Coral Bleaching: Possible Long Term Solutions

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Coral reef

Coral reefs may be damaged by warmer waters. Image by Julia Falkner.

According to a new modeling study published in Nature Climate Change, the use of Solar Radiation Management (SRM) can help in reducing coral bleaching. This is because SRM can lead to lower ocean temperatures, which in turn decreases the bleaching.

SRM use could be more effective in saving coral reefs than a drastic decrease in greenhouse gases emissions reduction.

Warmer and More Acidic Oceans

In recent years, scientists published several studies and reports which indicated that, overall, oceans are becoming warmer and more acidic.

According to some studies, the increase in the temperature of sea waters is linked to the warmer atmospheric temperature, caused by the anthropogenic emissions of greenhouse gases such as carbon dioxide (CO2). A higher CO2 concentration in the atmosphere will also lead to a higher acidity of the seas (ocean acidification), as when dissolved in water, carbon dioxide reacts to form an acidic molecule (carbonic acid).

Effects on Coral Reefs – Bleaching

Coral reefs are ecological communities typical of the tropical seas.

For some years many researchers have expressed their concern regarding the health of coral reefs due to changes occurring in the seas, as described above.

One of the most worrying phenomena is coral bleaching. This involves the expulsion into the waters of zooxanthellae, an algae species which normally lives inside the coral. According to some studies, coral bleaching affects more than 90% of the corals and can cause them serious damage, including death. Bleaching is favored by higher Sea Surface Temperatures (SSTs).

Possible Solutions

Several investigations were done to see which possible long term solutions and mitigation actions could be adopted to avoid or, at least minimize, coral bleaching.

Researchers from the University of Exeter (UK), the Carnegie Institute of Science (Stanford, US) and the University of Queensland (Australia) performed an interesting modeling study on this topic; they published their results in Nature Climate Change on the 25th of May 2015.

Sun

Solar Radiation Management can lower atmosphere temperature. Image by Violetta.

Solar Radiation Management (SRM)

Decoded Science spoke to Dr. Lester Kwiatkowski, leading scientist in the study.

“In our work we studied the potential of a geoengineering solution; more specifically, we considered the effect that Solar Radiation Management (SRM) techniques could have on SSTs and, hence, on coral bleaching.

The idea of SRM is to try to decrease the amount of solar energy reaching the Earth’s surface by placing an aerosol-like shield in the high atmosphere. The main limit of this technique is that its use does not cause a decrease in CO2 or other greenhouse gases in the atmosphere, but it just lowers its temperature.

If we consider the implications for the oceans, this means that we will have lower SST but we will also still have acidic waters.”

Different Scenarios

Dr. Kwiatkowski and his coworkers simulated two possible scenarios:

  • In the first one, no SRM was implemented; at the same time, however, global mean temperatures are stabilized at approximately 2 oC higher than pre-industrial levels. This is the most ambitious greenhouse gas mitigation scenario recommended in the Fifth Assessment Report published by the Intergovernmental Panel on Climate Change (IPCC) (Representative Concentration Pathway 2.6 – RCP 2.6).
  • In the second scenario, they considered a SRM situation with an aerosol derived from sulfur dioxide. The mean global temperature value is stabilized at a lower level in this scenario despite higher background greenhouse gas concentrations (Representative Concentration Pathway 4.5 – RCP 4.5).

In both cases, they modeled the SSTs from the year 2020 to 2070.

Oceans

Oceans are getting warmer and more acidic. Image by U.S. Fish and Wildlife Service.

SRM: Lower Ocean Temperatures

“In the first scenario, under RCP 2.6, the lower global temperatures are due to a drastic reduction in the greenhouse gas emission; in the second case, on the contrary, less strict cutbacks on the emissions were implemented.

Despite this, however, the SSTs we calculated were lower for the second scenario; this was due to the combined effect of RCP 4.5 and that of the SRM.” Dr. Kwiatkowski said.

Effect on Coral Bleaching

To estimate the effect on the bleaching, scientists had to consider also how CO2 concentrations influenced this. Dr. Kwiatkowski explains:

“With RCP 4.5 + SRM scenario, the water temperatures were lower, which is good. The CO2 concentration, however, was higher; this is bad for ocean acidification. For the RCP 2.6 scenario, it was the opposite.

However, even considering the higher CO2 levels, our calculations showed that coral bleaching decreased under the RCP 4.5 + SRM; this indicates that coral bleaching is more affected by the temperature than by the acidity of the waters. This was a surprise for us.

We did different simulations, considering different sensitivity of the corals to the acidity; even when we assumed the highest possible coral acid sensitivity, the RCP 4.5 + SRM still resulted in lower coral bleaching.”

Important Understanding

Coral bleaching is surely a very serious problem, which can affect corals and the whole marine ecosystem, as well as the local communities relying on them. It is, therefore, essential to understand how this happens, and which are the key parameters affecting it, to be able to implement remediation strategies.

According to Dr. Kwiatkowski:

“These results are important as they give us a better understanding of coral bleaching; we discovered aspects of this phenomenon that were unknown before. All of this can help us in developing measures which ideally reduce it, or at least minimize it.”

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