How cloud brightening might save the Great Barrier Reef

In this guest post, Olivia Baillie, a master’s student who took my Solar Radiation Modification course at UCL explains the potential of Marine Cloud Brightening to alleviate impacts on coral reefs.

Coral reefs are on the precipice of extinction, and human intervention in the climate may be the only way to save them. The ‘emissions gap’ between globally adopted climate policies, and the measures needed to limit warming to 1.5°C above pre-industrial levels is vast and expected to widen. With warming reaching 1.26°C in 2022, it is becoming increasingly clear that we are set to exceed our most optimistic climate goals, and could surpass 2.5°C of warming by 2100.

The speed and magnitude of this warming will devastate already vulnerable and heat-stressed communities and ecosystems, including coral reefs which are one of the world’s most sensitive (and important) ecosystems.

In addition to being one of the most diverse habitats on the planet, coral reefs support 25% of global fish species. They are also very important for human welfare and the economy. Over 3.3 billion people rely on fish for more than 20% of their daily protein intake, and the 70 million tourist trips to coral reefs each year are valued at $36 billion per annum.

Despite their vital importance, little has been done to protect corals against the deleterious impacts of climate change – particularly rising sea surface temperatures. Due to this, there has been increasing support and research into more ‘radical’ interventions, which some believe may be the only way to save the planet and its inhabitants.

Coral Bleaching

When ocean temperatures increase, especially if this occurs rapidly, corals become stressed and expel the tiny algae (called zooxanthellae) that live in their cells. The coral is then considered ‘bleached’ due to the stark white skeleton that becomes visible (Figure 1). Algae are essential to coral’s survival as they photosynthesise and provide the coral with energy. Without it, they starve and eventually die.

Figure 1. Comparison of healthy and bleached coral. Greenpeace.

Corals can recover however, this can take years and research has shown that they experience bleaching events 5 times as frequently as in 1980, primarily due to increased global sea temperatures. This coupled with an increase in their ‘normal’ temperature conditions means that corals are constantly stressed and do not have a chance to recover before the next bleaching event.

As a result, the global population of corals is declining rapidly and there is a very real possibility that reefs will disappear in our children’s lifetimes, even if we limit warming to 1.5°C.

2016 and 2017 Bleaching Events

The world’s largest coral reef, located off the north-eastern coast of Australia, experienced intense bleaching events in 2016 and 2017, with 29% and 22% of its corals dying respectively (Figure 2). Triggered by the extreme heatwave, 54.8% of the entire reef was ‘severely bleached’ after the 2016 event. The north fared particularly badly, reporting that 81% of the 522 sites surveyed were severely damaged.

Figure 2. The Great Barrier Reef, subdivided into three sectors by the severity of bleaching. Britannica.

The scale of the damage was unlike anything previously witnessed, both spatially and in terms of the very low percentage of corals that were not bleached (a mere 10% in comparison to more than 40% in previous events).

Reef Restoration and Adaptation Programme

In addition to widespread public dismay, the bleaching event of 2016 brought coral reefs to the forefront of scientific discussions on adaptability, climate change and ‘geoengineering’. The latter involves purposefully altering the climate to mitigate or counteract some of the effects of climate change.

One of the most promising, regional ideas to protect the Great Barrier Reef (GBR) was put forward by Dr Daniel Harrison, a senior lecturer at Southern Cross University. Harrison proposed the ‘Reef Restoration and Adaptation Programme’ (RRAP), which includes the use of marine cloud brightening (MCB). MCB involves spraying tiny particles of sea-water into the underside of clouds to make them more reflective.

Warm clouds are a collection of small water droplets, and a cloud with a higher number of smaller droplets is brighter than one with larger, fewer droplets (even if the total volume of water is the same). Droplets form around microscopic airborne particles called ‘aerosols’, which can be anything from sulphates to ash to sea salt.

MCB mimics this process by using sea salt (which is readily available and is not polluting) as cloud condensation nuclei to increase the number and decrease the size of clouds’ water droplets. This will result in brighter, more reflective clouds over the ocean.

Figure 3. Field trial vessel spraying sea salt into the air to brighten clouds. RRAP.

Whilst the idea of MCB may appear radical, it draws inspiration from our current environmental impact. We have inadvertently been brightening the clouds through our polluting activities and ship trails for decades.  The IPCC estimate the release of aerosols has offset 30% of warming caused by greenhouse gas emissions.

A promising, but incomplete solution

Harrison and his team conducted the first field trial in 2020 and proved MCB’s mechanism to be feasible.  A second trial the following year focused on understanding the behaviour of the atmosphere over the GBR during the summer, when the risk of bleaching is at its highest. It also provided insight into how MCB could be used most effectively along the reef.

MCB has often be criticised as a global geoengineering technique for its ‘patchiness’ - the inability to produce an even cooling effect across the planet. However, it has more recently been considered as a regional or local intervention to address specifical, smaller-scale issues.

One of the most enticing things about using MCB on a regional scale is that it can be turned on and off when heatwaves hit, providing emergency protection without distracting policymakers and governments from the root cause of warming: CO₂ emissions.

While MCB seems promising as a potential means of reducing the threat to the Great Barrier Reef, it is not a complete solution. The reef is threatened by a host of environmental problems, including land pollution, coastal development and ocean acidification, none of which can be combatted using MCB. It is vital that if any form of geoengineering is used to improve the reef’s health, it is not seen as a replacement for reducing emissions.

FIN