Coral and the Great Barrier Reef

By Mike Jonas – Re-Blogged From WUWT

I recently had a (fairly short) conversation with an acquaintance, who was stunned to discover that I did not think at all highly of the position that Professors Terry Hughes and Ove Hoegh-Guldberg at James Cook University, Queensland, have taken on coral science and the Great Barrier Reef.

“Have you seen the documentary Chasing Coral?” I was asked, “There seems to be a lot of evidence that the reef is dying.” (or words to that effect).

I agreed to watch the documentary (if I’d known it went for a whole hour I might not have agreed so readily), and to report back when we next happened to be at the local cafe at the same time – an event which occurs from time to time. A bit like coral bleaching, perhaps.

I found the documentary quite distressing, but not for the same reasons as my acquaintance. I wrote an appraisal of it (see below), but then thought .. if I’m going to get anywhere in the next conversation, I have to come prepared. So I wrote a set of notes, which follow. The question is – are these notes OK; are there major things I’ve got wrong or left out? I’ve cited a number of papers and articles, so before recommending one please see if I’ve cited it already. Each section typically consists of my notes followed by the material (in blockquote) that I used for them.

BTW, I looked for a ready-made document on coral and/or GBR, and couldn’t find one. If one exists – not too long – that might be a lot better. Otherwise, maybe these notes will be helpful to others.

My Notes on Coral and the Great Barrier Reef (GBR)

Chasing Coral (Documentary)

The documentary did look at other coral, but its major focus was on the GBR. The documentary’s video footage was necessarily anecdotal and covered only very small parts of the GBR – the reef is over 2,300 km (1,400 mi) long, with an area of ~344,400 sq km (~133,000 sq mi). The “experts” in the video were all of the one mind – that the reef is doomed by man-made climate change – and no attempt was made to find or report on any alternative view.

A very different picture can be obtained simply by reading some of the scientific evidence.

I’ll start with the long term picture …

Geological Time

The picture that emerges from the geological evidence is that corals first appeared at a time when Earth’s temperatures and CO2 levels were much higher than today’s. Or, possibly, if modern coral species did not begin until the Devonian period then they could have begun at a time when temperatures and CO2 levels were similar to today’s. But in any case they have lived through long periods (millions of years) when temperatures and CO2 levels were much higher than today’s.

“Paleozoic [~250-600mya] was the period of early life where plants, insects, fish, mollusks, corals and many more living organisms developed.”

Wikipedia puts start of corals ~240mya. [Devonian].

In the Paleozoic, CO2 levels reached ~25x today’s level. By the Devonian they were down to about today’s level, but between then and now reached ~9x today’s level. [NB. The geological data is very low resolution. There could have been large short-term variation within those periods which would not show in the long term chart.]

clip_image003 clip_image004

“During far most of Phanerozoic the planet’s temperature was significantly higher than in modern times.”

~240mya, sea levels were similar to today’s, but in the intervening period they went much higher. [Again, NB that the long term chart is low resolution].


“Coral reefs from the Jurassic have been found on the Sakhalin Peninsula at 60 degrees northern latitude, which is 30 degrees farther north, than where today’s coral reefs are found. Coral reefs require a minimum temperature of 20 degrees in order to grow.”

“For the past 55 million years the global surface temperature has declined by more than 10°C from a “hot house” condition into an “ice house” with increasing temperature variability as depicted in Figure 1 (Mya = millions of years ago). During the Cretaceous and Early Cenozoic, glaciers and ice caps were absent from both Antarctica and Greenland. Antarctica was covered in para-tropical vegetation and Greenland was home to crocodiles. More importantly for millions of years the oceans had been storing enormous amounts of heat. In contrast to near freezing temperatures today, Antarctic bottom waters averaged about 11°C, suggesting Antarctic coastal temperatures never dropped below 11°C even during the long polar nights. Amazingly the equator to pole surface temperature difference averaged just 10°C compared to the 30°C gradient measured today. Of particular interest, changes in carbon dioxide fail to explain the greatest proportion of these ancient temperatures.”



The Holocene is the current inter-glacial period. The end of the last glacial period (“glacial maximum”) began around 20,000 years ago. Temperatures and sea levels rose rapidly at the start of the Holocene, and during the Holocene Optimum (around 6-7,000 years ago), sea levels were about 2-3m above today’s sea levels.

The Holocene itself, including the supposedly threatening and supposedly man-made climate of today, is not at all unprecedented. The Holocene is just one of an irregular series of inter-glacial periods in the current ice age. The current ice age, the Quaternary or Pleistocene, has lasted for a bit over 2.5m years. The last inter-glacial period, the Eemian, occurred a bit over 100,000 years ago. It was 2-3 degrees warmer than today, sea levels were 4-6m higher, and it had greater rates of temperature change.

