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All of the important macromolecules that make up living organisms contain carbon, and much of the energy that organisms use to fuel their metabolic activities is stored in carbon-containing (organic) compounds. On land, biological processes move carbon directly between organisms and the atmosphere as terrestrial organisms take up carbon during photosynthesis and return it to the atmosphere through respiration and metabolism (Figure 57.13). In contrast, carbon dioxide moves into ocean waters from the atmosphere primarily by simple diffusion at the ocean surface; this dissolved CO₂ is the source of the carbon used by marine primary producers. Even taken together, however, the amounts of carbon in the atmosphere, in soils, and in living and dead organisms are dwarfed by the vast quantities of carbon stored in terrestrial rocks, in fossil fuels, in marine sediments, and in seawater in the form of carbonate ions (CO₃^–2) or bicarbonate ions (HCO₃^–) (see Figure 57.13).

Over the span of life’s existence on Earth, quantities of carbon were removed from active cycling when organisms died in large numbers and were buried in sediments lacking oxygen. In such anaerobic environments, with few detritivores to reduce organic carbon to CO₂, organic molecules accumulate and are eventually transformed into deposits of oil, natural gas, coal, or peat—the fossil fuels that modern humans use as a combustible source of energy. Humans have discovered and used these fossil fuels at ever-increasing rates during the past 150 years. As a result, CO₂, one of the final products of burning fossil fuels, is being released into the atmosphere faster than it is absorbed in the oceans or incorporated into terrestrial biomass. Measurements of CO₂ on the top of Mauna Loa, Hawaii, show that, even though atmospheric CO₂ varies seasonally because of the change in NPP, it has steadily increased over the last 55 years of data collection (Figure 57.14).

ATMOSPHERIC CO₂ AND GLOBAL CLIMATE CHANGE Carbon dioxide is a greenhouse gas, so we would expect increasing atmospheric CO₂ concentrations to trap more heat in Earth’s atmosphere and raise temperatures at Earth’s surface (see Figure 53.2). What evidence do we have that this is occurring? Measurements of gases in air trapped in the Antarctic and Greenland ice caps show that temperatures have been warmer when atmospheric CO₂ concentrations have been higher and cooler when they have been lower (Figure 57.15). For example, the atmospheric CO₂ concentration was very low at the end of the last glaciation (180 parts per million [ppm]), 18,000 years ago, when temperatures were much colder than they are today. In contrast, during a warm interval between 11,000 years ago and the start of the Industrial Revolution (1750), the concentration of CO₂ in Earth’s atmosphere was between 260 and 280 ppm. Today atmospheric CO₂ concentration is just over 400 ppm (see Figure 57.14), the highest recorded in the last 800,000 years. The increase in CO₂ (and two other greenhouse gases, CH₄ and N₂O) from the burning of fossil fuels has resulted in a roughly 1°C (1.8°F) rise in global temperatures compared with those from 1981 to 2010 (a comparison known as a “temperature anomaly”).

How global climates and ecosystems will change in response to this rapid warming is a subject of intense investigation. Global warming has already resulted in the shrinking of Arctic sea ice, which is currently the lowest ever recorded. As temperatures continue to rise and glacial ice melts, sea level rises (because of both thermal expansion of ocean waters and the addition of glacial meltwater), increasing flooding of coastal cities and agricultural lands, especially during extreme storm events. Nearly one-third of the world’s population lives in coastal regions, which make up only 4 percent of Earth’s total land area.

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Even though atmospheric CO₂ is increasing as the result of burning fossil fuels, less than half of the CO₂ released into the atmosphere by human activities remains in the atmosphere. Where does the rest of the CO₂ wind up? Much of it is absorbed by the oceans in inorganic forms. Over decades to centuries, the oceans exert a large influence over atmospheric CO₂ concentrations. Of the CO₂ absorbed by the ocean, some is used in photosynthesis by phytoplankton in the surface waters. These organisms remove dissolved CO₂ from water, thereby increasing the rate at which atmospheric CO₂ is absorbed by surface waters, but as you saw earlier, there are limits to this absorption given nutrient limitation in vast parts of the open ocean. In addition, many marine organisms (including clams, oysters, corals, and planktonic foraminiferans) incorporate carbon in their shells and other structures in the form of calcium carbonate (CaCO₃), which is synthesized by combining bicarbonate ions (HCO₃^–) and calcium ions (Ca²⁺) dissolved in seawater. When these organisms die, those shells and their embedded carbon sink to the ocean floor.

Today’s oceans absorb millions of tons of CO₂ from the atmosphere each day—more than at any time during the past 20 million years. As a result, water near the ocean surface is becoming more acidic. As CO₂ concentrations in the atmosphere rise, more of the gas diffuses into the water at the ocean surface, where it reacts with water to form carbonic acid (H₂CO₃). As levels of carbonic acid rise, the pH of seawater drops. This increase in acidity can have negative effects on many marine organisms, particularly corals. The combination of decreasing pH and increasing water temperature to which corals are being exposed results in *coral bleaching and sometimes the death of corals. Because so many other reef species depend on corals and the structure they provide, an entire reef community can collapse if its corals fail to thrive.

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But as you read in the opening story, the effects of CO₂ and ocean acidification vary depending on the organisms or ecosystems considered. Investigating Life: Food Webs in an Acidic and Warming Ocean describes research in Swedish by marine ecologists interested in the effects of ocean acidification and warming on an ecologically critical but predominately primary-producer-dominated ecosystem: estuaries. In this system, the researchers focused on benthic microalgae, an important food source for a diversity of sediment-dwelling herbivores, including crustaceans, snails, and worms. Benthic microalgae indirectly benefit from higher trophic levels, particularly omnivores that feed on macroalgae competitors and herbivores of the microalgae. Using experiments the researchers discovered that the biomass of benthic microalgae did not appreciably change under the elevated CO₂ and warming manipulations. As it turned out, the consumers mediated the effects of ocean acidification and warming on benthic microalgae, suggesting that consumers, or other strongly interacting species in food webs, have the potential to modulate the effects of climate change in complicated and unpredictable ways.

In terrestrial ecosystems, photosynthesis, principally in tropical forests, typically absorbs about the same amount of carbon that is released by terrestrial plants, microbes, and animals through metabolism. In more recent times, as atmospheric CO₂ increases, the photosynthetic consumption of CO₂ exceeds its metabolic production, which means Earth’s terrestrial vegetation is storing carbon that would otherwise be increasing atmospheric CO₂ concentrations—but as we saw in the FACE experiment, we cannot count on terrestrial vegetation to store the vast amounts of excess CO₂ that human activities produce. Furthermore, climate warming (another result of increasing atmospheric CO₂ concentrations, as you have already seen) increases plant metabolism and is thus likely to increase the flux of CO₂ from vegetation into the atmosphere.