During the last century the Earth has warmed by 0.6˚C and it is predicted that sea surface temperatures may rise by up to 1˚C over the next 50 years. At present, carbon dioxide (CO2) and methane gas levels are the highest they have been for at least 420 000 years. These two gases, together with water vapour, nitrous oxide and halocarbons, make up the five predominant greenhouse gases. On average, four metric tonnes of carbon dioxide are added to the atmosphere per person per year. While the actual increase in carbon dioxide may not seem like much, it represents an increase of 30% over the last 250 years. The IPCC (Intergovernmental Panel on Climate Change) currently monitors these changes in climate.
By 2100, some scientists predict that at the current rate of CO2 change, world temperatures will increase 2-4˚C with sea surface temperatures rising by 1-3˚C. There could lead to:
The greatly elevated level of atmospheric carbon dioxide (CO2) is having an effect on ocean carbonate chemistry. Corals rely upon converting carbonates dissolved in seawater into a skeleton by a process known as calcification. As atmospheric CO2 increases, seawater acidity increases and calcification becomes more difficult.
Currently, it has been observed that increased water temperatures block the photosynthetic reaction in the zooxanthellae algae that live in a mutualistic association with coral polyps. Less carbon dioxide is converted into sugars and there is a build-up of products that poison the zooxanthellae. To save itself, the coral spits out the zooxanthellae and some of its own tissue in a process called tissue sloughing, leaving the coral a bleached white. In tissue sloughing, the coral polyps detach from the coral skeleton. Without the zooxanthellae, the corals that remain slowly starve to death. Once the coral dies, fish and a multitude of other marine species are soon affected. If cooler water temperatures return, the bleached coral may recover with the algae able to recolonise corals and grow again.
Coral bleaching is a natural process. However, the rate of coral bleaching is increasing. The worst coral bleaching in 700 years struck the Great Barrier Reef in 1998, followed by an even worse one just four years later. Massive areas of corals were affected all over the world. In Australia alone, the 2002 bleaching saw nearly 60 per cent of the reef affected and in the worst hit areas, 90 per cent of the coral was bleached. To survive coral bleaching, some corals have a special ‘sunscreen’. Some corals also recover better from bleaching events than others because they have a different type, or clade, of zooxanthellae algae living in their tissues.
Sea sponges also appear to be susceptible to the effects of climate change. Several species of sponges have a complex community of microbes living within their tissues. Microbes such as cyanobacteria form a symbiotic relationship with the sponge, providing food for the sponge. These sponges are at risk from higher sea surface temperatures because this symbiotic relationship breaks down at 33˚C. This is the same temperature threshold at which corals experience bleaching. Both result from a breakdown in the symbiotic relationship between the host and its microbes.
Useful web link: Work by AIMS scientist Dr. Nicole Webster has shown that climate change, with its predicted elevated sea temperatures, would threaten the survival of sponges, particularly autotrophic species. Heterotrophic sponges may be less affected although the contribution of symbiotic microbes to their physiology is also significant.
Changes in water temperature can also affect fish reproductive cycles. There could be problems if there is a mismatch between when the larvae or young are produced and when their food, such as plankton, becomes available. Studies have also shown that fish found in warmer waters of the GBR, have a smaller maximum size, shorter lifespan and earlier maturation.
You may like to perform some investigations into the Effect of Global Warming on Marine Ecosystems and develop your scientific thinking skills with several structured discussions on Corals in the For Teachers section.
Useful web link: Other useful web links include the Adapting to Climate Change Publications; Human Impacts on Coral Reefs; Queensland Murray Darling Committee; and Climate Change and the Great Barrier Reef.