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The Grim Reaper and the Great Barrier (to be presented to the Board of Supervisors in January)
”The recent frequency and intensity of mass coral bleaching are of major concern, and are directly
attributable to rising atmospheric greenhouse gases.” [1]
Tony Noerpel
As I’ve discussed in my last article, an immediate consequence of human-caused climate change is
repeated bleaching and mortality of the world’s corals. Coral reefs are the rain forests of the ocean
comprising about 26% of the marine biodiversity. While many human activities stress corals, the main
cause of bleaching and mortality is human-caused climate change. As shown in Figure 1 healthy corals
bleach when exposed to about 4 degree-weeks of elevated temperatures [2]. NOAA defines a measure
of degree-weeks where one week with an elevated temperature of 2 degrees C would be 2 degreeweeks. If the temperature returns to normal soon enough the corals survive. But as the warm
temperatures persist the corals die. Coral polyps, an animal, form a symbiotic relationship with certain
types of single-celled, photosynthesizing algae known as zooxanthellae. Coral reef scientists describe
this intimately linked ecological unit as a holobiont [3]. As we saw last week bleaching occurs when the
corals purge themselves of the algae which give them their color. Death occurs if the bleaching
condition lasts too long.
Surface land temperature varies as a Gaussian or normal distribution around a mean temperature [4].
As the mean temperature climbs so do the extremes. However the variance is also increasing which
means the tails of the temperature distribution are fatter or extreme excursions are even more likely
and last longer. The sea surface is more sparsely sampled but I am assuming it should behave similarly.
If the global surface temperature were to recover, dead coral reefs might grow back in 15 years but only
if the reef does not get overgrown by competing sea weed as shown in Figure 2. If this happens then
coral larvae cannot find a solid surface to attach to and the reef cannot recover.
Figure 2 shows the Lizard Island section of Australia’s Great Barrier Reef, before and after the recent
coral bleaching event [5]. The image on the left shows a healthy, colorful, vibrant reef as of October
2014, while the image on the right shows the same reef overgrown by seaweeds as of May 2016
Corals in the Caribbean have been dying at an average rate of 1.4% per year so that only 50% remain [1].
Corals in the Great Barrier Reef are dying at a rate of 1.54% per year. 51% of the corals on the southern
part of the reef died and have been replaced by seaweed between 1985 and 2012 though what remains
of this section of the reef was not severely stress during the 2015-2016 El Nino event as shown in Figure
3. However this figure shows that the Northern part of the reef suffered a 67% mortality rate over 700
km (435 miles) in the latest event [6].
During the 2015-2016 El Nino Kiritimati or Christmas Island corals suffered through sea surface
temperatures 2.5 degrees above normal for ten straight months or 100 degree-weeks (see figure 4). As
of April, 2016 80% of the reef was dead, 15% bleached and only 5% was still healthy [7].
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The Lakshadweep atolls, about 120 miles off the south west coast of India, have sustained frequent coral
mass mortalities. The most severe events in 1998 [8] and 2010 resulted in around 90 and 70 % coral
mortality, respectively. Even though there has been modest recovery still up to 80 % of the benthic
cover on many reefs continues to be dominated by expanses of dead coral skeletons as shown in Figure
4 [9].
These mortality rates are being experienced while the average earth surface temperature has only risen
1 degree C or 1.8 degrees F above pre-industrial climate. This is half of the goal of 2 degrees C (3.6 F)
which world leaders committed to (non-binding and unenforceable) in Paris last year. It is hard to
imagine that corals can survive at that temperature given the rate of mortality at current temperatures.
Sadly, it is hard to believe that we can even limit the warming by 2 degrees C since we’ve only known
about the threat of human-caused climate change for 40 years and still have elected, pun intended, to
do nothing.
Coral reef mortality is geophysically catastrophic and will be permanently recorded in the sedimentary
rock record and future generations will forever judge us thereby.
Figure 1
Figure 1 bleaching does not necessarily mean death if it does not last too long. Warmer ocean
temperatures increase the likelihood of bleaching and in turn reduce the likelihood of recovery. Further
corals which are stressed by elevated temperatures and higher acidity are prone to disease, storms and
predation [2].
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Figure 2 bleached corals on the left and dead corals replaced by seaweed on the right. . Source: XL
Catlin Seaview Survey and http://www.joboneforhumanity.org/a_changing_climate_for_coral_reefs
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Figure 3 Mortality on the Great Barrier Reef. [8]
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Figure 4. The record El Nino of 2015 raised ocean water temperatures 2.5 degrees Celsius, killing 80 per
cent of the corals on Kiritimati’s atoll. Photo by Kristina Tietjen [9]
[1] Glenn De’ath, Katharina E. Fabricius, Hugh Sweatman, and Marji Puotinen, The 27–year decline of
coral cover on the Great Barrier Reef and its causes, PNAS,
www.pnas.org/cgi/doi/10.1073/pnas.1208909109 , October 30, 2012, vol. 109, no. 44.
http://www.pnas.org/content/109/44/17995.full.pdf
[2] http://ningaloo-atlas.org.au/node/193
[3] http://www.joboneforhumanity.org/a_changing_climate_for_coral_reefs
[4] James Hansen, Makiko Sato, and Reto Ruedy, Perception of climate change, PNAS, Published online
August 6, 2012, www.pnas.org/cgi/doi/10.1073/pnas.1205276109
[5] http://www.globalcoralbleaching.org/
[6] http://coralreefwatch.noaa.gov/satellite/analyses_guidance/global_coral_bleaching_201417_status.php
http://www.cnn.com/2016/11/28/asia/great-barrier-reef-coral-death/
[7] http://www.publish.csiro.au/MF/pdf/MF99078
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[8] Shreya Yadav, Pooja Rathod, Teresa Alcoverro, Rohan Arthur, ‘‘Choice’’ and destiny: the substrate
composition and mechanical stability of settlement structures can mediate coral recruit fate in postbleached reefs, Coral Reefs (2016) 35:211–222, http://link.springer.com/article/10.1007/s00338-0151358-z see also
http://www.joboneforhumanity.org/why_dead_coral_reefs_could_mark_the_beginning_of_dangerous
_climate_change
[9] http://www.theglobeandmail.com/news/national/kiritimatis-coral-reef-die-off-offers-clues-forconservationefforts/article32918565/