A new solution has been proposed for the forthcoming crisis of the coral reefs in warming tropical oceans: blot out some of the sunlight.

Scientists from the US, UK and Australia suggest a form of climate engineering called solar radiation management (SRM), which involves pumping fine particles known as aerosols into the stratosphere reflect incoming sunlight.

SRM has already been proposed as a mechanism for cooling the Artic to preserve its sea ice and glaciers, and for wide scale ‘whole earth’ cooling. But the most pressing purpose for the technology may be to save the world’s coral reefs.

If sea temperatures rise just 1°C to 2°C above the normal summer high, something gruesome happens to the coral reefs: they turn white in a process known as ‘bleaching‘.

This is because the high temperatures disupt the usual symbiotic relationship between living coral and the photosynthetic algae that sustains it. The coral expels the colourful algae, turning white in the process. Although the coral does not immediately die, it is deprived of its food supply and weakens, ultimately dying if the water remains hot for a prolonged period.

Even if we reduce emissions, the coral will still need to be saved

Lester Kwiatkowski – a researcher with both the University of Exeter in the UK and the Carnegie Institution for Science in the US – and colleagues have published their findings in Nature Climate Change.

Human-induced global warming because of the burning of fossil fuels and land-based emissions is set to raise temperatures enough by 2050 to bleach and degrade 90% of the world’s coral reefs, they argue – even if we succeeed in reducing emissions, they write:

“Widespread bleaching, affecting >90% of global coral reefs and causing coral degradation, has been projected to occur by 2050 under all climate forcing pathways adopted by the IPCC for use within the Fifth Assessment Report.

“These pathways include an extremely ambitious pathway aimed to limit global mean temperature rise to 2 °C, which assumes full participation in emissions reductions by all countries, and even the possibility of negative emissions.

“The conclusions drawn from this body of work, which applied widely used algorithms to estimate coral bleaching, are that we must either accept that the loss of a large percentage of the world’s coral reefs is inevitable, or consider technological solutions to buy those reefs time until atmospheric CO₂ concentrations can be reduced.”

And after 2050 things are only expected to get worse: by 2100 the entire reef ecosystem will be at risk. So, the authors argue, SRM is the obvious solution: introducing massive quantities of sulphate aerosols into the stratosphere to reflect radiation and whiten the skies.

Ecosystem at risk from multiple threats

This would not take away the need to reduce carbon emissions by switching to renewable sources of energy, and protecting terrestrial carbon sources like forests, woodlands, soils and peatlands: rising levels of CO₂ would also weaken corals by acidifying seawater.

“Coral reefs face a dire situation, regardless of how intensively society decarbonises the economy”, says Peter Cox, professor of climate system dynamics at the University of Exeter.

“In reality, there is no direct choice between conventional mitigation and climate engineering. This study shows that we need to accept that the loss of a large percentage of the world’s reefs is inevitable, or start thinking beyond conventional mitigation of CO₂ emissions.”

Dr Kwiatkowski has only lately dismissed at least one ocean geoengineering solution – to cool the sea surfaces by pumping up cold water from the ocean depths – because, in the long run, it might make the climate change crisis even worse.

At least one other group has proposed that some form of solar protection could be an answer, but another has suggested that at least some corals might adapt as they are colonized by algae that are resistant to the higher temperatures.

Coral reefs are the richest ecosystems in the oceans, and 500 million people depend on the living coral and its co-dependants for food, tourist income and coastal protection.

The tropical reefs have bleached before, in extremes of heat, but after a few years have recovered. Whether they could survive both a sustained rise in temperatures and the increasing acidification of the oceans that goes with higher carbon dioxide levels is another matter.

The paper: ‘Coral bleaching under unconventional scenarios of climate warming and ocean acidification‘ by Lester Kwiatkowski et al is published in Nature Climate Change.

Tim Radford writes for Climate News Network. Additional reporting by The Ecologist.