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Kiritimati’s coral reef die-off offers clues for conservation efforts


Beauty on the brink

Earlier this year researchers discovered most of the coral on Kiritimati, the world's largest atoll, was dead due to overheating from a record breaking El Nino. This week they're back and to their surprise there are tentative signs of life beneath the waves

For seven years, Julia Baum has been going to the coral atoll of Kiritimati to study one of the most astonishing wonders of the natural world.

But last week, she was dreading the possibility that she would find an underwater Eden in the act of disintegration.

"We'd already seen the mass grave that these beautiful coral reefs had turned into. Our fear in coming back was that in the past six months the entire structure would have crumbled," said Dr. Baum, an associate professor of biology at the University of Victoria, who spoke with The Globe and Mail from her research base on the island.

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Instead, she has found life among the ruins, where baby corals are struggling to rebuild after a massive climate-related die-off.

By analyzing what she has found, Dr. Baum hopes to glean information that will help scientists anticipate what's in store for reefs around the globe as oceans warm, and will inform efforts to help them survive.

For the more than 6,400 people who live on the island and depend on the reefs for subsistence fishing, what happens to the coral is more that just an academic question.

Kiritimati – also known as Christmas Island – is the world's largest atoll and host to one of the most pristine series of reefs in the world. Only the northwest side of the island is actively fished. What first drew Dr. Baum to this remote locale was the chance to compare and document what reefs are like when they are not disturbed by human impacts.

But last spring her research program took a dramatic turn when she and her team found that about 80 per cent of the corals on that atoll were dead because of bleaching. The culprit was the record El Nino of 2015 that raised ocean water temperatures as much as 2.5 degrees Celsius, well past the threshold that the reefs could comfortably endure.

Dr. Julia Baum hopes to glean information that will help researchers predict what’s in store for reefs around the globe as oceans warm.

Dr. Julia Baum hopes to glean information that will help researchers predict what’s in store for reefs around the globe as oceans warm.

Kristina Tietjen

"They were basically sitting in a hot-water bath for 10 months," Dr. Baum said. "That's never happened before anywhere that we know of on the planet."

El Nino, a periodic warming of equatorial waters in the Pacific, is a natural phenomenon. But evidence is mounting that climate change is making El Ninos more intense. That puts Kiritimati in a particularly vulnerable spot, where pulses of warm water overlaid against a backdrop of gradually increasing temperatures, rising sea levels and increased storm activity are threatening to kill off the coral while eating away the island.

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For Kim Cobb, a climate scientist at the Georgia Institute of Technology who studies the coral of Kiritimati to discern how conditions have varied there over thousands of years, the current situation offers a broader warning about what can happen when climate change nudges ecosystems – or human systems – over the brink.

"So much of it is a slow and steady march that's very difficult to see. But when we cross these thresholds, it's like a line in the sand and the system has an outsized response," she said.

A researcher surveys the reef on Kiritimati in 2013 before the El Nino bleaching event.

A researcher surveys the reef on Kiritimati in 2013 before the El Nino bleaching event.

University of Victoria

The mass die-off of coral at Kiritimati made headlines as a harbinger of things to come for reefs around the world. But the news is not all bleak.

In their first visit to the reef since documenting the die-off, Dr. Baum and her team have seen evidence that some coral managed to endure the El Nino heat stress. Her aim is to study this coral in the hope of understanding what gives some varieties their resistance to sudden temperature shifts.

It may also mean that there is hope for preserving reefs such as Kiritimati in the long term if the rate of global warming owing to greenhouse gas emissions can be successfully reined in.

"The big thing that I think we're seeing is the reef is trying to regain balance," said Kristina Tietjen, a research assistant on the expedition. "We just have to give it a chance to do that."

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The record El Nino of 2015 raised ocean water temperatures 2.5 degrees Celsius, killing 80 per cent of the corals on Kiritimati’s atoll.

The record El Nino of 2015 raised ocean water temperatures 2.5 degrees Celsius, killing 80 per cent of the corals on Kiritimati’s atoll.

Kristina Tietjen

Dr. Baum is careful to balance hope with the reality that the reefs of Kiritimati are facing a dire situation. Even if they can continue to withstand future heat stress, they also face growing threats from ocean acidification, another side effect of increasing carbon levels in the atmosphere, and humanity's unchecked exploitation of the world's oceans.

But in the short term, there is much to be learned from the message that nature is offering at Kiritimati, Dr. Baum said. Coral reefs are some of the most sensitive and diverse ecosystems on Earth. If they can be coaxed to hang on while humanity learns how to manage the planet, the outlook for all of life on the planet will be vastly improved.

"We could come out of the end of this century in a very positive state," Dr. Baum added. "But we have to start taking action now."


When coral are heated beyond their tolerance, the algae they contain break down and are expelled. This turns the colourful coral white – a phenomenon referred to as "bleaching." If temperatures return to normal quickly, the coral can recover, but if not, they are deprived of algae and eventually starve. An estimated 80 per cent of coral on Kiritimati was killed off by heat stress during the 10-month El Nino of 2015-16.

Danielle Claar

Studies of ancient coral at Kiritimati reveal climate patterns over a 7,000-year period, including the severity of El Nino, a cyclic warming ocean of Pacific equatorial waters. The effect of El Ninos can be discerned back to prehistoric times because the increased water temperatures and the higher rate of rainfall they bring have a measurable effect on coral chemistry. The data suggest El Ninos have become significantly more intense over the past century compared with the centuries that came before, an effect that researchers attribute to climate change owing to greenhouse gas emissions.

Danielle Claar/University of Victoria

An image of coral growing on the Kiritimati atoll before the massive die-off due to the record-breaking El Nino of 2015-16 shows a vibrant marine ecosystem. Corals are soft-bodied animals related to jellyfish that settle in one place and secrete calcium carbonate, forming structures that grow into impressive shapes. The living coral polyps occupy a thin layer on the surface of these structure. They live in a symbiotic relationship with algae, which convert sunlight into nourishment for the coral. The algae are also largely responsible for the colour of living coral.

Kieran Cox

Researchers at the University of Victoria have employed a variety of methods to monitor the health of different coral species on Kiritimati. Here, a ceramic tile has been deposited on the sea floor to provide a platform that young coral "recruits" can fasten onto to start a new colony. Most of the recruits will settle on the underside of the tile, where they are better protected from predators.

Kristina Tietjen

This week, researchers spotted a coral colony fighting for space amid a carpet of green algae that has taken over this mostly dead section of reef off


. Some varieties of coral have proved to be more resistant to heat stress than others. In the future, the cultivation of such varieties may help threatened reefs survive the long-term effects of climate change.

Kristina Tietjen

Children on


participate in a food web activity as part of an outreach project led by members of Julia


lab at the University of Victoria. Residents on the island face a double threat from climate change because of the degradation of coral reefs and the rise in the sea level. The island is part of the Republic of Kiribati, a series of 33 atolls and islands that collectively total about 800 square kilometres and is one of the poorest nations on Earth.

Maryann WatsoN

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