Research on the current and future impacts of human-induced climate change on reef-building corals is causing scientists and managers to become increasingly concerned about the future of coral reefs. A healthy reef ecosystem literally buzzes with sounds, activity and colors and is populated by incredibly dense aggregations of fish and invertebrates. In this respect, tropical reefs are more reminiscent of the African Serengeti than of the tropical rainforest they are often compared to, where the resident birds and mammals can be secretive and difficult to see. A coral reef can contain tens of thousands of species and some of the world’s most dense and diverse communities of vertebrate animals. Unfortunately, very few remaining coral reefs resemble this pristine condition; on most, corals and fishes are much less abundant than they were only a few decades ago. [More]
Climate Change and Coral Loss
There are many causes of local and global coral loss but human-induced climate change is one of the main and undeniable threats. Climate change is having negative effects on coral populations via at least three mechanisms.
Ocean warming can also indirectly kill corals by magnifying the effects of infectious diseases, which are one of the primary causes of coral loss, particularly in the Caribbean. The number, prevalence, and impacts of diseases of corals and many other types of marine animals have been increasing over the last 20-30 years. The severity of marine diseases could increase with temperature for several reasons. Because elevated water temperature causes corals physiological stress, it can also compromise their immune system, potentially making them more susceptible to infections. Additionally, increased temperature could also benefit bacterial and fungal pathogens, making them more fit and/or virulent.
A recent study found that anomalously high ocean temperatures greatly increased the severity of the coral disease white syndrome on the Great Barrier Reef. Disease outbreaks only occurred on reefs with high coral cover after especially warm years. The disease was largely absent on cooler reefs. The temperature increases required to trigger a white syndrome outbreaks were relatively modest as most disease outbreaks occurred on reefs where the temperature was only 1-2 °C warmer than usual. Other evidence also points to temperature as an important driver of coral epizootics. For example, some coral diseases such as black band disease become more prevalent or spread faster in the summer. However, not all coral epizootics are caused by anomalously high temperature. Some major outbreaks have occurred during relatively cool periods or years, such as white band disease, which decimated the then-dominant branching corals Acropora palmata and Acropora cervicornis in the Caribbean in the 1980s. Both species were recently listed as vulnerable under the U.S. Endangered Species Act.
The third and in many respects the greatest concern in the longer term, is that global change is causing the world’s oceans to become more acidic. By burning immense amounts of fossil fuels, humans, particularly North Americans, are rapidly increasing the concentration of carbon dioxide (CO2) in the atmosphere (by roughly 30% to date). A quarter of the CO2 produced by the burning of fossil fuels enters the ocean and reacts with water to form carbonic acid, acidifying the ocean. We have already lowered the pH of the ocean by about 0.1 unit which makes it more difficult and energetically costly for corals to secrete their calcium carbonate skeleton. Several experiments have demonstrated than even modest decreases in pH can slow coral growth, which will cause and compound a number of other problems. For example, it will reduce the ability of corals to compete with other species like sponges and seaweeds and to keep up with higher rates of sea level rise (due mainly to the thermal expansion of the ocean, but also to the melting of polar glaciers and ice caps). Coral populations might also recover more slowly from other climate change-related stressors like bleaching and infectious disease or from natural disturbances and mortality agents like storms or predation.[More]