Corals like it HOT

Corals like it hot

In the last hundred years temperatures in the GBR region has increased by around 0.60C [1],[2]  and the fragile reef hypothesis suggests that this has caused massive bleaching new to the GBR. It is effectively suggesting that all the corals are within one degree Celsius of their upper thermal limit.

It is a remarkable proposition that corals are so precariously positioned and ignores evidence from coral cores that the GBR water was about the same temperature today as in the 1700s.[3] It ignores the fact that most of the corals of the GBR also live in Indonesia and Papua New Guinea where the water is one or two degrees hotter (290C), and the corals grow faster  – approximately twice as fast as corals of the southern GBR where the water is coldest.[4] The corals of the warmer northern GBR grow almost 50 percent faster than the colder southern GBR. For every degree that the temperature increases, the corals grow over 20 percent faster.

Figure Average water temperature versus average calcification rate for Porites corals. (after Lough and Barnes, 2000).  Solid diamonds – Indo-Pacific reef data. Open triangles – Great Barrer Reef data.  Corals grow faster in hotter water.

That corals grow faster in hot water is hardly surprising. Tourists do not flock to cold climates to see coral reefs even though hardy and very slow growing corals live in very cold water such as in Scotland. It is perhaps unsurprising that scientists are worried that even these Scottish corals, living in almost freezing water, are also threatened by climate change.[5] They are also supposedly living in water close to their thermal limit. How this is concluded is a mystery considering that corals grow better when the water is warmer.

Interestingly, scientists have given the name, “Coral Triangle,” to a region which includes Malaysia, Indonesia and Papua New Guinea, which has the highest diversity of corals and fish as well as far higher coral growth rates than the GBR or anywhere else. Oceanographers and meteorologists have a different name for this region – the Indo-Pacific warm pool, so-named because it is the hottest major water mass in the world’s oceans. Its effect on the weather and climate is profound – hot moist air rising from the Indo-Pacific warm pool drives the Walker Circulation that is important to the El-Nino-La Nina phenomenon.  It is not a coincidence that the Coral Triangle and the Indo-Pacific Warm Pool are the same place – it is a place with very hot water, and corals like it hot.

The Coral Triangle: Home to the most diverse and fastest growing corals in the world. The water temperature is a degree or two hotter than most of the GBR, and very similar species live in these regions. Image from Google Earth

[1] McLean, J.D. (2017). An audit of uncertainties in the HadCRUT4 temperature anomaly dataset plus the investigation of three other contemporary climate issues. PhD Thesis.

[2] Hendy, E.J., Lough, J.M. and Gagan, M.K. (2003). Historical mortality in massive Porites from the central Great Barrier Reef, Australia: evidence for past environmental stress? Coral Reefs, 22(3), pp.207–215.

[3] See Endnote 2

[4] Lough, J.M. and Barnes, D.J. (2000). Environmental controls on growth of the massive coral Porites. Journal of Experimental Marine Biology and Ecology, 245(2), pp.225–243.

[5] The Newsroom (2016). Scotland’s deep-sea coral reefs in danger from climate change. [online] http://www.scotsman.com. Available at: https://www.scotsman.com/news/environment/scotland-s-deep-sea-coral-reefs-in-danger-from-climate-change-1-4296663.

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