Some Coral Reefs Could Weather Climate Change Better Than We Thought

Unprecedented ocean temperatures are triggering mass coral bleaching events across the world

by Liam Lachs and The Conversation
Originally Published: 
HAIKOU, March 18, 2021-- Photo taken on March 18, 2021 shows coral and schools of fish in the sea of...
Xinhua News Agency/Xinhua News Agency/Getty Images

Unprecedented ocean temperatures are triggering mass coral bleaching events across the world. This year, the world’s third-largest barrier reef, in Florida, is already being hit hard.

New research by my colleagues and I offer a glimmer of hope: coral reefs we studied in the Pacific appear to have increased their resistance to high ocean temperatures. But this can only improve their long-term futures if there is strong global action on reducing carbon emissions.

We know that corals will need to withstand rising ocean temperatures to survive under climate change. And we know reef-building corals are acutely sensitive to even small increases in temperature. What we don’t yet know is whether their “thermal tolerance” — essentially their ability to handle high temperatures — can keep pace with ocean warming.

In normal conditions, corals live in symbiosis with microscopic algae housed within their tissue. These algae give corals their beautiful colors and provide them with food through photosynthesis, just like plants (corals are animals, don’t forget).

However, this relationship breaks down when it’s too hot: the microalgae are expelled, leaving the corals stark white or bleached, which usually leads to death. Extreme temperatures can even kill corals outright, bypassing the gradual bleaching process.

Mass bleaching in the Maldives in 2016 led to many corals dying.

Stephen Bergacker, Author provided

In our new study published in Nature Communications on the coral reefs of Palau, a nation of more than 300 small islands in the western Pacific Ocean, we found that the tolerance of corals to warm conditions has likely increased over the past three decades.

Testing thermal tolerance

Palau experienced intense marine heat waves in 1998, 2010, and 2017, but interestingly, each successive event led to less coral bleaching. Such a phenomenon has also been recorded in Australia’s Great Barrier Reef, southeast Asia, and French Polynesia.

Is this evidence that coral communities are adapting to hotter temperatures? We set out to test whether thermal tolerance has likely increased for at least those reefs in Palau, and if so, how quickly.

Our international team of researchers designed a simulation study, drawing on 35 years of sea surface temperature data and historical observations of bleaching. We found that the thermal tolerance of the coral communities in Palau has likely increased at 0.1 degrees Celsius/decade. That’s slightly less than the increase in global temperatures (about 0.18 degrees Celsius/decade) but does suggest these coral reefs have an innate capacity for climate resilience.

Researcher conducting a 3D mapping survey of a coral reef in Palau.

Eveline van der Steeg, Author provided

How coral reefs are adapting to warmer oceans

More work is needed to pin down exactly what has happened, but there are various mechanisms that could explain this.

One involves the turnover of species. There are hundreds of different coral species, each with a unique evolutionary history and life strategy. Some, like branching Acropora, are fast-growing yet sensitive to temperature, while others, like massive Porites, are slow-growing but more stress tolerant.

Severe heatwaves can weed out the sensitive species, leaving the coral reef dominated by the tougher ones, which can compromise important ecological functions like reef growth and habitat provision for seafood species.

The second mechanism is genetic adaptation. Thermal tolerance is a complex trait that is likely influenced by thousands of genes, but most corals only have some of these. Following Darwin’s theory of survival of the fittest, natural selection can choose the winners under climate change. Over multiple generations and many rounds of selection, thermal tolerance genes can become more prevalent and thus increase the thermal tolerance of species populations.

The final explanation involves individual acclimatization. Even within the lifetime of a single coral, its ability to survive thermal stress events can change. As the saying goes, “what doesn’t kill you makes you stronger”, and so, being exposed to low-level thermal stress can later improve chances of survival under high-level thermal stress.

a) Community of corals with high species diversity; b) two corals of the same species with contrasting bleaching susceptibility; c) the symbiotic microalgal community housed within coral tissues.

Liam Lachs; Laurie Raymundo; OIST, Author provided

To make things more complicated, all of these processes — species turnover, genetic adaptation, and acclimatization — can also occur in the microalgae communities living within each coral. Scientists like me will need to disentangle the mechanisms that have driven potential shifts in thermal tolerance in Palau and elsewhere.

What does the future hold?

Can coral thermal tolerance continue increasing into the future? If so, then will it be fast enough to keep pace with ocean warming? Our study tackles these questions using high-resolution future temperature projections from 17 global climate models.

Projections of coral bleaching in Palau: if the Paris Agreement is achieved, in a middle-of-the-road scenario, and a worst-case scenario. An increase in thermal tolerance (blue line) buys some time, but coral reefs will still struggle unless climate change is halted.

Lachs et al, Author provided

Our analysis reaffirms the scientific consensus that, ultimately, the future of coral reefs depends on rapidly reducing carbon emissions. However, if coral thermal tolerance can continue rising, then bleaching could be avoided on some reefs or at least delayed.

Our study and others have identified reefs with some level of innate climate resilience. This might buy us some time, but securing a future for coral reefs still hinges on rapid climate action. As our oceans get hotter, fewer reefs will escape bleaching conditions.

There are promising conservation measures, restoration efforts, and more experimental interventions, such as selective breeding to increase thermal tolerance. All of these might help corals persist into the future, but reducing carbon emissions is ultimately the only sure bet.

This article was originally published on The Conversation by Liam Lachs at Newcastle University. Read the original article here.

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