Coral bleaching has been a regular feature of the Great Barrier Reef for the past 400 years, with evidence of repeated mass events dating back to well before European settlement and the start of the industrial revolution, according to new research.
The Conservative Party, which has long been highly sceptical of alarmist claims that so many of the Earth's ills have their foundations in human activity (especially emissions), has welcomed the new findings and the extra, obvious questions they raise.
The Australian reports, a study of coral core samples has extended the known history of bleaching by more than 350 years but warns it is becoming more frequent and may be approaching a “tipping point” beyond which reef survival is uncertain.
Published in Frontiers in Marine Science, the research by scientists from Glasgow and Edinburgh universities reconstructs temperature-induced bleaching patterns over 381 years spanning 1620-2001.
The findings are at odds with claims that mass coral bleaching is a recent phenomenon due to climate change.
As the chart shows, the decade of the Federation Drought (1890s to early 1900s) was a particularly bad period for coral bleaching on the Great Barrier Reef (GBR), as was the 1750s. But man’s emissions hadn’t even gotten out of the basement until well into the 1900s.
GBR bleaching may have less to do with man and his emissions and more to do with coral-stressing anomalies during pronounced El Ninos such as:
- Greater sun exposure (due to clearer skies and lower sea levels over the GBR in its El Nino spring and summer), and
- Stiller waters (instead of the usual nutrient and oxygen-rich flow from the central Pacific)
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During pronounced El Ninos, the normal easterly trade winds across the equatorial Pacific stall and reverse, causing warm water to well at, and near, the surface in the central and eastern tropical Pacific, with the western Pacific (and GBR waters) turning anomalously cool.
- Overall, the average surface temperature of the Pacific and globe is higher during El Ninos as the world’s largest ocean is not over-turning as much.
This reversal of the equatorial Pacific’s trade winds is associated with a number of things:
- It causes a sustained tidal effect over 2-3 seasons (generally decaying in April), which increases the height of the sea surface in the central and eastern Pacific and lowers the sea height in the western Pacific (by up to a half a metre of more) compared to neutral and La Nina years (when the healthiest corals grow up and out the most, eg the GBR).
The stormy and cloudy convergence zone of the Pacific’s Walker Cell/Circulation shifts east (from the Indonesian archipelago, New Guinea and the GBR) to beyond the International Date Line, causing anomalously cloudy skies there and anomalously clear skies over the GBR and much of the archipelago to our north.
- The GBR’s lower sea height and less cloud-covered skies mean sun exposure of the reef’s coral, particularly its tops, is at a maximum – a double whammy for sunburn when the sun is over the southern hemisphere (ie late September to mid-March).
- A third stressor, unrelated to sun exposure, is the stiller or slightly reversed surface currents in the western Pacific (and through/over the GBR) during pronounced El Ninos, which prevent the nutrient and oxygen-rich waters of the central and eastern Pacific from brushing past the “gills” of the reef’s corals so readily.
All these GBR coral stressors are at their maximums during pronounced El Ninos, are entirely unrelated to man or CO2, and occur just when the waters at and near the surface of the western Pacific (ie around the GBR corals) are anomalously cool – not warm.
As a healthier reef will tend to grow more (both up and out) in neutral and La Nina years – ie the good times – to be more sun-exposed and bleached in El Nino years, it could be that, rather than always being a sign of sickness, coral bleaching on our GBR may actually be a sign of prior robust growth and therefore good health.
This is a debate that scientists need to have – one which will not likely be progressed by grants of $444 million to reef activist causes.
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