Coral decline in the Indo-Pacific

The decline of coral reefs has entered the literary world: a recent New Yorker featured Kimiko Hahn’s poem The Fever about coral bleaching. Take a look before it disappears from the online site.

The poem includes the passage "I wonder if it’s, yet again, the ozone layer". Hopefully the author is playing off the history human impacts on the environment not suggesting ozone depletion is the cause of coral bleaching. UV light does play a role; thankfully no corals are growing on the shores of Antarctica.

The poem's appearance is well-timed. First, in the past month, there have been warm-water anomalies in Florida, in Okinawa (Ishigaki) and the Northern Mariana Islands. In each case, people on the ground reported coral bleaching was underway. The predictive ability never ceases to amaze me. You can see the temperatures maps at NOAA's Coral Reef Watch (the data is best visualized using Google Earth).

Second, the IPCC's full Working Group II report - that's the climate change impacts section - is now available. The impact of climate change on corals reefs is one of several "cross-cutting" themes including the impact of climate change on coral reefs. Rather than look flip through the 20 chapters of the full report for information about corals, you can just read the case studies at the end (scroll down, it is before the appendix). The summary is a pretty solid survey of the science; unfortunately, since the IPCC report was finished a while back and could not include some more recent results.

Third, a terrific meta-analysis by John Bruno and Elizabeth Selig of UNC published a few weeks ago in the free (yes, free to anyone) online journal Public Library of Science documents the decline of coral cover across the Indo-Pacific. Bruno and Selig went through the arduous task of analysing every survey of coral cover conducted in the past 40 years. They found coral cover across Indo-Pacific reefs may have declined from over 40% in the 1980s to closer to 20% today. The method of averaging the data over such a large area may be questioned; there may be bias in where we choose to study corals. But the study makes it very clear that coral cover is decreasing. The paper has caused quite a stir (see Coral Bones and Climate Shifts for some accessible discussion).

The decline is believed to be caused by the usual suspects: overfishing, destructive fishing practices, disease, pollution / sedimentation, and coral bleaching due to rising ocean temperatures. The gradual decline in coral cover may seem to point to the direct human threats rather than coral bleaching, the rationale being that coral bleaching events like the 1997/1998 event are widespread and should result in step changes in coral cover (punctuated equilibrium over gradualism?). A tempting argument, but the data does not support it.

In truth, individual events are never that widespread. Coral bleaching has yet to occur in concert across globe or even across the massive and diverse region in the Bruno and Selig study. For example, the famed 1997/1998 was not really global: events occurred in the Indian Ocean, the Caribbean, and parts of the Pacific, due in part to El Nino. The "Indo-Pacific" covers SE Asia and Australia east to Tahiti, an area largely not affected by the 1997/1998 El Nino (the W Pacific is, if anything, cooler during El Nino events; bleaching on the Great Barrier Reef happened when the Pacific flipped to La Nina conditions in 1998).

So, if bleaching was a major cause of a decrease in coral cover, a gradual decline, not a step change, in total average coral cover for a large region, or for the entire globe, is exactly what we should expect to see. That's not to say bleaching is "the" cause, rather one of the causes.

This year, the N Marianas and Okinawa. Next year?