Ocean Acidification: Consequences on Flora and Fauna

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Chelsea Zeller Dr. Andre Droxler ESCI 107 10 December 2012 Ocean Acidification: Consequences on Flora and Fauna The prosperity and health of our Earth is held within a delicate balance, a balance easily disrupted by any interference from natural and unnatural forces alike. The environment is currently in a state of gravely precarious instability pushed to this edge by careless human activity. The impacts within the atmosphere are evident – temperature changes, increased storm activity, and melting ice – but less obvious is how the upset balance of our Earth’s systems affects our oceans. While the oceans certainly play into the atmospheric changes we see, they are also experiencing serious consequences distinct from those on the surface. As CO2 concentrations in the atmosphere continue to increase, seawater chemistry is being seriously and detrimentally altered. In a process called ocean acidification, chemical reactions between absorbed carbon dioxide and H2O occur that reduce the pH of seawater, as well as decrease carbonate ion concentration and saturation states of calcium carbonate minerals (“Ocean Acidification,” PMEL). This calcium carbonate is necessary for the formation of skeletons and shells of many marine organisms. Therefore, acidification is damaging, and likely killing, large amounts of sea life (Horsey, 2012). The consequences of destroying these organisms that serve as a base for marine ecosystems will have expansive impacts that go far beyond harming a few species. To understand how CO2 affects the chemical balance in the ocean, it’s necessary to first note the historical changes in CO2, and the consequent decrease in seawater pH. While carbon dioxide is naturally present in the atmosphere as part of the Earth’s carbon cycle (the circulation of carbon among the atmosphere, oceans, soil, plants and animals), the current levels and the

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sustained increase in concentration far surpass their expected natural fluctuation (EPA, 2012). Since the Industrial Revolution, the burning of fossil fuels has substantially increased amounts of carbon dioxide in the atmosphere, as shown in Figure 1. In the past, concentrations fluctuated from a low of 180 ppmv and a high of 300 ppmv. However, current levels are approaching the 400-ppmv mark, with a rapidly accelerating rate of increase.

Figure 1: The regular variations in concentration of carbon dioxide over the past 400 thousand years are shown in blue. Fluctuations are mainly due to past glacial cycles. The record of the extreme increase since the Industrial Revolution was found from accumulating data from four different ice cores with varying spacing of samples (Rohde, 2012).

These rising levels of anthropogenic CO2 have a direct impact on seawater pH. As part of the carbon cycle, the ocean naturally absorbs carbon dioxide from the atmosphere and buffers the CO2 changes that occur there (“Ocean Carbon Uptake”). Over the past 200 years oceans have been compensating for the increases in CO2 and have absorbed 525 billion tons of carbon dioxide from the atmosphere, accounting for almost a half of all CO2 released from human activity

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(Feely, Sabine and Fabry, 2006). With increasing concentrations in the atmosphere, the current rate of absorption is almost 10 times the natural historic rate. While oceans have helped significantly slow global warming, the greater amount of CO2 is altering the chemical balance of seawater, causing pH to decrease. A decrease in pH means an increase in the acidity of water. Surface ocean pH is already 0.1 unit lower than preindustrial levels and is expected to become another 0.3-0.4 units lower by the end of the century (Orr et al., 2005). While 0.1 unit may seem like a small amount, it is actually a 30% decrease just since the 19th century. The relationship between CO2 and sea surface pH is shown in Figure 2. The past change is already very significant, and according to Ken Caldeira from Carnegie...
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