“Rates and patterns of global sea level rise (SLR) following the last glacial maximum (LGM) are known from radiometric ages on coral reefs from Barbados, Tahiti, New Guinea, and the Indian Ocean, as well as sediment records from the Sunda Shelf and elsewhere. These records provide estimated global and regional rates of SLR when combined with LGM and deglacial ice sheet history and geophysical models of regionally varying glacio-isostatic adjustment (GIA) to chang- ing land ice mass. For example, Lambeck et al. (2014) estimate mean global rates during the main deglaciation phase of 16.5 to 8.2 kiloannum (ka) at 12 mm yr−1 with more rapid SLR rates (∼ 40 mm yr−1) during meltwater pulse 1A ∼ 14.5–14.0 ka and slower rates during the Younger Dryas (YD) from 12.5 to 11.5 ka.”

“We find that sea level [at Barbados] tracked the climate oscillations remarkably well. Sea-level rise was fast in the early Allerød (25 mm yr-1), but decreased smoothly into the Younger Dryas (7 mm yr-1) when the rate plateaued to <4 mm yr-1> here termed a sea-level “slow stand”. No evidence was found indicating a jump in sea level at the beginning of the Younger Dryas as proposed by some researchers. Following the “slow-stand”, the rate of sea-level rise accelerated rapidly, producing the 14 ± 2 m sea-level jump known as MWP-1B; occurred between 11.45 and 11.1 kyr BP with peak sea-level rise reaching 40 mm yr-1. Following MWP-1B, sea-level rise rapidly decreased to 8 mm yr-1.”

“The relative sea level was 2.5 ± 0.5 m above the present level during the mid Holocene between 7000 and 6000 years ago.”

… and lots more …

Kobashi and Alley GISP2 Central Greenland Temperature Reconstructions

“The Eemian interglacial period [..] began 127,000 years ago and extended to 106,000 years ago. [..] It is estimated that the Eemian temperatures were 1–2° higher than the current ones (Kaspar, 2005). In England, the fossil studies show tropical fauna such as hippopotami, and this period has been called Ipswichian. In addition, the study of corals and foraminifera shows that the seas were 2–3° degrees warmer than the current temperature (Lea et al., 2000; Pelejero et al., 2010; Martrat et al., 2004). The Eemian sea level was 4–6 m higher than the current coastline, which probably was due to the melting of the Greenland ice sheet (Cuffey and Marshall, 2000). During this period the earth’s orbit around the sun was more eccentric and its perihelion coincided with summer in the Northern Hemisphere (today it corresponds to the aphelion). The inclination of the earth was also greater than the current 23°27′. This led to a seasonality and the heat was greater, with colder winters, than at present, corroborated by the study of the former coral reefs (Felis et al., 2004).”


The GBR has existed for about 20m years. But it has not survived in its current form over that period. On the contrary, it has essentially died many times, when sea levels dropped below the continental shelf, and recovered each time the sea level rose again. By “died” I mean that the reef as we know it died, because it was left high and dry. Graphs of the last 400,000 years of temperature suggest that the GBR as we know it would have died several times in that period. And there could have been many other such occasions over the last 20m years.

In spite of these large changes in sea level, some corals must have survived where there was still sea – presumably the outer edge of the continental shelf would have been one of those places. There are still corals in those (now very) deep places, but the mix of coral species in deep and shallow waters is quite different. So it seems likely that the reef changes its mix of coral species to deal with changing conditions – in other words, some species can die out from some areas and be replaced by others.

“The Great Barrier Reef is an extremely ancient, enormous host of living things, composed of living coral growing on dead coral dating back perhaps as much as twenty million years.”

“For large parts of that time, during periods of glacial activity, the area of the Great Barrier Reef was dry with large flat coastal plains. This area is at a depth of less than one hundred metres below sea level today.”

“Global sea level rose by about 120 m during the several millenia that followed the end of the last Ice Age (approximately 21,000 years ago) …”


[NB. Chart is in feet not metres]

“Even four times as deep as most scuba divers venture, the Great Barrier Reef blooms. A new exploration by a remote-operated submersible has found the reef’s deepest coral yet.

The coral Leptoseris is living 410 feet (125 meters) below the ocean’s surface, a discovery that expedition leader Pim Bongaerts of the University of Queensland called “mind-blowing.””

