Cloud cover could protect the Great Barrier Reef from sea surface temperature rises.
An overcast sky is rarely a welcome sight for snorkelers on the Great Barrier Reef. But a generous cover of clouds could be exactly what’s needed for the future survival of the magnificent corals that make the reef the biodiversity wonder that it is.
The Great Barrier Reef hugs the north-eastern coastline of Australia across more than 2600 kilometres, earning it the distinction of being the world’s largest coral reef system. A considerable portion of the nearly 3000 individual reefs and 900 islands lies within the protected waters of a marine park, yet human impact is still keenly felt.
Global warming has seen consistently higher sea surface temperatures over the past 45 years. These higher temperatures are the source of one of the greatest threats to reef survival – coral bleaching. Corals are made up of colonies of millions upon millions of tiny marine invertebrates that cement themselves together by secreting a calcium carbonate exoskeleton.
The majority of corals get their nutrients from photosynthetic unicellular algae that take up residence as endosymbionts within the cells of the coral tissue. The spectacular rainbow of colours that draws visitors to the Great Barrier Reef each year is the result of the many different algal species that reside in the reef’s corals.
When the corals face stressful conditions, they expel their algal symbionts and the result is coral bleaching. In the case of high temperature, temperatures 1–2°C above the long-term local average over a number of days leads to widespread bleach in the form of a mass bleaching event.
Predicting when a bleaching event might occur is not always easy. The precise mechanisms at play, especially at the local level, are complex, and whether cloud cover helps to buffer sea surface temperature has been a point of contention among researchers.
On the one hand, elevated levels of the greenhouse gases carbon dioxide, methane and nitrous oxide raise global air temperature, which increases evaporation of water from the sea. This in turn leads to higher levels of atmospheric water vapour – another potent greenhouse gas – potentially resulting in further air temperature rises. In spite of the warmer air temperature, this positive feedback situation could actually lower sea surface temperatures due to the cooling effects of evaporation counteracting the heating rays of the sun. On the other hand, a negative feedback system where water vapour in the atmosphere condenses into clouds, thereby blanketing the sea from further solar radiation and warming is also plausible.
By comparing sea surface temperatures during cloud-covered and clear skies, researchers from James Cook University and the Australian Institute of Marine Sciences now have solid evidence that clouds do indeed prevent the sea surface from overheating and may be protecting tropical reefs from bleaching.
The researchers used satellite observations of cloud cover, combined with average daytime sea surface temperature data from several near-surface temperature loggers in the central Great Barrier Reef. By overlaying the two datasets across four summers, including the warmer summers of 2005 and 2006, and the relatively cooler 2007 and 2008 summers, the team were able to observe the cause–effect relationship between cloud cover and sea surface temperature.
Across all four summers, periods of high cloud cover corresponded with lower sea surface temperatures, whereas sea temperatures climbed under clear skies. A change from overcast to clear didn’t produce an instantaneous warming of the sea. There was a three-day lag between a change in cloud cover and a resulting sea temperature change.
The researchers estimated that cloud cover alone accounted for up to 32.1% of the change in sea surface temperatures three days later.
Conversely, the effect of sea surface temperatures on cloud cover was more subtle. Warmer sea surface temperatures explained a smaller proportion (up to 21.6%) of the increase in cloud cover three days later.
By better understanding how much of a cooling effect clouds have on sea surface temperatures, more accurate predictions of coral bleaching events on a local scale should be possible. The authors also point out that satellite data on cloud cover may be an under-recognised and under-utilised resource for monitoring and understanding local factors at play during bleaching events.
Reference: Leahy SM, Kingsford MJ & Steinberg CR. (2013). Do clouds save the Great Barrier Reef? Satellite imagery elucidates the cloud-SST relationship at the local scale. PLoS One doi:10.1371/journal.pone.007040