How surprised would you be if I told you that it’s highly unlikely human-induced climate change will drive coral reefs to extinction? For many, I imagine, this would come as quite a shock. Over the last few decades, the story surrounding the steep decline of global coral reefs has been rightfully dismal. In just the last thirty years, more than fifty percent of coral reefs have died, with an expected ninety percent mortality by the end of the century. Given the vast number of ecosystem services that coral reefs provide, this projection should sound alarm bells throughout the global community. 

However, it is important to remember that scleractinian, or ‘reef-forming’, corals have been on this planet for nearly 250 million years. They’ve survived two of the ‘Big Five’ mass extinction events, including hundreds of thousands of years of volcanism and an extraterrestrial impactor that killed the dinosaurs. Coral reefs have also endured each warm excursion of the last 65 million years, where average tropical sea surface temperatures were many degrees higher than they are today. In fact, coral reefs are so adept at survival that their taxonomic composition and diversity has remained largely unchanged for the multiple ice ages of the last 500,000 years. 

The global distribution of today's coral reefs across the tropics.
The current distribution of coral reefs worldwide. Photo credit: Hugo Ahlenius via Flickr (https://www.grida.no/resources/7765)

The remarkable persistence of coral reefs through extreme climates in the geologic past is contrasted by their apparent fragility in the face of anthropogenic climate change. This paradox begs the question: how did coral reefs survive such adverse conditions over the last 250 million years? One viable answer is that anthropogenic climate change is unlike anything coral reefs have experienced before. This answer is unsatisfying, however, because it doesn’t address why coral reefs were able to survive such extremely warm conditions in the first place. So, how exactly are coral reefs still here?

One potential explanation for this conundrum lies in the idea of coral refugia, or locations where corals can proliferate when their typical environments become inhospitable due to increased ocean temperatures or storm intensity. Coral refugia are classically thought to be in areas such as the Red Sea or Bermuda, where ocean currents carry warm tropical waters to abnormally high latitudes that, in theory, are not as susceptible to extreme warming events or tropical storms. However, a far more ubiquitous coral refuge is thought to exist not farther from tropical coral reefs, but beneath them.

Mesophotic coral reefs, which are often seen as deep-water extensions of shallow-water reefs, are light-dependent corals that live below thirty meters water depth. These low-light adapted corals live close enough to the surface to harvest light, while living deep enough to escape the detrimental effects of extreme heat and tropical storms. While mesophotic reefs are not exempt from stressors associated with anthropogenic climate change, they make for a promising refuge for coral reefs under threat from climate change. Furthermore, most shallow water reefs are accompanied by an underlying mesophotic reef, and the two ecosystems share vital resources that scientists are yet to fully understand. This may include genetic information under times of stress, so that shallow-water species may (over generations) retreat to the depths until conditions become more favorable in their preferred environment.

While I personally find solace in the fact that corals may find refuge in the face of anthropogenic climate change, I remind myself that future coral reefs will be very different from the ones we see today. Mesophotic coral reefs are lacking many of the vital ecosystem services that human beings depend on from shallow-water systems, including storm protection, nurseries for fishery-dependent juveniles, and carbon sequestration. If coral reefs retreat from the shallows into the mesophotic zone, it will herald a difficult time for human beings. 

 

Peer Editor: Mikayla Feldbauer

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