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.
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.
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.
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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.
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.