In recent years, scientists have noticed signs of damage in coral reefs. These signs are very widespread-in fact they can be seen almost everywhere in the oceans around planet Earth. Some reefs appear damaged, while others are "bleached"- turned white. This disturbing realization has prompted many questions. Is there a single cause for this problem, or is the damage to coral more like "the death of a thousand cuts"- due to a number of causes? Do coral reefs have a future? Could cooling down the temperature of the world ocean buy more time for coral reefs?
These questions are challenging for scientists. In order to attempt to answer them, it is critical to gain a basic understanding of the coral polyp as a organism. Coral is a rather weird, soft little creature that builds a hard castle for itself out of limestone, a chemical that it absorbs from sea water. But the polyp is also dependent on photosynthesis for food and energy. It cannot perform this function itself, but depends rather on algae for it, algae that live with the coral. These biological realities dictate that coral can only exist in shallow areas of the ocean, areas in which sunlight can penetrate to the coral to permit photosynthesis. Coral must also have water temperatures that are not too high, and a reliable supply of calcium and carbon in the sea water.
Unfortunately, there are now numerous ways in which humans interfere with the coral life cycle. One problem is the flow of sediment and nitrogen from coastal human communities into the ocean. Sediment is a product of human development and land clearing; when it flows into the ocean from rivers it tends to bury and choke local coral reefs.
Nitrogen is a chemical waste product of human existence. Nitrogen flows into the ocean from human sewage waste, the manure of livestock or the fertilizer runoff from agriculture. This chemical then causes large algae "blooms" in the coastal ocean areas, killing coral by removing oxygen from the waters.
Global warming also damages coral in two distinct ways. Global warming has caused ocean temperatures to rise. The coral organism can only survive in a narrow temperature range, so this is a problem. But global warming (the process of human fossil fuel burning) also is raising the amount of carbon dioxide in the atmosphere. This carbon dioxide is gradually being absorbed into the ocean, a process that is turning the ocean acid.
Over the past 200 years, scientists estimate that human burning of fossil fuels has added the amazing amount of 550 billion tons of carbon dioxide to the world ocean! This process threatens coral survival, because there is a certain degree of acidity beyond which the coral organism can no longer absorb any calcium carbonate from sea water with which to build its own reef! In addition to these serious problems, human activities are damaging local reefs through ship anchor dragging, dynamite fishing and poison fishing. Those last categories of damage, however, seem to be more localized than global.
Why is coral damage a problem? The sharp worldwide decline in coral threatens human populations, because fish from coral reefs are a major food source for coastal communities in tropical areas worldwide. There are over a billion humans who consume fish from coral reef habitats. From a science perspective, coral reef decline also promises a wave of ocean extinctions of many other species of sea life, because countless species in the ocean depend on coral for survival.
In general terms, the coral organism is being threatened by the waste products of the human race. Sediment runoff, nitrogen runoff, acidification from atmospheric carbon, heat gain from atmospheric carbon, can all be described as waste products from human activity. These waste products are all unintentional-nobody is making a conscious effort to destroy coral reefs, but it is happening nonetheless. Scientists and economists looking at this situation view the twin consequences of global warming-heat gain and acidification-as the big long term dangers. Unfortunately, either consequence has the potential to eventually kill off most of the world's coral.
Would slowing down the heat gain give more time to save the coral? Not really, because acidification is equally deadly to coral. Not to mention, the only realistic way to slow the ocean heat rise is to reduce and halt fossil fuel burning-and since the human race shows no inclination to do that, it is apparent that the oceans will continue to warm for decades to come.
At this point it is becoming clear to those who study coral reefs that the situation is grave. A 2012 report revealed that the world's largest coral reef, the Great Barrier Reef of Australia, has lost half its coral cover since 1985. The trend continues. It now appears that coral may approach extinction worldwide in the 21st century. This has the look of inevitability, because of the fifty year lag between reducing carbon dioxide emissions (which has yet to occur) and any reduction in the rate of ocean warming and carbon dioxide absorption.