AIMS has issued an easy to read white paper on its home page, outlining its major findings related to coral reefs and climate change. This was apparently added on Dec 19, 2009 the same day as Jamie Walker’s “How the reef became blue again” piece in The Australian. Coincidence? I’ll excerpt some highlights below:
Climate change and the tropical marine environment
Tropical marine environments such as coral reefs and mangrove forests around the world are under unprecedented pressure due to climate change, changes in water quality from terrestrial runoff and overexploitation. Coral reefs are iconic tropical ecosystems represented by Australia’s irreplaceable Great Barrier Reef (GBR) and the less explored reefs off Western Australia. Corals thrive in locations which also happen to be near their physiological limits, making them sensitive to stresses caused by rising sea surface temperature and an increase in ocean acidity linked to rising carbon dioxide (CO2) in the atmosphere.
What we know
- The long-term average temperature for the waters of the Great Barrier Reef has increased by about 0.4oC since the 19th century and the Reef system has experienced two mass coral bleaching events (1998 and 2002) caused by long periods of coral exposure to unusually warm seawater.
- During the 1998 coral bleaching event, 42 per cent of shallow water coral reefs on the GBR bleached and an estimated 2 per cent died that year. This equates to approximately 400km2 of reef area.
- In 2002, the largest bleaching event on record, an even greater proportion of the Reef bleached (55 per cent) and an estimated 5 per cent died. This equates to approximately 1000km2 of reef area
- While these percentages may seem small, they can be localised and severe events.
- Some local extinctions of coral species in several parts of the Great Barrier Reef have been observed and appear to be linked to higher sea surface temperatures causing coral bleaching.
- Coral bleaching was again observed in the 2006 summer, particularly in the southern GBR, where local water temperatures reached around 1-2oC above the seasonal average.
- Coral reefs may take 10 to 20 years to recover from serious bleaching events that cause coral death.
- The amount of CO2 in the atmosphere is known to be increasing and the extra CO2does not stay just in the atmosphere with a significant amount dissolving into the ocean. The pathways it then follow under different conditions and the consequences of its accumulation in different environments is under-researched. Some scientists have proposed that the large portion of CO2 that is entering the ocean from the atmosphere is causing a shift downwards in seawater pH, making it more acidic.
- A growing body of experimental evidence is showing that seawater acidified to mimic potential future scenarios significantly impacts upon the health of some fish and coral species. There are many millions of species in the ocean and each will have different sensitivities to acidification and respond in different ways. No single species lives in isolation and how the effects seen at an individual species level translate to an ecosystem response is not understood. It has been speculated that acidified seawater may alter the makeup of marine ecosystems and weaken coral reef structures.
- It is known that heat stress causes corals to expel the symbiotic algae they host in their tissues. What is not sufficiently understood are the numerous mechanisms that may enable corals to adapt to new, warmer and potentially acidic conditions.
- Based on observations of an increase in hurricane and cyclone events in recent decades, even more severe storms and cyclones have been proposed to occur as our climate changes, though this remains a topic of debate.
The consequences of increasing atmospheric carbon dioxide
The amount of CO2 in the atmosphere has risen to the current level of 383 parts per million (ppm) from about 200 ppm in the days before the Industrial Revolution more than 200 years ago. Measurements of atmospheric CO2 taken from AIMS headquarters outside Townsville show broad agreement with this global figure (see page 1 of this document).
Under current IPCC projections and assuming no measures are adopted to reduce CO2emissions, atmospheric CO2 concentrations are likely to reach 500 ppm in the second half of this century. If that is the case, global temperature averages may increase a further 2oC and possibly more.
Coral reefs provide ecosystem services essential to our national identity and wealth. The GBR contributes more than $5 billion annually to the Australian economy.
While Australia’s coral reefs are well managed, they are not isolated from global atmospheric and ocean changes.