Environmental Threats To The Great Barrier Reef - Climate Change

Climate Change

Most people believe that the most significant threat to the status of the Great Barrier Reef and of the planet's other tropical reef ecosystems is climate change, consisting chiefly of global warming and the El Niño effect. Many of the corals of the Great Barrier Reef are currently living at the upper edge of their temperature tolerance, as demonstrated in the mass coral bleaching events of the summers of 1998, 2002 and 2006. In February 2007, the current threat of mass coral bleaching was assessed as being "low" due to a monsoonal climate system, although sites displaying some coral bleaching were monitored.

As demonstrated in 1998, 2002 and 2006, corals expel their photosynthesising zooxanthellae (which provide up to 90% of the coral’s energy requirements) and turn colourless, revealing their white calcium carbonate skeletons, under the stress of waters that remain too warm for too long. At this stage the coral is still alive, and if the water cools, the coral can regain its zooxanthellae. If the water does not cool within about a month, the coral will die of starvation. Australia experienced its warmest year on record in 2005. Abnormally high sea temperatures during the summer of 2005-2006 have caused massive coral bleaching in the Keppel Island group.

Most scientists studying the issue believe that climate change poses a massive threat to the future of the Great Barrier Reef. A draft report by the UN Intergovernmental Panel on Climate Change, the world's preeminent gathering of climate scientists, states that the Great Barrier Reef is at grave risk and will be "functionally extinct" by 2030, warning that coral bleaching will likely become an annual occurrence.

However, a few scientists hold that coral bleaching may in some cases be less of a problem than the mainstream believes. Professor Ridd, from James Cook University in Townsville was quoted in The Australian (a conservative newspaper) as saying; "They are saying bleaching is the end of the world, but when you look into it, that is a highly dubious proposition". Research by scientist Ray Berkelmans "... has documented astonishing levels of recovery on the Keppel outcrops devastated by bleaching in 2006." A related article in The Australian newspaper goes on to explain that; "Those that expel their zooxanthellae have a narrow opening to recolonise with new, temperature-resistant algae before succumbing. In the Keppels in 2006, Berkelmans and his team noticed that the dominant strain of zooxanthellae changed from light and heat-sensitive type C2, to more robust types D and C1."

Nevertheless, most coral reef researchers anticipate severely negative effects from climate change already occurring, and potentially disastrous effects as climate change worsens. The future of the Reef may well depend on how much the planet's climate changes, and thus, on how high atmospheric greenhouse gas concentration levels are allowed to rise. On 2 September 2009, a report by the Australian Great Barrier Reef Marine Park Authority revealed that if carbon dioxide levels reached 450 parts per million corals and reef habitats will be highly vulnerable. If carbon dioxide levels are managed at or below 380 parts per million they will be only moderately vulnerable and the reefs will remain coral-dominated.

Global warming may have triggered the collapse of reef ecosystems throughout the tropics. Increased global temperatures are thought by some to bring more violent tropical storms, but reef systems are naturally resilient and recover from storm battering. Most people agree that an upward trend in temperature will cause much more coral bleaching; others suggest that while reefs may die in certain areas, other areas will become habitable for corals, and new reefs will form. However, the rate at which the mass bleaching events occur is estimated to be much faster than reefs can recover from, or adjust to.

However, Kleypas et al. in their 2006 report suggest that the trend towards ocean acidification indicates that as the sea's pH decreases, corals will become less able to secrete calcium carbonate. In 2009, a study showed that Porites corals, the most robust on the Great Barrier Reef, have slowed down their growth by 14.2% since 1990. It suggested that the cause was heat stress and a lower availability of dissolved calcium to the corals.

Climate change and global warming are one of the greatest threats to the reef. A temperature rise of between two and three degrees Celsius would result in 97% of the Great Barrier Reef being bleached every year. Reef scientist Terry Done has predicted that a one-degree rise in global temperature would result in 82% of the reef bleached, two degrees resulting in 97% and three degrees resulting in "total devastation". A predictive model based on the 1998 and 2002 bleaching events has concurred that a temperature rise of three degrees would result in total coral mortality.

Climate change has implications for other forms of life on the Great Barrier Reef as well - some fish's preferred temperature range lead them to seek new areas to live, thus causing chick mortality in seabirds that prey on the fish. Also, in sea turtles, higher temperatures mean that the sex ratio of their populations will change, as the sex of sea turtles is determined by temperature. The habitat of sea turtles will also shrink.