Coral Reefs and climate change
by Roger Grace
As summer draws to a close, our thoughts may soon be drifting to scenes of coral sand beaches, coconut palms, blue lagoons and coral reefs. With the approach of winter comes the season to escape to the tropics for a couple of weeks diving where the water is always warm and clear.
We may, however, be seeing the last few years of healthy, lush and diverse coral reefs as we like them. It might be a good idea to take that tropical dive holiday sooner rather than later, because corals are facing an increasing threat from rising sea temperatures on a global scale.
Global warming, an aspect of climate change believed to be caused largely by an increase in greenhouse gases, especially carbon dioxide, through the burning of fossil fuels such as coal, oil products and gas, is pushing up sea temperatures to a point where they are causing unprecedented problems for corals.
The worldâs weather is going crazy. The recent abundance worldwide of severe floods, cyclones, storms, blizzards, droughts and heat waves, and resulting forest fires, closely follows predictions of climate change and increasingly severe weather events. The concept of the â100-year floodâ is no longer a valid basis on which to design engineering works. We can now expect the â100-year floodâ to occur several times within a decade!
When sea temperatures rise as little as one degree Celsius above the normal summer high level, many corals suffer from a phenomenon known as coral bleaching. The corals turn white and many die.
Coral bleaching occurs when corals become stressed. Stress can result from high temperatures, high UV light, pollution, sediment laden flood waters, and bacterial infection, and all have been implicated in coral bleaching. By far the most widespread cause of mass coral bleaching, however, is high temperature.
Mass coral bleaching events are a relatively new phenomenon, but have been increasing globally since the early 1980s, and have been reported from all major coral reef areas of the world. Coral bleaching is now more serious than previously recognised threats to coral reefs such as crown-of-thorns starfish, hurricanes, pollution, sediment and agricultural runoff, damage from boat anchors, souveniring and fishing. Unlike the other threats, which tend to be rather local in their immediate effect, coral bleaching is damaging reefs on a global scale.
To understand coral bleaching we first need to understand a little about coral biology and how corals manage to live where they do. Corals are remarkable in that they live in clear tropical waters containing very little plankton or nutrients, the stuff that most bottom-dwelling animals and plants require to live and grow. Corals can live in nutrient-poor seas because of a special adaptation – a symbiosis between the coral animals and microscopic algae called zooxanthellae (pronounced zoo-zan-thel-ee).
These special algae occur as microscopic single cells scattered through the living tissues of the coral animal. Like most plants, they require both light and nutrients in order to survive. Clear seas and high sun angles of the tropics ensure that they get sufficient light. The coral animals, by way of their excretory products, provide the algae with the nutrients they need. In return, sugars and other complex products created by the photosynthetic activities of the tiny plants are the main food source of coral animals.
Living tissues of corals, and their skeletons of calcium carbonate, are generally colourless or have only pale colours. The microscopic algae within the coral tissues are responsible for most of the brownish and greenish colours of living corals.
When high sea temperatures strike, and we are talking of only one degree Celsius above normal, something goes wrong with the microscopic algae in the coral tissues, and they start producing free-radicals of oxygen. Many corals react by spitting out their symbiotic algae. The algae take most of the colour of the corals with them, so the coral turns white or pale and appears âbleachedâ.
Bleaching can occur if sea temperatures rise for only two or three days. If the temperature then drops again, many corals will recover. The few microscopic algae remaining within their tissues will multiply and recolonise the rest of the coral colony. If, however, the temperature remains high for several weeks, the coral animals have lost their main source of food and will become weakened by starvation.
In this poor condition corals are unable to grow or to resist attacks by micro-organisms. Many bleached corals soon suffer from small seaweeds growing on their surface. When this happens the coral is unlikely to recover.
Bleaching can occur in organisms other than hard corals. Any of the coral relatives that contain zooxanthellae can suffer from bleaching. The large clownfish anemones and some types of soft corals and gorgonians can sometimes bleach. Some types of hard corals are more resistant to bleaching than others. The fast-growing stags-horn corals, mainly species of Acropora, easily bleach and usually die, whereas the slow-growing massive corals such as Porites, which is the one commonly containing the colourful Christmas tree worms, sometimes bleach but usually recover.
Over successive bleaching events we may see a shift in the species composition of the reefs, and some adaptation to the higher temperature. But by and large, a major bleaching event results in the deaths of 20% or more (over 50% at Moorea in 1994, and 60 to 80% on parts of the Great Barrier Reef in 1998) of the corals on a reef. If this was just a one-off loss there would be little problem. After all, hurricanes and crown-of-thorns starfish kill probably more than that in one hit. The reefs eventually recover from those events.
The problem is that as temperatures rise, mass bleaching of corals is becoming more frequent, is increasing in intensity, and becoming more widespread in occurrence. If 20% of the corals are lost every three or four years, then the coral reefs are eroding away faster than they are growing. Clearly this is not sustainable.
The death of coral reefs can have profound effects. Many island nations rely on coral reefs nearby for their social and economic wellbeing. Coral reefs provide the physical structure and most of the food upon which the whole reef ecosystem depends. Many fisheries rely on coral reefs. Tourism, based on coral reefs, blue lagoons, and coral-sand beaches, is a major money-spinner for many island nations.
An eroding reef would open up low-lying land to severe erosion. Most human habitation on Pacific and Indian Ocean islands is concentrated on easily-eroded unconsolidated land less than two metres above sea level, protected from erosion only by a fringing or a barrier coral reef. The impact of the loss of protective reefs due to mass coral bleaching in these areas would be totally devastating and further aggravated by a projected sea level rise of about 30cm over the next 50 years as a result of thermal expansion of the waters due to global warming.
Coral reefs may be the first of the worldâs major ecosystems to suffer serious effects from man-induced climate change. Increasing CO2 in the atmosphere reduces the ability of corals to lay down aragonite, the main type of calcium carbonate making up the coral skeleton. Rising temperatures are increasing the incidence of coral bleaching. Rising sea levels allow waves to erode degraded coral and threaten the coastlines with serious inundation. It all points to serious problems for coral reefs around the world.
While industrialised nations procrastinate about controlling carbon dioxide emissions caused by burning fossil fuels, the poorer oceanic nations are literally going under. Governments must urgently get real about pursuing alternative non-polluting energy sources, and getting away, as fast as possible, from reliance on burning of gas, oil, and coal for our energy needs.
There is only one planet we can live on. It is our precious earth.