Global warming has been causing the "bleaching" of coral reefs. A bleached coral reef has lost its photosynthesizing symbiotic organisms, called zooxanthellae. It may look white as a ghost — as in the picture above — but it is not yet dead. If the zooxanthellae come back, the reef can recover.
With this year’s record high temperatures, many coral reefs are actually dying:
• Dan Charles, Massive coral die-off reported in Indonesia, Morning Edition, August 17, 2010.
DAN CHARLES: This past spring and early summer, the Andaman Sea, off the coast of Sumatra, was three, five, even seven degrees [Fahrenheit] warmer than normal. That can be dangerous to coral, so scientists from the Wildlife Conservation Society went out to the reefs to take a look. At that time, about 60 percent of the coral had turned white – it was under extreme stress but still alive.
Caleb McClennen from the Wildlife Conservation Society says they just went out to take a look again.
DR. CALEB MCCLENNEN: The shocking situation, now, is that about 80 percent of those that were bleached have now died.
CHARLES: That’s just in the area McClennen’s colleagues were able to survey. They’re asking other scientists to check on coral in other areas of the Andaman Sea.
Similar mass bleaching events have been observed this year in Sri Lanka, Thailand, Malaysia, and other parts of Indonesia.
For more, see:
• Environmental news service, Corals bleached and dying in overheated south Asian waters, August 16, 2010.
It’s interesting to look back back at the history of corals — click for a bigger view:
Corals have been around for a long time. But the corals we see now are completely different from those that ruled the seas before the Permian-Triassic extinction event 250 million years ago. Those earlier corals, in turn, are completely different from those that dominated before the Ordovician began around 490 million years ago. A major group of corals called the Heliolitida died out in the Late Devonian extinction. And so on.
Why? Corals live near the surface of the ocean and are thus particularly sensitive not only to temperature changes but also changes in sea levels and changes in the amount of dissolved CO2, which makes seawater more acid.
We are now starting to see what the Holocene extinction will do to corals. Not only the warming but also the acidification of oceans are hurting them. Indeed, seawater is reaching the point where aragonite, the mineral from which corals are made, becomes more soluble in water.
This paper reviews the issue:
• O. Hoegh-Guldberg, P. J. Mumby, A. J. Hooten, R. S. Steneck, P. Greenfield, E. Gomez, C. D. Harvell, P. F. Sale, A. J. Edwards, K. Caldeira, N. Knowlton, C. M. Eakin, R. Iglesias-Prieto, N. Muthiga, R. H. Bradbury, A. Dubi and M. E. Hatziolos, Coral reefs under rapid climate change and ocean acidification, Science 318 (14 December 2007), 1737-1742.
Chris Colose has a nice summary of what this paper predicts under three scenarios:
1) If CO2 is stablilized today, at 380 ppm-like conditions, corals will change a bit but areas will remain coral dominated. Hoegh-Guldberg et al. emphasize the importance of solving regional problems such as fishing pressure, and air/water quality which are human-induced but not directly linked to climate change/ocean acidification.
2) Increases of CO2 at 450 to 500 ppmv at current >1 ppmv/yr scenario will cause significant declines in coral populations. Natural adaptive shifts to symbionts with a +2°C resistance may delay the demise of some reefs, and this will differ by area. Carbonate-ion concentrations will drop below the 200 µmol kg-1 threshold and coral erosion will outweigh calcification, with significant impacts on marine biodiversity.
3) In the words of the study, a scenario of >500 ppmv and +2°C sea surface temperatures “will reduce coral reef ecosystems to crumbling frameworks with few calcareous corals”. Due to latitudinally decreasing aragonite concentrations and projected atmospheric CO2 increases adaptation to higher latitudes with areas of more thermal tolerance is unlikely. Coral reefs exist within a narrow band of temperature, light, and aragonite saturation states, and expected rises in SST’s will produce many changes on timescales of decades to centuries (Hoegh-Guldberg 2005). Rising sea levels may also harm reefs which necessitate shallow water conditions. Under business-as-usual to higher range scenarios used by the IPCC, corals will become rare in the tropics, and have huge impacts on biodiversity and the ecosystem services they provide.
The chemistry of coral is actually quite subtle. Here’s a nice introduction, at least for people who aren’t scared by section headings like “Why don’t corals simply pump more protons?”:
• Anne L. Cohen and Michael Holcomb, Why corals care about ocean acidification: uncovering the mechanism, Oceanography 22 (2009), 118-127.