Recently, the mass coral bleaching event occurring on Australia’s Great Barrier Reef (GBR) has had extensive media coverage, bringing to light the severe consequences of the changing climate. But what is coral bleaching? And why is this year set to be particularly bad?
Although the climate is slowly warming globally according to vast scientific consensus, there are patterns of weather which influence particular areas of the planet, causing more severe weather events at certain periods of time. El Niño is a complex interaction between ocean and atmospheric temperatures which result in certain patterns of climate which prevail for 9-12 months at a time, occurring every 2-7 years.
Put simply, it occurs when winds pushing water from East to West across the Pacific become slightly weaker, meaning that water that piles up on the western side becomes less piled-up than in ‘normal’ conditions. Movement of cooler water from the deep replaces the water blown away by the wind in the East. But as the wind becomes weaker this process also becomes less pronounced, so the surface temperature of the eastern side of the ocean increases. This results in a positive feedback loop, whereby the warmer ocean temperatures make the winds less strong, which makes the ocean temperature warmer and so on. The cycle in full is known as the El Niño Southern Oscillation (ENSO) cycle, consisting of the warm-phase El Niño, and cool phase La Nina.
Currently, we are in the middle of an El Niño cycle. Along with causing heat waves and droughts in Thailand, the climatic effects brought about by the El Niño cycle have detrimental effects on coral reef ecosystems. Extended periods of warm weather, lasting several weeks, are known to cause bleaching in corals. The first recorded mass coral bleaching event (bleaching of a wide range of coral genera across a large geographic area) occurred during an El Nino year on the Great Barrier Reef in 1979-1980. Since then, there have been mass coral bleaching events on every coral reef ecosystem in the world, in every decade. The most destructive year of coral bleaching was in 1998, which was also the world’s first global mass bleaching event. During 1998 almost every reef ecosystem in the world experienced some degree of bleaching, with many areas experiencing large amounts of mortality which they have still not recovered from. In all, about 16% of the hard corals in the world died in 1998.
Bleaching is the process by which corals expel their symbiotic algae, known as zooxanthellae, which normally provide around 80% of the corals’ energy when in a healthy relationship. Without this zooxanthellae, corals become susceptible to natural stresses like predation and disease, and risk dying.
Along with corals, other animals such as anemones, giant clams, and sponges also contain zooxanthallae and can undergo bleaching
Along with anthropogenic (human-caused) stresses, such as nutrient run-off, sedimentation and physical disturbances, these stresses combine to overwhelm a coral’s coping mechanisms, causing mass mortality and an ecosystem phase shift which is near impossible to fully recover from.
Koh Tao has been affected by the global bleaching events of 1998, 2010 and 2014 to different extents. The 1998 bleaching event wiped out nearly all of the corals in Shark Bay, an ecosystem which is still recovering but may never revert back to its original state. So far this year we have begun to witness bleaching on both the natural reef and our artificial structures.
The ENSO cycle will always pose a threat to reefs around the world, yet human-induced climate change is exacerbating the problem. As temperatures remain high for the coming months, we expect to see more bleaching on our reefs, but whether there will be lasting impacts remains to be seen. In the meantime, we will continue to work on our artificial reefs and monitor the state of reef health through our EMP surveys. We will keep you updated as to the extent of the bleaching event in the coming months.