Travel Reference
In-Depth Information
Changing Environments & Coral Bleaching
The idyllic symbiosis between coral and zooxanthellae evolved to perfectly match the en-
vironmental conditions of the past. But corals don't like change, and they are currently be-
ing hit with rates of change unparalleled for at least 400,000 years.
Bright sunlight and warm waters are required to support coral reefs, but it's a fine line
between warm enough and too warm. Around the turn of the last century (mainly 1998 and
2002 for the GBR, as late as 2010 elsewhere), spikes in water temperatures caused the
densely packed zooxanthellae to go into metabolic overdrive, producing free radicals and
other chemicals that are toxic to the coral host. The corals' response was to expel their
zooxanthellae, to rid themselves of the damaging toxins. Water temperatures must return to
normal before the small numbers of remaining zooxanthellae start to reproduce and thus re-
instate the corals' live-in food factory. But if the heat wave persists for more than a few
weeks, the highly stressed corals succumb to disease and die, their skeletons soon becom-
ing carpeted with fine, shaggy algal turfs. A 2013 study found that about 10% of GBR cor-
al deaths over the last three decades followed these episodes, known as coral bleaching.
And an important reality is that climate change isn't occurring in isolation: this study also
highlighted that storm waves and outbreaks of the coral-eating crown-of-thorns starfish are
big killers of corals (each around 40%). The effects are cumulative, and with projections of
an increase in the severity of cyclones and the frequency of coral-bleaching events, we are
likely to see more incidents of broad-acre coral death under a changing climate.
It takes one to two decades for a healthy coral reef to bounce back after being wiped out.
So far, damaged GBR sites have shown remarkable resilience to damaging events, but the
future might not be so rosy, as more frequent events driven by climate change repeatedly
decimate reefs before they can fully recover. In other parts of the world, some reefs have
suffered the added insults of decades of pollution and overfishing. By those means, former
coral areas have become persistent landscapes of rubble and seaweed.