“The 410-foot distance is surprising for the Great Barrier Reef, where scuba divers find stunning coral displays at depths down to 100 feet. But corals are known to live deep elsewhere. In the Gulf of Mexico, researchers have found the coral Lophelia pertusa thriving 2,620 feet (799 m) down. Lophelia doesn’t need sunlight to survive. In Puerto Rico, light-dependent corals survive as far down as 500 feet (150 m).”

Leptoseris and Lophelia do not appear in the Australian Geographic list of coral species that “… are all found in the outer reef at Heron Island, at about 1–3m depth, as well as in other shallow reef zones of the Great Barrier Reef.”.

Coral Resilience

Although some scientists and others are very ready to forecast the end of the GBR, all of those forecasts are based on quite mild changes in conditions – mild relative to the changes that the reef has gone through in its past. One thing that is very clear is that these scientists and others are underestimating coral’s resilience. They are also overstating the importance of coral bleaching. Bleaching is one of coral’s defence mechanisms, and there is a world of difference between a bleached coral and a dead coral.

“Despite increasing confirmation of the Adaptive Bleaching Hypothesis and its ability to explain coral resilience, most people are unaware of its debate within the scientific community. The ability to rapidly adjust to changing environments by modifying their symbiotic partnerships has been the key to their success for millions of years. As one expert wrote, the “flexibility in coral–algal symbiosis is likely to be a principal factor underlying the evolutionary success of these organisms”.”

“… when a reef is reported as ‘bleached’ in the media, that often leaves out a critical detail on how severe that bleaching is, at what depth the bleaching has occurred and if it’s going to cause permanent damage to the coral at that site.”

“Reefs around the world experienced bleaching in 2016 and 2017, and while the northern part of the Great Barrier Reef did experience some severe bleaching, this condition did not affect the entire Reef and there are currently encouraging signs of recovery at a variety of key tourism sites. Recent photos taken in June and July 2018 show healthy, vibrant coral at numerous locations that suffered during the back-to back coral bleaching events in 2016 and 2017 including Fitzroy Island, Moore Reef and Saxon Reef near Cairns, among other locations.”

Great Barrier Reef starts to recover after severe coral bleaching, survey of sites between Cairns and Townsville shows

[..] The [Australian Institute of Marine Science]’s Neil Cantin said they were surprised to find the coral had already started to reproduce.” (Sep 2017)

“An amazing example of coral resilience has been found in the Gulf of Aqaba, west of the Arabian mainland. Specimens of the coral there have been placed into tanks where they are exposed to rising temperatures and sub-optimal pH levels. Scientists reported that most of the variables measured, such as energy metabolism or the building of a skeleton were actually improved. One explanation for these surprising results where such corals are observed in the stress tests to be not only surviving, but thriving, is that coral in the Gulf of Aqaba is highly evolved due to historical extreme changes in the climate of the region. The ramifications of such discoveries may be significant. Associate Professor David Suggett suggests that events such as these teach us that corals are surviving in waters that are really hot, very acidic and have very little oxygen. He reminded his 40 person audience that these are the same conditions that have been predicted under climate change.”

[From the report on a lecture by Associate Professor David Suggett M.Sc., Ph.D.

Marine Biologist, University of Technology, Sydney, at the NSW Southern Highlands branch of the Royal Society, Jun 2018]

2016-7 Bleaching

Some scientists are very ready to ascribe the 2016-7 severe bleaching of the GBR to rising water temperatures. But perhaps the main cause was a fall in sea level. A fall in sea level is prima facie a greater danger to coral than a rise in temperature.

CO2 levels and pH also appear unlikely to be the cause. As shown above, corals evolved in, and/or survived through, a period of much greater CO2 concentration than today’s. They have also survived through a period of much lower sea surface pH than today’s.

“CLIMATE change sceptic and geophysicist Professor Peter Ridd has questioned research that blames global warming for devastating coral bleaching on the Great Barrier Reef.

Prof Ridd also says the work of a US schoolteacher, who claims a drop in sea level caused by the El Nino phenomenon might have caused bleaching, should not be discounted.

Prof Ridd, of James Cook University’s Centre for Tropical Water and Aquatic Ecosystem Research, has re-entered the fray in a simmering climate war.


“I think the sea level (issue) could account for some of the bleaching but I’d be surprised if it accounts for all of it,” Prof Ridd said.

He said wind died off during El Nino events, causing sea levels to drop.”

“It is puzzling why the recent 2017 publication in Nature, Global Warming And Recurrent Mass Bleaching Of Corals by Hughes et al. ignored the most critical factor affecting the 2016 severe bleaching along the northern Great Barrier Reef – the regional fall in sea level amplified by El Niño. Instead Hughes 2017 suggested the extensive bleaching was due to increased water temperatures induced by CO2 warming.

