Guest post by David Middleton

 

EXECUTIVE SUMMARY

Climate change is internationally-recognised as one of the biggest threats to coral reefs around the world, including the Great Barrier Reef. For the last three years, coral bleaching, due to ocean warming associated with climate change, has impacted coral reefs worldwide. Mass coral bleaching events occur during extended periods of elevated sea surface temperatures and have the potential to result in significant and widespread loss of coral.

The current mass coral bleaching occurring in tropical regions across the world since 2014 is the longest mass bleaching event ever recorded. This is a global event triggered by record-breaking sea surface temperatures caused by climate change and amplified in 2016 by a strong El Niño. The ocean is warmer than at any time since the instrumental record began. For Australia’s Great Barrier Reef, this resulted in the worst ever coral bleaching in 2016.

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Final report: 2016 coral bleaching event on the Great Barrier Reef

There’s no doubt that the 2016 coral bleaching event and associated coral mortality was the worst in recorded history since at least 1980.  However, no one was really paying much attention to coral bleaching before the early 1980’s.

Insufficient sea temperature data exist from the Great Barrier Reef to indicate changes in the long-term means over recent times. However, an analysis of air temperature records from Townsville shows that mean January February air temperatures above 29°C occurred 6 times between 1980 and 1995, 5 of which coincided with bleaching events at nearby Magnetic Island. Prior to 1980 however, these conditions had occurred only 4 times in the 53 years since 1927, all occurring in the 1930s (Jones 1995; Jones et al. in press).

Guldberg et al., 1999

“This is a global event triggered by record-breaking sea surface temperatures caused by climate change and amplified in 2016 by a strong El Niño.”

Was it “triggered by record-breaking sea surface temperatures caused by climate change and amplified in 2016 by a strong El Niño”?  Or was it caused by a strong El Niño and amplified by a sharp local fall in sea level from 2014-2016?

2016 was the GBR’s second warmest year “on record”…

The SST data for the GBR are derived from NOAA’s ERSST v4… (for whatever that’s worth).

2016 was the second hottest “year on record” for the GBR… However, bleaching occurs in summer.

At this point, I asked myself: “Self?  Why are they using NOAA’s ERSST v4?  Aren’t there any weather stations in or around the Great Barrier Reef?”

There are actually quite a few weather stations.

Agincourt Reef #3 has fairly complete weather records going back to 1991 and it experienced “high” mortality rates… But it hasn’t participated in Gorebal Warming since at least 1993.

Original data…

Air temperature only…

I have only looked at Agincourt Reef #3 in detail.  However, my cursory review of Thursday Island, Lizard Island, Cape Bowling Green and Square Rocks didn’t support any significant warming over recent years in the GBR.

So… What is anomalous about 2014-2016?  A really strong El Niño and a sudden, rather sharp local fall in sea level.

Sea level fall and reef mortality

It’s a well-known fact that coral reefs react poorly to falling sea level… Subaerial exposure of the reef is generally fatal.  However sea level fall on the order of 0.5 meter can literally shut reefs down.

A re-examination of 46 recently published U/Th reef flat ages from Heron and One Tree reefs in the southern Great Barrier Reef (GBR) identified several distinct Holocene reef growth phases with a clear 2.3-kyr hiatus in lateral reef accretion from 3.9 ka to 1.5 ka. An analysis of all available published radiocarbon reef flat ages (165) from 27 other mid-outer platform reefs revealed a similar hiatus between 3.6 ka and 1.6 ka for the northern, south-central and southern GBR. However, no hiatus in reef flat growth was observed in reefs from the central GBR with ages ranging from 7.6 ka to 0.9 ka. Increased upwelling, turbidity and cyclone activity in response to increased sea-surface temperature (SST’s), precipitation and El-Nino Southern Oscillation variability have been ruled out as possible mechanisms of reef turn-off for the mid-outer platform reefs. Rather, a fall (~ 0.5 m) in relative sea level at 4–3.5 ka is the most likely explanation for why reefs in the northern and southern regions turned off during this time.

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Successive phases of Holocene reef flat development: Evidence from the mid- to outer Great Barrier Reef (PDF Download Available). Available from: [accessed Oct 2, 2017].  LINK

Jim Steele has written about the effects of sea level fall on coral reefs in two excellent articles here and here.

Correlation of 2016 GBR coral cover loss to 2014-2016 sea level fall

The coral cover loss was most severe in the Lizard Island Transect Area, just north of Agincourt Reef #3.  Using the color bar scale in Figure 8, I assigned a numerical value  to each color, ranging from 1 (very low) to 8 (high).  I then calculated the change in the rating from early to late 2016.  I then posted sea level profiles from University of Colorado’s Interactive Sea Level Time Series Wizard along a north-south transect.

To enumerate the severity of the 2014-2016 sea level fall, I calculated the slope since 2012.

		2016  Change in Coral Cover Rating
Lat, Long	Slope Since 2012	Lagoon	Reef Front	Average
-11, 144	-0.8114	-5	-1	-3
-12, 144	-0.6106	-5	-1	-3
-13, 144	-1.6792	-3	-3	-3
-14, 145	-1.747	-5	-3	-4
-15, 146	-1.2204	-2	-4	-3	Lizard Island
-16, 146	-0.6637	0	-2	-1	Agincourt Reef #3
-17, 147	0.0762	0	-2	-1
-18, 147	0.5102	-1	-3	-2
-19, 148	0.5055	-1	-3	-2
-19, 149	0.4738	-1	0	-0.5
-20, 150	0.136	0	0	0
-21, 152	0.0594	0	0	0
-22, 152	0.4375	0	0	0
-23, 152	0.6094	0	0	0

Figure 11

Conclusion

It is highly likely that a localized sea level fall played a greater role in the severity of the 2016 coral bleaching and mortality than Gorebal Warming did.

References

[1] Great Barrier Reef Marine Park Authority 2017, Final report: 2016 coral bleaching event on the Great Barrier Reef, GBRMPA, Townsville.

[2] Ove Hoegh-Guldberg, Ray Berkelmans and James Oliver.  1999.  Coral bleaching: implications for the Great Barrier Reef Marine Park.  CRC Reef Research: The Great Barrier Reef – Science, Use and Management.

[3] Dechnik, Belinda & Webster, Jody & Webb, Gregory & Nothdurft, Luke & Zhao, Jian-xin. (2016). Successive phases of Holocene reef flat development: Evidence from the mid- to outer Great Barrier Reef. Palaeogeography, Palaeoclimatology, Palaeoecology. 466. . 10.1016/j.palaeo.2016.11.030.