Unusually cold weather on the southern Great Barrier Reef has triggered localized bleaching on the reef crest at Heron Island. Low tide, strong winds and unusually cold wintry weather in July 2007 has caused the top few centimeters of coral in the intertidal sections of the reef to bleach. Cold bleaching has been reported before at Heron Island by others during 2003, and is very similar to bleaching that occurs when water gets too warm.

So, is this event a sign of global climate change?

I don’t think we have enough evidence to say this right now. Some models, however, suggest that the southern Great Barrier Reef may experience colder winters with a weakening if the south Pacific gyre, which runs down the east coast of Australia and normally pushes warm water southward. Certainly, colder years tend to follow strong El Nino (warm) years. So far we have seen winter bleaching on the southern Great Barrier Reef in 1999, 2003 and now in 2007. In the three cases, the preceding years 1998, 2002 and 2006 were very warm years and saw extensive coral bleaching on the southern Great Barrier Reef.

Perhaps a passing meteorologist could work this one out for us. If there is a connection to climate change, then we might need to consider the effect of corals stressed out in summer that are then stressed again in the following winter.