If, as the song goes, you always hurt the one you love, humans must really love coral reefs. Reefs seem to be under siege from all sides. Between overfishing, nutrient pollution, climate warming, and ocean acidification, it’s no wonder marine biologists express so much concern for the future. Not all research has yielded doom and gloom, of course. Some species have shown the ability to deal with acidification or to adapt over a few generations; some high-latitude reefs limited by cold waters could see growth. But, on balance, the outlook is still decidedly bleak.
In the ongoing quest to see into the crystal ball a little more clearly, one route is to study how climate has impacted coral reefs in the past. A new study published in Science finds that climatic variability during the Holocene—the period since the end of the last ice age—strongly affected reefs in the Pacific. And the researchers believe it’s a preview of what’s to come.
The group studied reef sediment cores from three locations off the southern coast of Panama. The sites were chosen for their differing relationship with the upwelling in the Bay of Panama—where deep water rises to the surface. One location is smack in the middle of the upwelling zone, another is on the edge, and the third is outside it. The water that seasonally rises from the deep ocean is more acidic than surface water because CO2 has accumulated during its long journey along the seafloor.
That seasonal acidity is a source of stress for the coral reefs there, but so are the warm ocean temperatures in the eastern Pacific that come with El Niños. When the water is too warm, corals can “bleach”, expelling the photosynthetic symbionts that live inside them. If the corals don’t recover quickly, algae can take over. This reduces the diversity and structure of the reef to a carpet of green.
The sediment cores were dated using carbon-14 as well as a uranium-thorium method commonly used for corals, and the researchers noted how the mix of species changed through time. In each core, there was a thin layer where very little coral growth had taken place. And the corals that were present were in pretty poor shape.
The dating revealed this thin layer actually accounted for about 2,500 years, beginning around 4,000 years ago. For some reason, the coral communities had collapsed for more than two millennia before resuming growth.
Reefs elsewhere in the Pacific—near Costa Rica, Australia, and Japan—went through similar growth “hiatuses” at nearly the same time. Previously, each had been chalked up as one-off, regional events. But now researchers argue they seem to be part of the same, larger story.
A little over 4,000 years ago, the El Niño Southern Oscillation (ENSO) began behaving oddly, with El Niños occurring more frequently and much more strongly than usual. That was amplified by increased variability in the position of the Intertropical Convergence Zone—the boundary that separates the atmospheric circulation in the Northern and Southern Hemispheres and drives the equatorial monsoons.
There are still some open questions, but since stronger El Niños (and more frequent heat stress events) are expected as climate warms, the researchers believe that this period tells us something about how Pacific reefs will respond. They don’t appear to be as resilient to climatic variability as we might hope. The researchers write, “Global climate change is probably driving eastern Pacific coral reefs toward another regional collapse.”
They do, however, end on a positive note. Since the reefs were able to recover after 2,500 years of collapse, perhaps they’ll similarly bounce back once we get around to dealing with our greenhouse emissions. Of course, temperature isn’t the only problem facing reefs today, so, as the saying goes, past performance is no guarantee of future results.