The Great Barrier Reef came into being millions of years ago.  It was worn down, and grew back repeatedly over those eons.  But now all the factors that let the reef grow are changing at a rate the Earth has never before experienced.  This time the reef may degrade  below a crucial threshold from which it cannot bounce back.

The fast change in the world’s climate is a particular threat.  Warming temperatures at the corals and increased exposure to the sun’s ultraviolet rays lead to a stress called bleaching.  The colorful algae become toxic and are expelled.  Fleshy seaweeds may then choke out the remains.

Heat has also contributed to a 60-year decline in ocean phytoplankton that can gobble greenhouse gases.

Recent massive flooding in Australia sent huge amounts of sediment and toxin-laden waters onto the Reef.

And the oceans are absorbing more and more greenhouse gases from the environment, resulting in a dramatic increase in acidity.  This makes the reefs ‘bones’ more fragile and brittle so that waves, storms, diseases, pollutants and other stressers can break them.

Thanks to National Geographic for information on this issue.   See the May 2011 issue for lots more detail.