Wednesday, November 4, 2015

Into the El Niño sauna

by Kim Cobb

I’ve been waiting 18 years for this moment.

You see, I study the El Niño-Southern Oscillation (some would say I’m obsessed with it), and this is the largest El Niño event since 1997/98, which itself was the largest El Niño on record. During that event, I was a beginning graduate student on a large research cruise to Christmas Island. I had no context for the conditions I was witnessing at the time – the white corals, the huge swells on the leeward side, the constant rain. Nobody on the cruise fully grasped the magnitude of the event that would peak that very month.

Fast-forward 18 years, and I am leading the first of several field expeditions designed to document the effects of this massive El Niño event on the coral reefs I’ve been studying for my entire career. The event itself will be gone in the blink of an eye, but its effects on coral reef ecosystems will likely last a decade or more.

At 2N, 157W, Christmas Island lies at the epicenter of the current El Niño event. NOAA has sounded the alarm on a global coral bleaching event that is currently underway, and Christmas Island has been under an Alert Level 2 (indicative of “Mortality Likely”) for the last three months (Figure 1). With ocean temperatures projected to remain warm over the next 1-2 months before cooling into the spring, most of the corals here will likely bleach. When corals lose their colorful, symbiotic algae, they lose a critical source of energy and become much more vulnerable to disease and infection. If bleaching lasts for more than a few months, corals are unlikely to survive the episode. In 1997/98, I witnessed widespread bleaching and mortality – although nobody was around to do an official survey. And over the last 18 years, I’ve watched the reefs recover, slowly but steadily.  
Figure 1. Map of coral bleaching projections for the last week, from NOAA Coral Reef Watch, showing a high likelihood for bleaching at Christmas Island.
In my lab, we use geochemical variations locked in coral skeletons from Christmas Island to reconstruct a history of El Niño-Southern Oscillation (ENSO) variability over the last centuries to millennia. Our lab’s data show that recent ENSO variability is significantly stronger than ENSO variability over the last 7,000 years, indicating that greenhouse gases may be driving an intensification in ENSO extremes (see related publication here). But what happens when the archive I rely on to study past El Niño events is threatened by a very strong El Niño event? I don’t know, but this winter, I intend to find out.

Ever since spring 2014, I’ve been waiting for the fledgling El Niño event to take shape. In late August of this year, after a decisive push from the atmosphere in the right direction, the breath-holding gave way to a flurry of preparations for the upcoming field season. Over the course of three planned expeditions, my lab’s resources, and those of my collaborators, will be stretched to the very limit (and beyond, in some cases).

Our mission for the current trip is multi-fold, and incredibly ambitious.

Our top priority is to service and/or install a wide variety of environmental logging devices, including a high-end weather station, as well as ocean temperature and salinity loggers, some of which were installed in Summer 2014. We will collect hundreds of rainwater and seawater samples to document how this event has changed the geochemical and isotopic properties of the environmental waters - changes that the corals record in their skeletons and that we rely on to document past El Niño events. With this trove of data, we hope to quantify how the physical and chemical environment changed before, during, and after the event.

Our second objective is to document the health of the coral reefs on Christmas Island by systematically photographing transects of corals previously photographed by our collaborator Julia Baum and her students on previous expeditions to the island. When her team was here in July 2015, her graduate student Danielle Claar photographed a variety of bleached corals, estimating total bleaching at roughly 30% (Figure 2). We expect much more extensive bleaching this time around, as ocean temperatures have only increased since that time.
A bleaching Acropora colony in upper left, along with fully bleached Pocillopora colonies, in July 2015. Photo taken by Danielle Claar.
The field team includes my new NOAA Postdoctoral Fellow Alyssa Atwood, my graduate student Pamela Grothe (a seasoned Christmas Island expeditioner), and long-time undergraduate researcher Shellby Miller. Yes, this is an all-female research team!
From left:  Pamela Grothe, Kim Cobb, Alyssa Atwood, and Shellby Miller.
Stay tuned for updates from the field (internet permitting - I blame the unusually slow internet on El Niño, of course).