Coral reefs are among the most diverse ecosystems on the planet, with species estimates ranging into the millions and new creatures discovered each year . Located in shallow, tropical marine waters, coral reefs are composed of calcium carbonate structures that form a symbiotic relationship with photosynthetic algae, allowing both the coral and the algae to share the necessary elements for life: food and shelter.The abrupt climate change the earth is now experiencing is having major impacts on this complex ecosystem worldwide.
Coral reefs are slow growing and need very specific conditions to survive, making them extremely sensitive to changes to environmental factors such as temperature and pH. Currently, 30% of the world's corals are in a degraded state, and an estimated 60% will be damaged by the year 2030 as a result of environmental changes and other distrubances. Predicted temperature and carbon dioxide levels expected in the next century due to climate change have not occurred since modern day reef systems evolved thousands of years ago,  and may threaten or even destroy much of the world's coral reef beds.
Impacts on Reefs
While daily temperature fluctuations put stress on corals, extreme temperature events can be devastating to an entire reef system. During El Nino Southern Oscillation events (ENSO), sea surface temperatures are higher than normal. Warmer water temperatures during a strong ENSO event in 1997-1998 caused coral bleaching in every ocean, and resulted in a global extinction of 16% of the world's known corals .
Coral bleaching occurs at high temperatures when the algae within the coral tissues die. Without the algae, which provide food for the coral, growth and reproduction cease. If the stress is for a short period, corals can recover. If, however, the stress is long-term, the corals tend to die . In the Indian Ocean, during the 1998 warming period, 80% of the corals bleached, with 20% of the corals dying . Rising ocean surface temperatures in the next century will lead to more bleaching events and the further coral reef degradation.
Rising carbon dioxide (CO2)levels in the atmosphere will also cause problems for coral reefs. Over the next century, a large amount of CO2 will be absorbed into the oceans, due to increased levels in the atmosphere, will result in lower pH, and the water becoming slightly more acidic. This will reduce the carbonate concentration in the water, a substance needed by corals to form their hard skeletons . Over the long-term, the decrease in pH levels will lead to a decline in reef abundance, with some reefs perhaps being dissolved in the more acidic oceans of the future .
Coral Reef Dependants
Disappearing coral reefs also threaten other species that depend on them. The Great Barrier Reef (GBR) is the largest coral reef on the planet, and home to numerous fish species found no where else in the world. In 1998, a mass coral bleaching event occurred, resulting in a change in fish communities in several parts of the GBR. While some fish species survived, others were scarcely found after the warming occurred leading to lower biodiversity .
People in coastal areas are also dependant on coral reefs. Along the coast of eastern Africa , the fishing industry employs over 100,000 fishermen. With reefs currently in decline, more than half of the workers may lose their livelihood . In Papua New Guinea, a reduction in reefs caused 75% of the local fish populations to decrease, many of them to half of their original levels . For coastal towns and villages around the globe, declining reef beds will lead to less fish to feed families, and increased pressure on fish populations that are already strained.
Coral reefs are also a major tourist attraction. Around the world, tourist industries have developed around visitors coming to observe and explore coral reefs. This ecotourism industry may be lost with the slow decline and possible death of coral reefs. For places like The Maldives in the Indian Ocean, where 45% of the country's economy is based on tourism, such a loss would be devastating .
Far reaching implications
With both higher temperatures and increased acidity affecting coral reef structures, climate change is profoundly impacting this ecosystem. Reefs are not just important to biodiversity - they are also a major component of general coastal health. The destruction of reefs leaves beaches, estuaries, lagoons, and shorelines more exposed . Coastal areas are already becoming more vulnerable to erosion and waves, and with compromised coral reefs the impacts of erosion and storms on coastlines will increase, threatening not only marine systems but terrestrial ones, too.
What we can do
Coral reefs around the world are subjected to many disturbances. Over fishing, coastal development, tropical storms, and introduced disease are all taking their toll on reef communities . Add climate change to these problems and the future looks bleak, but there is hope.
In many countries, Marine Protected Areas (MPAs) have successfully been implemented to protect threatened reefs. In addition to allowing reefs a chance to grow and recover, MPAs protect fish and other organisms dependent on coral reefs for food and shelter . While climate change doesn't recognize the boundaries MPAs provide, protecting these ecosystems may help them recover from other stresses more successfully while we collectively work towards reducing our greenhouse gas emissions. With millions of people dependant on healthy coral reef ecosystems for food and livelihood, we all have a responsibility to maintain these vitally important ecosystems .
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