Coral Reefs in the Warming Ocean

Coral reefs are integral ecosystems that are extremely vulnerable to the effects of climate change. For World Ocean Day, we’ll look at why the ocean is so important to all life on Earth and how global warming threatens it. In response, we’ll look at how an MDPI researcher is approaching coral reef restoration, which is vital for protecting marine life.

The 1987 Brundtland Report highlighted that the ocean lacked a strong voice and was often side-lined in discussions about the environment. Despite the ocean making up 70% of the planet, nations were reluctant take responsibility for it.

With this in mind, World Ocean Day was proposed at the Earth Summit in 1992 and was initiated by The Ocean Project in 2002. In 2022, it worked with over 10,000 organisations and businesses to coordinate over 150,000 events in over 140 countries.

In 2023, it will be held on 8 June, and aims to spread awareness of how human activity impacts the ocean and to develop a global movement in response. Additionally, the initiative promotes the #30×30 petition campaign, which encourages nations to commit to protecting 30% of the ocean by 2030.

Blue planet

The ocean plays a key role in sustaining climatic, atmospheric, and ecological processes.

The sun evaporates water, which condenses to form clouds that release moisture as rain. All life depends on this water cycle. Because water absorbs and loses heat slowly, it helps to balance global temperatures by absorbing heat in summer and releasing it into the atmosphere for circulation in winter.

It is home to at least 230,000 known species in varying habitats and nearly half the human population depends on it for their livelihoods.

The ocean has helped slow the shocks of climate change too by absorbing 30% of human-emitted carbon dioxide. However, this, amongst other factors, is negatively impacting the ocean.

Climate change

Rising levels of carbon dioxide and other greenhouse gases are preventing the heat radiating from Earth’s surface from being released into space. This heats the atmosphere and increases the radiation on Earth. Due to the ocean’s large surface area, it is the main sink of this extra heat. Water also has a high heat capacity, meaning it absorbs a lot of heat before its temperature increases. Thus, the ocean is helping to slow the speed of global warming, by absorbing excess heat, but this is at the cost of its carbon dioxide absorption.

The cooler the ocean’s water is, the greater its capacity to dissolve carbon dioxide. As rising greenhouse gas emissions warm the ocean, it will be able to store less carbon dioxide, thus creating the conditions in which global warming could escalate at increasing speeds.

Ocean heatwaves are part of the natural variability of the climate. However, “global warming is triggering more frequent and more extreme marine heatwave events”. This puts intense stress on marine life that is already having to adapt to rising temperatures.

Alongside temperature increases, the ocean has become 30% more acidic over the past 50 years, dead zones are forming, in which entire areas of the ocean are without life because of deoxygenation, and plastic pollution will last hundreds if not thousands of years whilst leaching toxic chemicals. For more information about plastic pollution, which was the focus of World Environment Day 2023, see our article “Plastic Pollution Needs Addressing”.

All these changes are making coral reefs, a small but integral part of ocean ecosystems, extremely vulnerable.

Coral reefs

Corals reefs cover less than 1% of the ocean floor, yet they support over a quarter of all marine life.

They make up an intricate ecosystem of species that interact with each other and the environment. Reefs are centred around coral, which is a class of animal that is related to jellyfish and sea anemones. Stony coral has a hard skeleton that forms the bedrock of reefs, containing hundreds of thousands of polyps that draw calcium from seawater and solidify into skeletal structures. Only the surface is living, with the mass beneath being this calcium carbonate skeleton. Other corals include octocorals, which have no skeleton and grow fleshy plants.

Within the polyps are microscopic algae called zooxanthellae. The coral provides a habitat, and the algae provides food for the coral through photosynthesis. For this process, corals live in clear and shallow waters to receive sunlight. Coral polyps are transparent; it is the algae that colours them.

Why are they so important?

Coral reefs serve “as a centre of activity for marine life”, including many species of fish, sponges, and crabs. They provide habitats and food for over 1 million aquatic species.

Losing them would cause a domino effect with unpredictable consequences, drastically changing food chains and lowering populations of coral-dependent fish. This would hugely impact commercially harvested fish populations and therefore coastal economies. According to the UN, approximately 1 billion people depend on coral reefs for their food and livelihoods.

Furthermore, they protect coastal infrastructure from storms and floods by acting as a buffer for waves. Losing them would push coastal towns further inland and damage tourism industries.

