The Great Barrier Reef is doomed again. A recent widely publicised scientific study reports the dramatic finding that it has lost half its coral in the last 27 years. Forty-eight precent of the loss is attributed to storm damage, with bleaching and crown-of-thorns starfish being responsible for 10% and 42% respectively. The average annual rate of coral loss over the 27-year period was estimated to be 3.38% and growth was put at 2.85%, leaving a net decline of 0.53% per year. Further effort and research on starfish control is suggested to be the most promising means of reversing the decline. Elimination of the loss due to starfish would leave a net gain of 0.89%.
While the news reports present the appearance of scientific precision and certainty, examination of the study itself reveals a number of doubtful assumptions, undisclosed conditions and instances where strong conflicting evidence is unmentioned. Examples of this include:
The margin of error in visual surveys of coral cover is high and unassessed; yet, they are presented to hundredths of a percent without any qualifying explanation, as if they are precisely accurate. Coral cover is highly variable between reefs and over different areas or at different years on the same reef. Visual estimates of the percentage of coral cover can differ significantly, depending on where, when and by whom the observations were made. Also, many of the observers doing the surveys upon which this study is based were inexperienced students primed by learned expectations of threats to the reef.
The reef is vast and in any given year surveys sample only a small portion. The reported sudden decline in coral cover in the last couple of years is almost certain to have been exaggerated by surveys made to assess the damage from severe cyclones crossing the reef in 2009 and 2011, with few or no surveys in unaffected areas in those years.
The study states, "Cyclone intensities are increasing with warming ocean temperatures…."
This statement is unsubstantiated and contrary to available evidence. The most definitive recent studies find no increase in tropical cyclone frequency or intensity. On the GBR severe cyclone activity for the past century has also been well below the level for the preceding century. The study also states:
The recent frequency and intensity of mass coral bleaching are of major concern, and are directly attributable to rising atmospheric greenhouse gases.
No evidence exists for this claim. The mass-bleaching events of recent decades have coincided with surface water warming resulting from periods of extended calm associated with strong El Niño events. This impedes normal evaporative cooling as well as wave driven mixing. There is no evidence of any increase in the frequency or strength of El Niño events, and climate models project increased wind speeds from warming, not more calms. The report further states:
Water quality is a key environmental driver for the GBR. Central and southern rivers now carry five- to ninefold higher nutrient and sediment loads from cleared, fertilized, and urbanized catchments into the GBR compared with pre-European settlement.
No actual measurements of pre-European sedimentation rates exist. These are only estimates and extrapolations from unverified proxies which may or may not represent what is claimed. What is certain is that the inshore areas of the GBR are heavily blanketed in sediments that have accumulated over thousands of years and turbidity in coastal waters is overwhelmingly governed by re-suspension of these sediments through wave action, not by current day runoff from the land.
The most widely cited study purporting to show a large increase in sedimentation after European settlement was based on an increase in barium in coral skeletons just after 60,000 head of cattle were introduced into the Burdekin area in 1870. This was attributed to an increase in erosion caused by the cattle. But this period also coincided with the ending of an extended period of extreme drought and no explanation has ever been offered for why the barium level has subsequently decreased despite the million head of cattle now in the same catchment.
The assumption that levels of turbidity in flood runoff events are almost entirely attributable to farming and grazing is unwarranted, and it is readily observable that runoff turbidity from crop and grazing areas is often markedly less than from undisturbed natural areas. Crops and grasses are simply better at retaining soil than is either the rainforest or open eucalyptus woodland they have replaced. Sediment-trapping by dams and cessation of the widespread annual burning practiced by the pre-European inhabitants of the area can also be expected to have reduced sediment outflows.
There is good reason to expect that agriculture and grazing may well have resulted in a net reduction in levels of sediment discharge, compared to pre-European condition. The claims of multi-fold increases in sedimentation are simply speculation wrapped in techno-waffle and presented as fact.
As for nutrients from land, the total estimated annual runoff of fertiliser into the GBR lagoon would only amount to something in the order of one part in 150 million even if dumped into the lagoon at one time. However, the lagoon is continually being flushed by ocean currents every few weeks.
Dr Walter Starck has a PhD in marine science including post graduate training and professional experience in fisheries biology. He is the editor and publisher of Golden Dolphin, a quarterly publication on CD focusing on diving, underwater photography and the ocean world.