Seawater temperature trends, Great Barrier Reef, Queensland

Bill Johnston, former Research Scientist, NSW Department of Natural Resources, (scientist@bomwatch.com.au) and David Mason-Jones (david@journalist.com.au)

Reassessing Climate Claims on the Great Barrier Reef

This paper critically reassesses claims that the Great Barrier Reef (GBR) is experiencing unprecedented warming and ecological decline due to climate change. Drawing on direct observational data, recent peer-reviewed literature, and physical principles of heat exchange, it challenges prevailing narratives of catastrophic warming, instead proposing a more constrained and evidence-based interpretation of reef temperature trends.

At the heart of the argument is a careful analysis of sea surface temperatures (SST) observed along the GBR by The Australian Institute of Marine Science. Contrary to widely circulated models suggesting sharp and continuous warming, the study finds that temperature increases over recent decades are modest, episodic, and largely bounded within historical variability. The paper draws attention to a critical statistical issue: many reports selectively use data that begins in the 1980s or later, omitting the earlier mid-20th-century cooling period. By using the full record from the 1870s onward, this study shows that current SST levels are not exceptional by historical standards.

A central theme is the physical constraint of evaporation, particularly the role of latent heat flux in regulating sea surface temperature. Drawing from classic thermodynamic theory and modern atmospheric science, the paper argues that ocean temperatures are inherently self-limiting due to the strong negative feedback of evaporative cooling. When SSTs rise, evaporation increases sharply, effectively capping further heating. This mechanism — while long recognized in meteorology — is often overlooked in policy reports and climate impact projections.

Several coral bleaching events are examined in light of this physical limit. The author notes that mass bleaching is not uniformly correlated with the highest recorded temperatures, nor does it follow a simple linear relation with SST anomalies. Instead, complex interactions between cloud cover, water clarity, depth, and preceding weather conditions often play a more decisive role. In some instances, reported bleaching occurred during relatively mild thermal stress, calling into question the attribution of such events solely to anthropogenic warming.

The paper is careful not to dismiss the reality of reef stress or the importance of stewardship. Rather, it argues that climate change is only one of several stressors, and possibly not the dominant one. Overfishing, poor water quality from runoff, and physical damage from cyclones remain significant threats. By focusing narrowly on temperature trends — and overstating their novelty — public discourse risks obscuring more tractable management issues.

From a policy standpoint, the paper urges a rebalancing of narratives: from alarmism rooted in model projections, to stewardship grounded in empirical observation. It makes a strong case that effective reef conservation must be rooted in realism, not rhetoric. Resources would be better spent on improving water quality, enforcing sustainable fishing practices, and supporting local scientific monitoring rather than pursuing large-scale climate interventions whose effects are slow and diffuse.

In sum, this manuscript provides a robust counterweight to dominant climate-fragility discourses surrounding the Great Barrier Reef. It brings data, physics, and field observation back to the centre of the discussion. While acknowledging that climate change is a relevant variable, the paper resists its inflation into a catch-all explanation for complex ecological dynamics. Its call for intellectual humility and empirical grounding is timely and welcome.

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Part 9. Rabbit Flat roadhouse, Northern Territory, Australia

Dr Bill Johnston

Former NSW Department of Natural Resources research scientist and weather observer.

scientist@bomwatch.com.au

Situated east of the Great Sandy-Tanami Desert, 450 km along the Tanami Track south from Halls Creek in Western Australia and 597 km north of Alice Springs, the Rabbit Flat roadhouse is one of the most isolated of the 112 Australian Climate Observations Reference Network – Surface Air Temperature (ACORN-SAT) sites used to monitor Australia’s climate. The site was selected by the Bureau of Meteorology (BoM) as a weather reporting station in December 1969 and useful data commenced in 1970.

The Rabbit Flat roadhouse beside the Tanami Track, is the site of a fresh-water bore east of the Tanami Desert. Maximum temperature (Tmax) is used to homogenise ACORN-SAT sites at Halls Creek, Victoria River Downs and Tennant Creek (solid lines). Dashed lines indicate stations used to homogenise Tmax at Rabbit Flat.

