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Local Triggers
Coral bleaching may be caused by a number of factors, including local and global triggers. Although overshadowed by large scale coral bleaching events, local triggers remain huge threats to coral reef health. Pollution, one such trigger, poses one of the biggest threats to coral reef health. Anthropogenic activities, such as coastal development, dredging, agricultural and deforestation activities, and sewage plant treatment operations, can lead to land-based runoff and pollutant charges. Both runoff and pollutant changes possess serious consequences, chiefly the eutrophication of the local bodies of water. Another human caused trigger are oil spills. Oil spills destroy local biodiversity, and coral reef biodiversity is no exception. When oil spills occur, the oil can damage a reef's egg and sperm, which interrupts the reproductive success of the coral reefs.

Global Triggers
Despite the local contributors to coral bleaching, global problems pose an equal or even bigger threat to coral reef health. Under increased carbon dioxide concentration expected in the 21st century, corals are accordingly expected to become increasingly rare, due to the increase in sea level temperature. Anthropogenic processes that increase sea level temperature include deforestation and fossil fuel emissions, and natural processes include El Niño events.

Deforestation
Deforestation, as a process, is harmful for the global ecosystem. However, the biggest effects of deforestation are observed through greenhouse gas emissions. Trees, through photosynthesis, uptake carbon dioxide. Through this process, trees establish themselves as terrestrial carbon sinks and inhibit global warming. Without them, carbon emissions would run rampant, and temperatures would soar. When these trees are mass-harvested during deforesting events, a large amount of carbon sinks are removed from the ecosystem. Consequently, a greater strain is placed upon other carbon sinks, namely the ocean. An accumulation of carbon in the ocean results in warmer temperatures, leading to unfavorable conditions for coral reefs to recover from bleaching events.

Another effect of deforestation is soil erosion. When trees are cut down, the surface sediments are disrupted and run off into nearby streams and waterways. These waterways run off into the ocean, carrying the leftover soil with them. When the soil reaches the ocean, it blots out the sunlight that zooxanthellae in coral reefs are dependent on. Without the zooxanthellae, the reefs take on the pale coloring that is associated with coral bleaching.

Fossil Fuel Emissions/Ocean Acidification
Another trigger of coral reef bleaching is the use of fossil fuels by humans. Burning fossil fuels releases harmful substances into the atmosphere, namely carbon dioxide. As a result, a greater burden is placed upon the oceanic carbon sink to handle a greater amount of emissions. When the ocean absorbs more carbon dioxide emissions, a harmful process known as ocean acidification occurs. Ocean acidification leads to an increase of HCO3- ions and a decrease of CO3(2-) ions. These changes in the chemical makeup of the oceans leads to an decrease in the pH of the entire ocean itself, causing the water to become more acidic. As a result, it becomes more difficult for corals to formulate skeletons.

Besides ocean acidification, burning fossil fuels possesses even more detrimental effects to coral reef health. The zooxanthellae that coexist within coral reefs can only survive in a temperate environment, and they are consequently fragile. When fossil fuels are burned, they trap heat inside the atmosphere, causing global warming. When the ocean is warmed, coral reefs face catastrophic effects. A vast majority of coral reefs bleach at 1°C above the average temperature of the hottest month at that location. In addition, the same reefs will perish if exposed to 2 °C above the average temperature. In such a delicate environment, zooxanthellae have difficulty surviving, leading to increases in the amount of bleached reefs.

El Niño
While anthropogenic events play a large role in the degradation of coral reef health, naturally occurring events like El Niño also are harmful to coral reefs. El Niño is a naturally occurring phenomenon that warms the seas. It occurs every 3-7 years. Similarly to fossil fuel emissions, El Niño warms the ocean waters. This results in unfavorable conditions for zooxanthellae, which results in the bleaching of the affected coral reefs.