Coral reef bleaching

Off the coast of Australia, the corals on the Great Barrier Reef are bleaching and dying.

One key reason is the heat. A quick glance at the history of sea surface temperatures in the region shows why the bleaching has become more common and widespread in recent decades.

13 thoughts on “Coral reef bleaching

    1. Of course bleaching varies from year to year, dependent on local temperatures. But a rise in temperatures increases the risks dramatically, which is why bleaching is now far more common than, say, before the 1980s. The coral scientists say that other factors play a role but are much less important than temperature. I suspect any temperature threshold will be location dependent, based on local factors.

  1. Certainly bears watching.

    A few observations:

    * I’m struck by how the rate of increase appears to have decelerated after 1970 or so compared to 1900 to 1970 though radiative forcing was much more significant after 1970. Buoy measurements didn’t really start til 1980 or so, so there is some uncertainty with that trend.

    * high temperatures are indeed correlated with coral die off. But then too are low temperatures. How many fewer coral deaths have occurred from presumably fewer low temperatures?

    * high temperatures are also correlated with another coral death factor: high levels of sunshine – cause and effect?

    * “which is why bleaching is now far more common than, say, before the 1980s. “ Do you have evidence that this is so? How common were coral surveys and measurements in 1980 compared to now?

    * Guard against confirmation bias – many factors of coral bleaching ( Wiki ):
    – increased temperatures
    – decreased temperatures
    – oxygen starvation
    – increased solar irradiance
    – increased sedimentation
    – bacterial infections
    – changes in salinity
    – herbicides
    – low tide and exposure
    – cyanide fishing
    – elevated sea levels
    – mineral dust
    – four common sunscreen ingredients!

    * Regarding sunscreens, I have a doctor friend who frequents tropical locations where he snorkels. He was concerned about coral death in the places he visited. I told him only half jokingly that he was the one killing the corals by bringing his own toxic effects directly to them like the sunscreen cause identified above.

    * Most coral are symbiotic organisms with algae or protozoa which provide the coral with their metabolic energy of photosynthesis along with phytoplankton. How much do coral benefit from increased carbon dioxide for photosynthesis?

    * coral have been around, though not identically, for about a half a billion years! Were they so delicate, it doesn’t seem likely that they would have survived orbitally induced warm periods ( HCO, Eemian ) which lasted for many millenia or the much higher temperatures before that which lasted tens of millions of years.

    * spatially, coral appear to have been quite adaptable. They populate various regions, except for the coldest and darkest waters. These regions experience a still very large range of temperatures, including some normally very warm waters indeed.

    1. This comment is a Gish Gallop. It would be better to stick to one or two main points you have questions about.

      1. Gish Gallop

        When I hear that term, I tend to reflect on my first exposure to it which was on so called skeptical science. I’m guessing that if you’re not an Aussie, that’s where you picked it up also.

        The real world is complex, compound, confounded, non-linear, and multifactoral. The real world is a Gish Gallop and often simplicity or limited scope is in error.

        1. Veron (2009):

          The five mass extinction events that the earth has so far experienced have impacted coral reefs as much or more than any other major ecosystem. Each has left the Earth without living reefs for at least four million years, intervals so great that they are commonly referred to as ‘reef gaps’ (geological intervals where there are no remnants of what might have been living reefs). The causes attributed to each mass extinction are reviewed and summarised. When these causes and the reef gaps that follow them are examined in the light of the biology of extant corals and their Pleistocene history, most can be discarded. Causes are divided into (1) those which are independent of the carbon cycle: direct physical destruction from bolides, ‘nuclear winters’ induced by dust clouds, sea-level changes, loss of area during sea-level regressions, loss of biodiversity, low and high temperatures, salinity, diseases and toxins and extraterrestrial events and (2) those linked to the carbon cycle: acid rain, hydrogen sulphide, oxygen and anoxia, methane, carbon dioxide, changes in ocean chemistry and pH. By process of elimination, primary causes of mass extinctions are linked in various ways to the carbon cycle in general and ocean chemistry in particular with clear association with atmospheric carbon dioxide levels. The prospect of ocean acidification is potentially the most serious of all predicted outcomes of anthropogenic carbon dioxide increase. This study concludes that acidification has the potential to trigger a sixth mass extinction event and to do so independently of anthropogenic extinctions that are currently taking place.

  2. The earth is currently experiencing the third global coral bleaching event with the first two in 1998 and 2010. Each of those lasted less than 12 months. This one is now in its 34th month.

    While that long list posted by Turbulent Eddie includes a variety of factors that can cause bleaching or bleaching-like responses, there is no way such local factors can cause bleaching around the world for almost 3 years. It is being driven by the high ocean temperatures during the three warmest years on record. You can see more on this event at:

    Dr. Mark Eakin, Coordinator
    NOAA’s Coral Reef Watch

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