrates, cyclealmost in step with carbon.Carbon dioxide is a key component of nature’sthermostat. If the carbon cycle removes too much CO 2from the atmosphere, the atmosphere will cool; if thecycle generates too much, the atmosphere will getwarmer. Thus even slight changes in the carbon cyclecan affect climate and ultimately the types of life thatcan exist on various parts of the planet.Some carbon atoms take a long time to recycle.Over millions of years, buried deposits of dead plantmatter and bacteria have been compressed betweenlayers of sediment, where they form carbon-containingfossil fuels such as coal and oil (Figure 4-29). This carbonis not released to the atmosphere as CO 2 for recyclinguntil these fuels are extracted and burned, oruntil long-term geological processes expose these depositsto air. In only a few hundred years, we have extractedand burned huge quantities of fossil fuels thattook millions of years to form. This is why fossil fuelsare nonrenewable resources on a human time scale.Oceans play important roles in the carbon cycle.Some of the atmosphere’s carbon dioxide dissolves inocean water, and the ocean’s photosynthesizing producersremove some. On the other hand, as ocean waterwarms, some of its dissolved CO 2 returns to theatmosphere, just as carbon dioxide fizzes out of a carbonatedbeverage when it warms. The balance betweenthese two processes plays a role in the earth’saverage temperature.Some ocean organisms build their shells and skeletonsby using dissolved CO 2 molecules in seawater toform carbonate compounds such as calcium carbonate(CaCO 3 ). When these organisms die, tiny particles oftheir shells and bone drift slowly to the ocean depths.There they are buried for eons (as long as 400 millionyears) in deep bottom sediments (Figure 4-29, left),where under immense pressure they are convertedinto limestone rock. Geological processes may eventu-78 CHAPTER 4 Ecosystems: What Are They and How Do They Work?