Cleaner air is (inadvertently) harming the Great Barrier Reef - Earth - Environment

Researchers found when the atmosphere was calm and skies clear, sulfate aerosol reduction made the biggest difference to coral bleaching. Credit: One Tree Island/ Robert Ryan

Air pollution is now recognized as one of the greatest threats to human health, contributing to an estimated 4.2 million premature deaths in 2019, according to the World Health Organization.

In and around port cities, ships are a major air pollution source. So, in 2020, the International Maritime Organization (IMO) took significant action to reduce air pollution and protect human health.

The IMO placed heavy restrictions on the amount of sulfur allowed in shipping fuel because it produces sulfur dioxide (SO₂) and sulfate particulate matter, which are linked to respiratory issues like asthma, wheezing and chest tightness.

They also cause environmental impacts through acid rain, which damages forests, crops and waterways.

The IMO policy has been successful, but has inadvertently introduced a new environmental problem: fewer particles in the atmosphere mean more of the sun's energy reaches the oceans.

Our new study, published in the journal Communications Earth & Environment, shows that this effect is also likely exacerbating coral bleaching at Australia's Great Barrier Reef.

How ship exhaust impacts the climate

Ships are the largest human-made source of sulfate aerosol over the world's oceans.

While these tiny particles can be harmful, they have the protective ability to reflect sunlight back to space. They can also help form clouds, which also reflect sunlight, helping cool the sea surface.

Since 2020, the IMO's mandate that shipping fuel must contain less than 0.5% sulfur, reduced sulfate aerosol from ships by about 80%.

But as the air became cleaner, the oceans suddenly began receiving more of the sun's radiation.

Sulfate aerosol reduction has been linked to other startling consequences, including reducing lightning and accelerating global temperature increases.

Coral reefs have recently been identified as the first global tipping point to be exceeded by climate change.

So, our research team wanted to specifically understand what the change in ship sulfate means for Australia's Great Barrier Reef.

More sunshine means more stress for corals

Coral bleaching is caused by heat, when warming oceans push corals beyond their thermal tolerance, causing them to expel the symbiotic algae they rely on for food and color.

Extra sunlight on the ocean surface warms the water even further and increases stress in the corals, especially during hot, calm and stagnant conditions (sometimes referred to as "doldrum conditions").

To understand the impact of the IMO's sulfur reduction on the Great Barrier Reef, we used a regional chemistry-climate model to calculate how much extra sunlight reached the ocean surface leading up to the 2022 mass bleaching event.

Under the IMO's sulfur restrictions, we found that sulfate aerosol from ships masked around three watts per square meter (W/m²) of incoming solar radiation.

If the same event had occurred prior to the ship sulfur regulations, sulfur aerosols would have masked 14 W/m² of solar radiation.

This means that up to a whopping 11 W/m² extra solar radiation reached the Reef through cleaner air.

The additional radiation equates to a sea surface temperature increase of 0.15°C, or an approximate 10% increase in coral bleaching severity.

And this is just the impact of local shipping—globally, cleaner ship fuels are expected to double the rate of global warming this decade, unmasking the heat from the greenhouse gases we've continued adding to the world's oceans.

Weather conditions matter

Our study also found that the impact of the local sulfur reduction on the reef wasn't the same for all weather conditions.

Periods dominated by cloud cover and strong south-easterly trade winds saw aerosols transported away from the reef more efficiently.

In these cases, the influence of ship emissions on the reef was much weaker.

However, when the atmosphere was calm and skies were clear—with the coral at its most vulnerable—the sulfate aerosol reduction made the biggest difference.

Increased urgency to consider novel interventions

The combined local and global impacts of the new regulations likely mean more bleaching stress for the Great Barrier Reef under cleaner air conditions.

Our research is also developing a potential intervention that emulates aerosol cooling of the ocean cooling effect using sea salt instead of sulfur.

Known as Marine Cloud Brightening, nano-sized sea-salt crystals—produced by spraying seawater over the ocean—make the clouds and sky more reflective, potentially mitigating severe coral bleaching.

Australia is leading the way in researching this futuristic technology.

Modeling suggests that using this technique over the Great Barrier Reef may delay the decline of the ecosystem by decades, buying precious time to for serious action to reduce greenhouse gas emissions.

The Great Barrier Reef is already facing intense pressure from repetitive marine heat waves, with mass bleaching recorded in six of the last eight summers.

But we also need to ensure that everyone has access to clean air, because a healthy environment is a fundamental human right.

For ecosystems vulnerable to climate change and enhanced solar radiation, like the Great Barrier Reef, action on air pollution must be matched by urgent greenhouse gas emission reductions and accelerated research into scalable interventions.

Only by doing this can we support this world heritage-listed ecosystem to survive and thrive in a sustainable future. 

Provided by University of Melbourne 

by Robert Ryan, Daniel Harrison and Robyn Schofield, University of Melbourne

edited by Lisa Lock, reviewed by Robert Egan 

Source: Cleaner air is (inadvertently) harming the Great Barrier Reef