When water temperatures rise a degree or two, corals expel their symbiotic algae – zooxanthellae – and turn white. It happens frequently on a local scale, and the affected corals usually recover when the sea cools again.
Summer sea surface temperature just one degree warmer than usual, lasting for 4-6 weeks, is enough to start the bleaching – corals clearly live, and thrive, at temperatures close to those that bleach and kill them.
When much of the sea surface around the tropics remains warmer for a prolonged period, the bleaching spreads, corals die and reefs are heavily damaged. The last really major global bleaching event occurred in 1997-98, while a less extensive one occurred in 2010.
And now, starting before the current El Nino emerged and compounded by the huge, warm ‘Pacific Blob’, we are well into another global bleaching event. With the current El Nino now growing into something fierce, this bleaching event could last a couple of years, long enough to kill and crumble a lot of the corals.
NOAA has produced a remarkable, satellite-based interactive global coral watch website, where you can observe the conditions change over the next few months. It’s scary, but it’s fascinating.
Coral bleaching isn’t new, but the 1997-1998 event was the greatest in hundreds, perhaps thousands of years. Now we have another of the same or greater magnitude. Again most reefs will be damaged. Last time, global loss was about 10%. This time it will be as much or more. It is a devastating prognosis.
Some protective efforts that would reduce other sources of stress on the reefs are possible, for instance reducing both pollution and overfishing, especially of algal grazers. Closing reefs to all fishing would be best. Unlikely perhaps, but not impossible.
We can as well look for other lines of hope. For instance, corals vary latitudinally in their tolerance of thermal stress, and there appears to be a genetic basis to this variation. In tanks at the National Sea Simulator at Melbourne, biologists are growing different strains of coral species under conditions of warmer temperatures and higher acidity, hoping to select for those more tolerant of the coming conditions, planning then to seed them back on the reefs. Of course this is worth doing.
Yet the Great Barrier Reef covers 135,000 square miles and since 1985 half of its coral cover has been lost; little remains of Caribbean coral reefs; the coral reefs around Hawaii and elsewhere in the Pacific have never been under such stress. The future of coral reefs is really bleaker than ever.
For now, the Pacific Blob will eventually dissipate. The new El Nino will slowly play itself out. Sea temperatures will drop back at least in the direction of normal. Parts of coral reefs not too damaged should slowly recover once again.
But we are warned: sea temperatures rising because of climate change rather than El Ninos will not revert to past ‘normal’ temperatures. At best their rise can be slowed by enlightened global agreement to curtail the emission of greenhouse gases,
The current global coral bleaching event is growing as we head into December’s UN Conference on Climate Change in Paris – a conference that is perilously close to being our last chance to take significant global action. Threatened, damaged, bleaching, crumbling coral reefs are not a bad metaphor for the state of our planet’s climate to remind global politicians that no longer can we afford politics as usual.
They – we – really have to do it this time.