Our weather map lately has been a series of blobs – three or four tropical waves lined up between Africa and Central America, each with its load of rain and thunder and the possibility of a tropical storm.  However, the middle of the Atlantic is covered by a much bigger blob – a huge mass of hot air and Saharan dust.

This dust cloud rose in the north African desert around the middle of June.  By July 13, its head was moving past Jamaica but its tail hadn’t quite left  the African coast.  It’s a monster.

Saharan dust has been riding the trade winds for centuries.  In winter, it reaches as far south as Brazil, and in summer it gets as far north as Florida.  It has its good points.  Sahara dust gives us beautiful sunsets, adds needed minerals to our soil, and blocks the development of tropical storms in its path.

The dust clouds have been getting bigger and coming more often since the 1970’s, and they also have their bad points.  In addition to reduced visibility and the dirt that seems to get on and into everything, Saharan dust is probably dangerous to the health of both people and marine life.

Dust of any kind is hard on people with breathing problems.  Medical researchers believe there is a link between increases in Saharan dust and rising levels of serious breathing problems, especially asthma, in places like South Florida, Cuba, Barbados and Trinidad.

In the process, they found that Saharan dust isn’t just dust.  Samples contained pesticides, heavy metals and a surprising number and variety of bacteria, viruses and fungus spores.  Apparently thick dust clouds shelter these from the sun’s UV rays, which ordinarily would kill them on their trip across the Atlantic.

Some marine scientists suggest that the nasty stuff riding on Saharan dust may also hold a key to some of the unexplained disasters on Caribbean reefs.  Peak years for dust match neatly with mass deaths of sea fans, staghorn and elkhorn corals and spiny sea urchins, and with widespread coral disease and bleaching events.

There isn’t necessarily a cause-and-effect relationship between things that happen at the same time, and we still don’t know the exact cause of these deaths.  However, a dust cloud is certainly an efficient vehicle for spreading disease or poison over a wide area quickly – most of these events took only about a year from start to finish.

It’s also worth noting that all the coral problems blamed on dust happened in or just after a major El Nino event.  That’s where we are now.  NOAA’s Coral Reef Watch notes that sea surface temperatures in the Caribbean are already well above normal, and are following the same pattern that produced mass bleaching in 2005 – the worst year on record.

Bleaching happens when corals spit out the algae that normally live in their cells, giving them colour and helping to feed them.  Thermal stress – water that’s too warm – is the main cause of bleaching.  Healthy coral can survive a short period of bleaching but if it continues for more than a week, the coral will die.  The coral reefs in Montego Bay are already stressed by decades of overfishing, silt, pollution and algae growth.  At this point, their best hope is a hurricane.