But why are coral reefs so at risk?

Global warming and coral bleaching

When stressed by changes in temperature, light, or nutrients, coral expel the algae living in their tissues. This is called coral bleaching because they turn completely white. When this happens, they are not dead, but they become extremely vulnerable as they’ve lost their primary food source whilst already under a lot of stress.

Once corals die, “reefs rarely come back”; they struggle to reproduce and quickly deteriorate.

Global-warming-triggered effects, such as ocean warming and increasingly intense and frequent heatwaves, are leading to “more frequent coral bleaching events worldwide”.

Saving coral reefs

The current paradigm for tackling coral reef loss is the 50 Reefs Initiative. This UN-backed approach involves focusing on the 50 coral reefs sites that are most likely to survive in the future. This involves conservation efforts and the aim to repopulate the neighbouring reefs over time.

The MDPI article “Coral-Focused Climate Change Adaptation and Restoration Based on Accelerating Natural Processes: Launching the “Reefs of Hope” Paradigm” takes a critical view of this approach. Given we are in an emergency, the author, Austin Bowden-Kerby, claims we should scrutinise the current paradigm and help bolster it.

Let’s look at how this author has used open access to share his proposals for restoring coral reefs.

A grave situation

Bowden-Kerby claims the approach primarily “fails to protect the more stress-adapted corals that are the best sources of adaptive capacity for future conditions”.  He stresses that the conventional coral reef restoration methodology is unsuitable because increasing temperature stress must be accepted as an inevitability.

The corals that are surviving in the hottest temperatures are at their limit, so, if we lose these, we lose those best-suited to survive global warming and the changes it will trigger.

Coral reefs are often called the “rainforests of the ocean”, so the author bases his approach on reforestation techniques.

Reefs of Hope

Reforestation techniques focus on replanting bigger tree seedlings into smaller patches. The larger trees become nuclei for recovery, attracting life and facilitating succession. This establishes attractive habitats and encourages structural complexity. Rather than conserving the trees, it helps re-establish the ecosystems that the trees support.

Because of the “individual variation” observed in coral bleaching events, that some coral die and some survive, the larger surviving corals in hotter conditions are to be transplanted. This prioritises moving the climate-adapted and bleaching-resistant corals to cooler areas before they eventually succumb to rising temperatures. Here are the steps he proposes:

  1. Translocate the corals from hot reefs approaching the maximum temperature thresholds. Aim for a diverse mix.
  2. Move them to nursery banks in cooler areas. These are areas for coral to grow whilst protected from storms, predators, and other stressors.
  3. Harvest and trim them to produce second-generation coral with the adaptations of heat protection and coral-bleaching resistance.
  4. Create genetically diverse “nucleation patches” of resistant coral, which will attract life and build complex ecosystems.
  5. Additionally, establish patches of these adapted coral amongst populations of bleaching-sensitive corals to share symbionts and spread resistance.

Restoring coral

Like in reforestation, this would help restore structural complexity and gather diverse lifeforms. This is because larger corals trigger the “grazing halo phenomenon”, in which herbivorous fish or sea urchins shelter in corals, graze outwards, and create a barren algal-free zone around the coral. These cleared zones are ideal for recruiting coral larvae, thus triggering the spread of coral reefs.

Choosing the strongest coral may mean choosing a less ideal coral for certain environments or aims, but sacrifices must be made. As the author highlights, continuing to plant corals without looking at the facts of global warming is like “continuing to plant trees for reforestation in the face of an approaching firestorm”.

With this approach, we ensure there are still coral reefs, and all the life they support, as temperatures inevitably rise and heatwaves become more frequent. It focuses on a small part of the ocean that supports a large portion of it, hopefully creating more time for us to tackle global warming.

World Ocean Day

MDPI makes all its research immediately available worldwide, giving readers free and unlimited access to the full text of all published articles. As World Ocean Day highlights, protecting the ocean requires a coordinated and concentrated global effort. Open access supports this by ensuring vital work by researchers like Bowden-Kerby is available to all.

Are you looking to contribute to this global effort? Why not consider submitting to our journal Oceans; see our full list of journals for more.

Coral reefs are fragile yet integral parts of the ocean, meaning they are fragile yet integral parts of our world. It’s critical that we protect them if we want to protect ourselves.