Climate change has affected, and is still affecting coral reefs not only in the Philippines but also those in the other parts of the global ocean through global warming, coral bleaching and ocean acidification. When the influence of the thermal stress, coral bleaching and ocean acidification combines with the local threats mentioned in the previous section (Direct Human Pressures), nearly 80 percent of the Philippines’ coral reefs are rated at high or very high threat, with more than half in the very high threat category.

Both global warming and ocean acidification are the results of rising level of greenhouse gases and one of such greenhouse gases is carbon dioxide.

Ocean Acidification:

The Carbon Cycle; Source: Photo by AIRS on Flickr

Carbon dioxide dissolves in the seawater as part of the carbon cycle and this helps to maintain the delicate pH level of the seawater. However, the rising level of carbon dioxide results in higher-than-optimum amount of carbon dioxide to dissolve in the seawater, which disrupts the pH balance of the seawater by making it more acidic and toxic to the coral reefs and other marine organisms.

Reef-building corals are very vulnerable to ocean acidification as the acidified seawater interferes with their ability to form hard skeletons, causing the corals to die.

Global Warming and Coral Bleaching:

As a result of global warming, the temperature of seawater is constantly rising and corals cannot survive in the warming seawater as the water’s rising temperature kills the algae called zooxanthellae that live in their tissues. Zooxanthellae ensures the corals’ survival by providing them with essential nutrients from photosynthesis and helping them to remove wastes which keeps them healthy.

In addition, zooxanthellae are responsible for the beautiful colours of healthy corals. When zooxanthellae gets killed by the warming seawater, this loss of the zooxanthellae leaves the coral in a starky white appearance, instead of their brilliant colourful appearance.

Bleached Corals; Source: NBC News

This phenomenon is commonly known as coral bleaching. If the coral lose too much of their zooxanthellae or go too long without them, coral bleaching can eventually lead to the coral’s death.