According to the ACORN-SAT catalogue, there was “no indication of any moves between 1969 and 1998”. As the roadhouse weather station site (BoM ID 15548) had become overgrown, an automatic weather station (AWS) was established about 200 m west on 13 November 1996 (BoM ID 15666). Daily maximum temperature (Tmax) for both sites was merged as per ACORN-SAT, summarised as annual averages, aligned with annual rainfall and analysed using BomWatch protocols.

Step-changes indicative of site changes, were detected in the merged dataset in 1986 and 2016, but not in 1997 after datasets were merged.

The magnitude of the change in 1986 (+0.87^oC (±0.17_sem)) is consistent with the Stevenson screen having moved from the vicinity of the roadhouse, where surrounds were possibly watered, to a more open position where conditions were generally warmer. Google Earth Pro satellite images show the up-step in 2016 (+0.89^oC (±0.23_sem)) resulted from erecting a 2.4m-high, chain-wire security fence, probably in 2015. Neither change could be attributed to the climate.

While adjustments for site changes are justified and necessary, it is vital that changepoints be detected objectively, and that adjustments directly match the magnitude of the effect.

ACORN-SAT V1 (to 2017) and V2.5 (to 2023) detected a significant changepoint in 1987, and another in 1997 corresponding with the merge. However, AcV2.5 failed to detect the significant changepoint in 2016. Adjustments made by AcV1 and AcV2.5 in 1987 were not the same, and in both cases were disproportionate to the rainfall-adjusted magnitude of the effect. Adjustments made by ACORN-SAT for merging the data in 1997, were also not the same.

The failure of AcV2.5 to adjust for installation of the security fence, which, as they paid for it, they must have known about, compounds previous errors, thus creating non-significant trends in homogenised data of 0.18^oC/decade and 0.14^oC/decade respectively.

A Google Earth Pro satellite image of the Rabbit Flat AWS on 18 August 2023. The Stevenson screen in the upper-right of the enclosure appears to be less than 4m from the 2.4m-high security fence. Fencing the site in 2015, groundcover destruction using herbicide, and other disturbances have warmed the site independently of the climate.    

Homogenisation adjustments by AcV1 and AcV2.5 in 1987 were not the same as the magnitude of the change they aimed to correct. Also, data were adjusted for a changepoint in 1997 that was not statistically significant. In addition, the significant site change in 2015 the BoM must have known about was ignored as though the effect on data reflected the climate.

The Bureau’s homogenisation methods cannot be independently replicated and it seems doubtful that statistical tests of differences with highly correlated neighbours are sufficiently sensitive to discriminate between step-change effects that are real and verifiable, and site and instrument changes that have no effect.  

ACORN-SAT also undertakes no post hoc tests to verify outcomes and check assumptions underlying the trend verses step-change model. ACORN-SAT adjustments resulted in non-significant positive trends of 0.18^oC/decade for AcV1, and 0.14^oC/decade for AcV2.5. In contrast, BomWatch protocols, which include verification and assumption tests, found that adjusting for step-changes and rainfall simultaneously left no residual trend that could be attributed to CO₂, coalmining, electricity generation or anything else.     

Implications

The Bureau’s incessant marketing of fake record temperatures, record-hot years, heatwaves and in particular, runs of warm days, ignores that since 1970, Tmax at Rabbit Flat increased 1.76^oC (±0.23_sem) due to site changes alone. However, ignoring that the site had warmed independently of the climate, it was reported in March 2019 that Rabbit Flat smashed the national heat-record previously held by Marble Bar.

It was claimed that maximum temperature of 38.4^oC on Tuesday 26 March 2019 ended the longest-run of 115-days of maximum temperatures over 39^oC in Australia’s history. However, the Rabbit Flat site was not operating when the record of 106-days was set at Marble bar between November 1921 and March 1922.

ACORN-SAT has exceeded its use-by date. Site-summary and ACORN-SAT metadata has been poorly researched. Statistical tests of differences between comparator datasets that are not homogeneous, and faulty ACORN-SAT-site data, can find changepoints that make no difference, while not detecting those that do. As the methodology is fundamentally flawed, not scientific and cannot be replicated independently, the project should be abandoned in its entirety.   

Reference:

Trewin B.C., 2001. Extreme temperature events in Australia. PhD thesis, School of Earth Sciences, University of Melbourne.