Pacific coral reefs have naturally increased their heat tolerance

Although coral reefs are home to remarkable levels of biodiversity, they currently face unprecedented declines due to marine heatwaves and subsequent mass coral bleaching and mortality events. Thus, to survive climate change, coral communities need to adapt to progressively more intense and frequent heatwaves. 

Yet, the rate at which the thermal tolerance of coral reefs can increase naturally and whether it can match the rapid pace of ocean warming has remain largely unknown.

Some corals have increased their heat tolerance

By investigating historic mass bleaching events, a team of researchers led by Newcastle University has found that coral reefs in Palau, an island nation located in a remote part of the Pacific Ocean, have increased their thermal tolerance and adjusted to higher ocean temperatures. 

This could reduce future bleaching impacts of climate change, but only if global carbon emissions are cut down.

Future trajectories 

Drawing on decades of field observations, the experts modeled a variety of possible future coral bleaching trajectories for Palauan reefs, each with a different simulated rate of thermal tolerance enhancement. 

The investigation revealed that if thermal tolerance continues to increase during this century, major reductions in bleaching impacts are possible. However, the severity of future coral bleaching depends on carbon emissions reductions. 

For instance, high-frequency bleaching can be fully mitigated for some reefs under low-to-middle emissions scenarios that would fulfill the Paris Agreement. If the emissions remain higher though, such bleaching impacts are unfortunately unavoidable.

The scientists tested different levels of global action to mitigate climate change and reduce carbon emissions, by taking into account four emissions scenarios and their impact on future coral bleaching projections. 

Natural resilience 

The analysis showed that the thermal tolerance of corals in Palau has increased at a rate of 0.1 degrees Celsius per decade since the late 1980s, suggesting that natural mechanisms – such as genetic adaptation or acclimatization of corals and their symbiotic microalgae – may have contributed to the enhancement of thermal tolerance.

“Our study indicates the presence of an ecological resilience to climate change, yet also highlights the need to fulfil Paris Agreement commitments to effectively preserve coral reefs,” said lead author Liam Lachs, a marine scientist at Newcastle. 

“We quantified a natural increase in coral thermal tolerance over decadal time scales which can be directly compared to the rate of ocean warming. While our work offers a glimmer of hope, it also emphasizes the need for continued action on reducing carbon emissions to mitigate climate change and secure a future for these vital ecosystems.”

Climate action is still needed

“We know that coral reefs can increase their overall thermal tolerance over time by acclimatization, genetic adaptation or shifts in community structure, however we know very little about the rates at which this is occurring,” added co-author James Guest, a research fellow in Coral Reef Ecology at the same university. 

“This study uses data from a remote Pacific coral reef system and estimates the rate of increase in tolerance since the late 1980s. The results provide some hope that reefs can keep up with increasing temperatures, but only if strong action is taken on climate change.”

Yet, as co-author Simon Donner – an expert in Climate and Coastal Ecosystems at the University of British Columbia – argued, this resilience can also come at a cost, in terms of reduced reef diversity and growth. 

“Without sharp cuts in greenhouse gas emissions over the next two to three decades, the reefs in the Pacific won’t provide the resources and protection from waves that Pacific peoples have depended upon for centuries,” he concluded.

The study is published in the journal Nature Communications.

More about Pacific coral reefs

Pacific coral reefs are among the most diverse and complex marine ecosystems in the world. 

Location

They are predominantly found in the Pacific Ocean, stretching from the eastern coasts of Australia to the islands of Polynesia, and from the Philippines down to the Solomon Islands.

Biodiversity

The Pacific reefs are home to thousands of marine species, from small colorful fishes like clownfish and damselfish to larger creatures like sharks and rays. 

Coral reefs, in general, are believed to support 25% of all marine life, despite covering less than 0.1% of the ocean’s floor.

Coral bleaching

Like other coral reefs globally, Pacific coral reefs have been subjected to the damaging effects of coral bleaching, primarily caused by rising sea temperatures due to climate change. 

Coral bleaching occurs when corals expel the algae (zooxanthellae) that live inside their tissues, causing the coral to turn completely white.

Threats

Aside from coral bleaching, these reefs face various threats, including overfishing, pollution, sedimentation, coastal development, and the increasing acidity of oceans.

Importance

Coral reefs act as nurseries for many marine species. They also play a vital role in coastal protection by reducing the impact of waves and storm surges. Furthermore, they are of significant economic importance for many Pacific Island nations due to tourism and fisheries.

Conservation

Numerous efforts are underway to conserve and protect Pacific coral reefs. This includes the establishment of marine protected areas, research on coral resilience and restoration, and local community involvement in reef management.

Cultural significance

For many Pacific Island communities, coral reefs have significant cultural and spiritual importance. They are often integrated into local customs, legends, and daily life.

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By Andrei Ionescu, Earth.com Staff Writer

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