by Kim Cobb





Every once in a while, the universe reminds you that you are small. Very small.





Several weeks ago, my students and I set out on what was supposed to be a 7-day expedition to Christmas Island, a remote island in the central equatorial Pacific, to document the effects of the current El Niño event on the coral reef there. We could not have known that we would face major ordeal after major ordeal, all while confronting a mass mortality event of staggering proportions on the island’s pristine coral reefs.





to revisit sites that we’ve been monitoring for the last two years, service temperature and salinity logging devices, and drill some coral cores that span the bulk of the 2015/2016 El Niño event. The mission’s goals were simple enough:to revisit sites that we’ve been monitoring for the last two years, service temperature and salinity logging devices, and drill some coral cores that span the bulk of the 2015/2016 El Niño event.





We joined a team from Julia Baum ’s lab, who had been diving intensively for two weeks prior to our arrival, taking detailed photographs of the reef and collecting tissue samples from any survivors. The goal of their work is to decipher the recipe for coral resilience to extreme temperature stress, in order to aid the reefs of tomorrow weather future such extremes.





We landed to grim reports of extensive coral mortality, confirming our worst fears that the 9 months of continuous ocean warming associated with the largest El Niño event in history had taken their toll. Of the sites affected by the current global bleaching event, Christmas has been in the grip of extreme temperature stress for the most time, by a long shot.





My first dive was shocking. Above water it looked like the same island I’d been visiting for 18 years. But underwater it was a wasteland. As I descended to depth that day, my eyes would see things that my heart and mind couldn’t yet process.





Algae-coated dead coral on my first dive of the expedition, to 30ft on Christmas Island's south reef, April 2, 2016. In this entire view, there is only one small coral still alive - a half bleached/half dead Porites colony in the lower left. Credit: Kim Cobb. The reef was almost completely dead, with all but a few of the hardiest Porites colonies – mostly the smallest size class – coated in red/brown algae. These corals had lost their valiant battle against the elevated ocean temperatures months ago, most likely. I busied myself with the task at hand, retrieving a Conductivity-Temperature-Depth unit off the reef, swapping out the smaller package of sensors we’d co-located with the CTD, taking momentary solace in the fact that the instruments were still intact on the reef. Aside from their huge cost, their memory banks would tell us exactly how warm it had been on the reef in the preceding six months, across the peak of the event. It would have been a crippling loss, scientifically, had they been washed away. We have been swapping in new CTD units at this site since 2014, amassing one the longest, most detailed records of ocean conditions in this region. The reef was almost completely dead, with all but a few of the hardiest Porites colonies – mostly the smallest size class – coated in red/brown algae. These corals had lost their valiant battle against the elevated ocean temperatures months ago, most likely. I busied myself with the task at hand, retrieving a Conductivity-Temperature-Depth unit off the reef, swapping out the smaller package of sensors we’d co-located with the CTD, taking momentary solace in the fact that the instruments were still intact on the reef. Aside from their huge cost, their memory banks would tell us exactly how warm it had been on the reef in the preceding six months, across the peak of the event. It would have been a crippling loss, scientifically, had they been washed away. We have been swapping in new CTD units at this site since 2014, amassing one the longest, most detailed records of ocean conditions in this region.

The Conductivity-Temperature-Depth sensor after recording ambient conditions on the reef from Nov 2015 until April 2016, surrounded by dead coral. Credit: Kim Cobb.



My mind still reeling from that first dive, I was sure my second dive would be better. We were diving a site that I have visited on every single field expedition I’ve ever conducted, where the largest Porites coral colonies grow. These colonies are decades old, rising 1m or more off the floor of the reef, locking a remarkably accurate history of ocean temperatures in their skeletons. I had drilled one such colony for the TV documentary “ Years ofLiving Dangerously ”, wielding a huge hydraulic drill in the glare of two underwater cameras.





When I jumped in the water, it was clear my optimism was misplaced. The reef I knew like the back of my hand was unrecognizable. Of the five larger Porites colonies I had tagged and photographed in November – all still alive at that point, if not bleached – four were completely dead, and one was partially dead, hanging on by a thread. I was so overcome with emotion that I shed a few tears into my mask.





all dead. Favia: all dead. Montipora: all dead. In fact, the only things that seemed to be alive and well were, once again, smaller colonies of Porites averaging well under 1ft in diameter. Every good story needs a hero, and these corals were just that. It was as if nobody had told them that a record-breaking El Nino was still underway. My eyes were repeatedly drawn to these small pockets of color on the reef, while my science brain kicked into overdrive planning new science around these stalwart survivors. I would go on to tag and photograph these individuals, and collect small tissues samples that I took some time to swim the reef, taking in the destruction. Pocillopora:all dead. Favia:all dead. Montipora:all dead. In fact, the only things that seemed to be alive and well were, once again, smaller colonies of Porites averaging well under 1ft in diameter. Every good story needs a hero, and these corals were just that. It was as if nobody had told them that a record-breaking El Nino was still underway. My eyes were repeatedly drawn to these small pockets of color on the reef, while my science brain kicked into overdrive planning new science around these stalwart survivors. I would go on to tag and photograph these individuals, and collect small tissues samples that Danielle Claar , from the Baum lab, will sequence in the hopes of uncovering the secret to their unlikely survival.





When we return to the reef later this year, I will take small drill cores from these survivors, in order to document the story of this El Niño event from the perspective of the corals by analyzing the geochemical variations in their newly-laid skeletons. Such samples will allow me to make an apples-to-apples comparison to coral records of past mega-El Niño events, like the 1997/98 event, and to mega-El Niño events of the past centuries to millennia. And by comparing recent El Nino activity in the coral record against a long baseline of natural variability in older coral records, we hope to understand if and how climate change is affecting extremes in the El Niño-Southern Oscillation. Our preliminary results, published before the onset of this winter’s record-breaking event, suggest that El Nino events have become stronger as a result of anthropogenic climate change (see article here ).





I hope that the reefs at Christmas Island have the time they need to recover before the next big El Niño hits. It will take ten years or more for the reef to crawl back to even a shadow of its former self. Over this period, we will document its recovery in detail, as an opportunity to learn more about life after death for a reef crippled by temperature stress.