FAQ on Climate Change & Corals

The corals’ polyps grow by creation of calcium carbonate cups called calices in which the polyp sits. Every couple days a new calicix is formed on top of the old one separated by wall called septa. Over many generations this extension forms the large calciferous structures of corals and ultimately coral reefs. The polyps deposit aragonite (CaCO₃) from the calcium ions in the seawater, forming a massive limestone exoskeleton in the end.

The rate of deposition varies greatly between species and environmental conditions and can vary from a few millimeters to several centimeters per year. This is light dependent, with nighttime production 90% lower than that during the middle of the day.

Essentially, corals build the habitat that is the home for the fishes and other marine species that live on the reef and that people eat and extract useful natural products from such as cancer fighting medications. Life in the ocean wouldn’t be as interesting and rich without corals and lots of species would probably disappear. From an economic point of view, coral-linked recreational activities are bringing constant revenues to the economy of the places where tropical corals exist. And, more than this, coral reefs constitute a natural barrier, slowing down hurricanes and typhoons intensity.

Ocean acidification is the process of continuing decrease in the pH of the Earth's oceans (increasing acidity), caused by their up take of carbon dioxide from the atmosphere perhaps better thought of as the oceans becoming less alkaline because the pH of the surface ocean is presently 8.1, i.e. slightly alkaline or basic on the 0-14 pH scale. Dissolving CO₂ in seawater increases the hydrogen ion (H⁺) concentration in the ocean, and thus decreases ocean’s pH.

The oceans have taken up nearly one third of the total anthropogenic CO₂ produced in the past 200 years. Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.179 to 8.104 and models are forecasting a pH as low as 7.824 in 2100. While these changes sound small a drop of 0.3 pH units amounts to a doubling in H⁺ concentration. This is because pH is measured on a logarithmic scale like earthquake intensity where each unit change in pH means a 10-fold change in H⁺ concentration.

As the concentration of CO₂ of the water increases, the pH decreases and the balance between bicarbonate and carbonate shifts more and more towards bicarbonate as the ocean attempts to buffer the drop in pH by combining H⁺ with CO₃^2- to produce HCO₃^-. As the carbonate ion concentration decreases it becomes more difficult for the corals to extract the CO₃^2- from the seawater to build their skeletons. It is presently unknown how species vary in their ability to cope with the decrease in carbonate ion concentration in a process unknown as acclimation.

Coral bleaching occurs when the corals lose their color, due to stress-induced expulsion of the symbiotic unicellular algae. The corals forming the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae living within their tissues. Zooxanthellae give the coral its particular coloration and, in stressful conditions, corals may force out their zooxantheallae, which leads to a lighter or completely white appearance.

Once bleaching begins, corals tend to continue to bleach even if the stressor is removed. If the coral colony survives, it often requires weeks to months for the remaining symbiont population to return to a normal density.

Changes are always happening in the nature and climate change is one of them. Climate change is any long-term important change in the “average weather” that a given region experiences. Average weather may include average temperature, precipitation and wind patterns. It involves changes in the variability or average state of the atmosphere over decades to millions of years.

Different factors can impact a climate change: variations within the Earth system like changes in the extent of the snow and ice cover at high latitudes, changes in the amount of volcanic activity associated with the movement of the continents and variations in solar energy received at the Earth’s surface due to changes in Earth’s orbit. What we call climate change nowadays is the raising of the global temperature, rising sea level and increasing acidity of the ocean that are mainly due to human activity during the industrial and post-industrial eras related to the emission of massive quantities of CO₂ into the atmosphere.

Climate change implies the changing of several factors, such as temperature, CO₂ concentration in the water, sunlight and UV radiation that are likely to have an impact on corals. Predicting the consequences on the corals and the rest of the coral reef ecosystem is complicated.

Coral bleaching can be provoked by natural factors, such as high sea surface temperature coupled with high irradiance in summers, low wind, and exposure at low tide. The concern is that the frequency of occurrence of conditions causing bleaching will increase with rising seawater temperature to a point where the corals no longer have time to recover between the events.

Some species of coral are known to be more sensitive to bleaching than others. The field of ocean acidification is too young to know how different species will respond in falling pH.

Yes, there is evidence that some corals can take up more temperature tolerant zooxanthellae from the seawater and thereby become more resistant to bleach. Little is known about the ability of corals to adapt to declining pH but there is hope that some acidification resistant species will be found.