Climate Change Effect On Coral Reefs

Coral reefs and climate change
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. 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.

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 rather than the natural climate change. Climate change is having negative effects on coral populations via at least three mechanisms.

First, ocean warming is directly reducing coral cover through coral bleaching. Reef-building corals contain plant-like organisms called zooxanthellae that live symbiotically within their tissue. Zooxanthellae provide their coral host with food and oxygen and in return, the zooxanthellae receive nutrients, carbon dioxide, and an enemy-free shelter. This symbiotic relationship evolved tens of millions of years ago and has been critical to the success and evolutionary radiation of corals and to the development of reef ecosystems.

When summertime water temperatures are just a degree or two warmer than usual for a few weeks, this critical yet delicate symbiotic relationship breaks down and the zooxanthellae are expelled, often leading to the coral’s death. (The greater the magnitude or duration of the warming, the greater the mortality and effect on coral populations.)
The phenomenon is called “coral bleaching” because the coral animal appears to turn white after the zooxanthellae loss. This is because without their zooxanthellae symbionts, which contain various photosynthetic pigments, corals are nearly transparent and the white, external calcium carbonate skeleton that the coral polyps live on becomes plainly visible.

Carbon dioxide, methane, and other greenhouse gases trap heat, leading to global warming. For example, in Palau, more than 90% of the corals on some reefs bleached and at least 50% perished. Even some isolated reefs were impacted. In the Maldives, in the east Indian Ocean, bleaching caused coral cover to plummet to only about 5%.

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.

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.

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.

This post is a summary of the article written at Encyclopedia of Earth website. Read the complete article at http://www.eoearth.org/article/Coral_reefs_and_climate_change webpage.