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Reef Emissions Affect Climate

Image of reef

Reefs produce aerosols that affect rainfall locally.

By Graham Jones & Zoran Ristovski

Coral reefs produce a natural aerosol that creates clouds over the ocean and keeps sea surface temperatures stable – with implications for both reefs and rainforests.

In the 1970s, researchers from the Australian Bureau of Meteorology found very large concentrations of aerosol particles in the air above the Great Barrier Reef. At the time they guessed that the coral of the reef was responsible, but just what the aerosols were composed of was a complete mystery.

When we first read this research paper we knew we had to start some research on the climate aerosol dimethylsulfide (DMS) in coral reefs. Previous research indicated that the coral algae contained the DMS precursor substance DMSP.

Our first study was onboard a voyage through the Great Barrier Reef led by Dr John Church of CSIRO Marine and Atmospheric Research. The ship’s Master set anchor as close as he could to Bait Reef and Old Reef, and we detected high levels of atmospheric DMS.

This led to a major study at the Orpheus Island Marine Station, north of Townsville, where we measured the first seasonal variation of dissolved DMS in coral reef waters at Pioneer Bay fringing reef. We measured atmospheric DMS over this reef, and also found that Acropora species (staghorn coral), the most common coral in the Indo-Pacific, contains the highest concentration of DMSP of all corals.

We then went on to do further studies at the Nelly Bay fringing reef at Magnetic Island, which is more stressed by nutrients than the other reefs we studied. We found that less DMS was emitted from these human-impacted reefs.

In the past 10 years we have shown that coral mucus contains the highest concentrations of DMS on Earth and that, unlike phytoplankton in the ocean, they produce it continuously every day because it is essential for the normal metabolism of the algae in their tissues.

The role of DMS as a regulator of climate first came to light when scientists tried to explain how clouds were formed over the ocean. In order to form clouds, DMS must be emitted into the atmosphere and oxidised to form small sulfate aerosol particles. These tiny particles start attracting water vapour and form cloud droplets (Fig. 1). This is how stratocumulus clouds form over the ocean and keep temperatures cool by reflecting sunlight back into space (a climate feedback).

We believe that this is how clouds form over coral reefs. If these aerosol particles are not there then clouds will not form even though there is lots of moisture.

Recently we have proved that coral reefs in the Great Barrier Reef do in fact produce bursts of aerosol particles that are similar to the DMS–sulfur substances we have been measuring from reefs in the central Great Barrier Reef. However, we need to know whether this source of reef aerosol particles is strong enough to have a significant effect on the regional climate of coastal Queensland.

Busy highways are large sources of tiny particles coming from the exhausts of the vehicles travelling along them. However, the Great Barrier Reef emits as many DMS aerosol particles as 50 highways running side-by-side along the coast of Queensland.

What is very important is that particles coming from highways are toxic while the Great Barrier Reef ones are not. Given that DMS aerosol emissions influence cloud formation and precipitation, the potential significance of this natural source of DMS is obvious.

We believe that this process significantly affects the regional climate of northern Queensland, where most of Australia’s coral reefs and rainforests occur. We also believe that it is no coincidence that much of Australia’s rainforest lies adjacent to the most northern reefs, since the sulfur aerosol particles produced by the reefs can produce cloud droplets that lead to rainfall if conditions are suitable. Hence the reef and the rainforest live in a type of symbiosis, just like the algae in the coral.

However, a disturbing aspect of our recent research is that the corals shut down their production of atmospheric DMS when they are stressed by elevated temperatures and sunlight, potentially affecting this regional climate regulation. This suggests that the mass coral bleaching episodes in the Great Barrier Reef may reflect a shutdown in the production of these aerosol substances from the corals since we now know that enhanced levels of sunlight are part of the coral bleaching phenomenon.

Sunlight over the reef has been increasing over the past decade. This is exactly what we would expect if corals were shutting down the production of atmospheric DMS and cloud cover decreased over the reefs during mass coral bleaching episodes.

Further support for this link between DMS, clouds and sea surface temperature has recently come from scientists at the Australian Institute for Marine Science, who have discovered that coral reefs in the western Pacific and Great Barrier Reef are more likely to bleach than coral reefs in French Polynesia, Melanesia and Micronesia in the central Pacific, where the reefs are in a much more pristine environment and the cloud–sea surface temperature feedback is more able to function, keeping the waters cool.

We are now beginning to think that this natural process can affect our northern rainforests, since we now know that the coral-derived DMS in the cloud can also potentially affect rainfall on the adjacent land.

Graham Jones is an Associate Professor in climate science at Southern Cross University, Lismore. Zoran Ristovski is an Associate Professor in atmospheric science at Queensland University of Technology, Brisbane.