Bleaching exaggerations

The major bleaching events 1998, and 2002, and 2016/17/20/22 – hugely exaggerated.

So how much coral bleached in these major bleaching events that have been studied. According to the Great Barrier Reef Marine Park Authority,[2] the supposedly first-ever mass coral bleaching event in 1998 caused “moderate to high” bleaching on 74 percent of the inshore reefs, which altogether represent about one percent of the corals of the GBR. For the offshore reefs, representing the other 99 percent of the GBR, 21 percent of reefs experienced moderate to high bleaching.

But how much died? Remember most bleached corals fully recover. According to GBRMPA, “most reefs recovered fully, with less than five per cent of inshore reefs suffering high coral mortality.” In other words, five percent of one percent of the GBR experienced serious coral death. Not especially serious and much less than what a decent sized cyclone would do. It is notable that the long-term monitoring of the coral (Figure 7.1) showed no change in the coral cover in 1998 notwithstanding all the fuss in the world’s media.

The supposedly second bleaching event ever, in 2002, only four years after the first, was claimed to be indicative that the effect of climate change was rapid and disastrous. The debate about the effect of climate change was influenced by news headlines all over the world. According to the GBRMPA,[3] this was slightly worse than the 1998 event: “About 41 per cent of offshore and 72 per cent of inshore reefs had moderate or high levels of bleaching . . .”. Nevertheless, according to GBRMPA, “reef recovery was generally good, with fewer than five per cent of the reefs suffering high mortality.” It is difficult to put a firm figure on the total amount of coral that died in the 2002 bleaching event but, by 2004, the total coral cover on the GBR had dropped by twenty percent although much of this fall was due to other factors such as cyclones and crown of thorns starfish.[4]  By 2012, coral cover was massively reduced by two very destructive cyclones (TC Hamish and Yasi) reducing coral cover to about half its value in 2000. By 2016, however, this had bounced back completely.

Coral cover for the Great Barrier Reef. Despite supposedly unprecedented bleaching events in 2016, 2017, 2020, and 2022 the reef is at record high levels of coral in 2022. See section on coral statistics on this site. Note there is also no fall in coral cover in 1998 or 2002 but there is a fall in 2016, mostly due to loss in the Northern region. This data only includes shallow water coral  – see text about the deep water coral.

The 2016 bleaching event again made massive headlines around the world such as –

Great Barrier Reef coral bleaching at 95 per cent in northern section, aerial survey reveals. (ABC news)[5]

This time, scientists gave solid statistics of how much coral died. It was claimed that the GBR lost 30 percent of its coral (Hughes et al., 2018[6]), most of which occurred in the Northern Region where many reefs were devastated, and typical mortalities were over 50 percent. This looks terrible at first glance.

The true story is buried deep in the appendix of this paper. The fundamental problem is that the survey reported by Hughes et al. (2018) used a methodology that sampled only corals in the shallowest water – less than two metres – notwithstanding that corals live down to well over 50 metres below the surface. As already mentioned, the conditions of light winds and clear cloudless sky, which causes bleaching, mean that it is the water in the top few metres that heats the most and thus it is the corals in the shallow water that are by far the most susceptible to bleaching. Hughes et al. (2018) thus surveyed the relatively small percentage of corals near the surface that are most at risk. The results were thus completely skewed.

With far less fanfare or world-wide publicity, the data on the deeper corals (between five and 40 metres depth) has now become available (Frade et al., 2018)[7] and the mortality was found to be around 10 percent for the far Northern Region where bleaching was worst and where often 50 percent of the surface corals died. It would be safe to assume that the mortality of deep water corals for the Central and Southern Regions, which had very low mortality at the surface, would have been close to zero. Thus, taking the areas of these three regions into account, the total mortality for the entire GBR for the corals between five and 40 metres is around three percent.

Surprisingly, figures are not available for the relative proportion of corals at different water depths but, if it is assumed that shallow water corals represent 20 percent of the coral (with 30 percent mortality) and the deeper corals representing 80 percent of the corals (with three percent mortality), then the total mortality for the entire reef was eight percent. This should be considered as an upper estimate as it is most likely that shallow water coral is far less than the 20 percent assumed here (Harris et al., 2013)[8].

Considering the capacity of corals to recover, a drop in cover of eight percent should not be regarded as cataclysmic. For example, the Southern region of the GBR increased coral cover by 250 percent in just six years between 2011 and 2016 following massive coral loss after cyclones (Yasi and Hamish).

There were supposedly major events in 2017, 2020 and 2022, but the record high coral cover for 2022 demonstrates that the loss of coral in those events must have been minimal. How can lots of coral have died when 2022 had record amounts of coral. Coral takes 5-10 years to recover – it is not like grass that can grow back in a month or two. The inescapable conclusion is that there was massive exaggeration of the latest bleaching events.

[1] Mobasheri, M.R. (1995). Heat transfer in the upper layer of the ocean with application to the satellite sea surface temperature (SST) measurement. PhD Thesis.

[2] Australian Institute of Marine Science (2016). Coral bleaching events. [online] Available at:

[3] See Endnote 6

[4] De’ath, G., Fabricius, K.E., Sweatman, H. and Puotinen, M. (2012). The 27-year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences, 109(44), pp.17995–17999.

[5] McCutcheon, P. (2016). Great Barrier Reef coral bleaching at 95 per cent in northern section, aerial survey reveals. ABC News. [online] 28 Mar. Available at:

[6] Hughes, T.P., Kerry, J.T., Baird, A.H., Connolly, S.R., Dietzel, A., Eakin, C.M., Heron, S.F., Hoey, A.S., Hoogenboom, M.O., Liu, G., McWilliam, M.J., Pears, R.J., Pratchett, M.S., Skirving, W.J., Stella, J.S. and Torda, G. (2018). Global warming transforms coral reef assemblages. Nature, 556(7702), pp.492–496.

[7] Frade, P.R., Bongaerts, P., Englebert, N., Rogers, A., Gonzalez-Rivero, M. and Hoegh-Guldberg, O. (2018). Deep reefs of the Great Barrier Reef offer limited thermal refuge during mass coral bleaching. Nature Communications, 9(1).

[8] Harris, P.T., Bridge, T.C.L., Beaman, R.J., Webster, J.M., Nichol, S.L. and Brooke, B.P. (2012). Submerged banks in the Great Barrier Reef, Australia, greatly increase available coral reef habitat. ICES Journal of Marine Science, 70(2), pp.284–293.

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