In contrast in Coral Mortality Induced by the 2015–2016 El-Niño in Indonesia: The Effect Of Rapid Sea Level Fall by Ampou 2017, Indonesian biologists had reported that a drop in sea level had bleached the upper 15 cm of the reefs before temperatures had reached NOAA’ Coral Reef Watch’s bleaching thresholds. As discussed by Ampou 2017, the drop in sea level had likely been experienced throughout much of the Coral Triangle including the northern Great Barrier Reef (GBR), and then accelerated during the El Niño. They speculated sea level fall also contributed to the bleaching during the 1998 El Niño. Consistent with the effects of sea level fall, other researchers reported bleaching in the GBR was greatest near the surface then declined rapidly with depth. Indeed if falling sea level was the main [driver] in 2016’s reef mortalities, and this can be tested, then most catastrophic assertions made by Hughes 2017 would be invalid.

Indeed the Great Barrier Reef had also experienced falling sea levels similar to those experienced by Indonesian reefs. Visitors to Lizard Island had reported more extreme low tides and more exposed reefs as revealed in the photograph above, which is consistent with the extremely high mortality in the Lizard Island region during the 2016 El Niño. Of course reefs are often exposed to the air at low tide, but manage to survive if the exposure is short or during the night. However as seen in tide gauge data from Cairns just south of Lizard Island, since 2010 the average low tide had dropped by ~10 to 15 cm. After previous decades of increasing sea level had permitted vertical coral growth and colonization of newly submerged coastline, that new growth was now being left high and dry during low tide. As a result shallow coral were increasingly vulnerable to deadly desiccation during more extreme sea level drops when warm waters slosh toward the Americas during an El Niño.”

[The whole article is worth reading]

“Although some researchers have raised concerns about possible negative effects of rising CO2 on ocean surface pH, there are several lines of evidence demonstrating marine ecosystems are far more sensitive to fluxes of carbon dioxide from ocean depths and the biosphere’s response than from invasions of atmospheric CO2. There is also ample evidence that lower pH does not inhibit photosynthesis or lower ocean productivity (Mackey 2015). On the contrary, rising CO2 makes photosynthesis less costly.

Furthermore in contrast to researchers arguing rising atmospheric CO2 will inhibit calcification, increased photosynthesis not only increases calcification, paradoxically the process of calcification produces CO2 and drops pH to levels lower than predicted by climate change models. A combination of warmer tropical waters and coral reef biology results in out-gassing of CO2 from the ocean to the atmosphere, making coral reefs relatively insensitive to the effects of atmospheric CO2 on ocean pH.

Sixty million years ago proxy evidence indicates ocean surface pH hovered around 7.4. If surface pH was in equilibrium with the atmosphere, then CO2 concentrations would have hovered around 2000 ppm, but there is no consensus that CO2 reached those levels. However as will be discussed, there are biological processes that do lower surface pH to that extent, despite much lower atmospheric CO2 concentrations.”

Rate of Change

All of the above indicates that corals have survived much more extreme conditions than are expected to occur because of man-made climate change. But much of the alarmism has pointed to the rate of change, claiming that it will be too fast for corals to adapt. A typical statement would be “These corals aren’t going to adapt at an unlimited rate”.

But are things really changing that fast now, compared to times past? That’s often difficult to assess because we can measure things today with high precision and high resolution, whereas data from the distant past tends to be of low precision and low resolution.

There is a clue in the rate of sea level rise. The sea level has been rising at something like 2mm per annum since the Little Ice Age ended about 350 years ago. Scary charts of global mean sea level like this …

… are produced to try to make us believe that something frighteningly different is happening now.

But if you look carefully, you will see that (a) the sea level rise highlighted in the chart began in 1890, long before mankind started putting serious quantities of CO2 into the atmosphere, (b) the average rate of sea level rise is only 1.8mm pa, and (c) there is little recent acceleration. It is also worth noting that high-resolution recent data has been spliced onto low-resolution older data – a definite no-no in proper science.

Compare the sea level rise rate in that chart with some of the sea level rates from times past, as cited above. eg: 12mm pa from 16.5 to 8.2 ka (that’s 6 times today’s rate for more than 8,000 years), ~40mm pa during a meltwater pulse ~14.5-14.0 ka, 25mm pa in the early Allerød and a “plateau” of 4mm pa (still double today’s rate) in the Younger Dryas.

So some things at least must have happened a lot faster in the not-so-distant past than they are happening today, without wiping out the coral.

And finally …

… if you want a really cynical view of reef alarmism, read Jo Nova:


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