Barely a month passes without a news headline claiming that the Great Barrier Reef (GBR) is imperilled by climate warming. But by how much has the water temperature changed in the last 150 years since carbon dioxide concentrations started to rise? Atmospheric temperatures have been measured at many locations around Australia and the world for over a century – but for water temperatures, the historic data is hopelessly sparse. Some data is available from ship logs – many ships, especially steamers, recorded water temperature associated with engine operation. But there were very few boats along the GBR coast 150 years ago. In addition, the data was often taken by thermometers with dubious calibration or accuracy. The measurements were never meant to be used to determine small changes in temperature over a century.

A rare exception to this dearth of temperature data for the GBR is a remarkable record from 1871 which has, until now, never been analysed. This water temperature record was taken during the Australian Eclipse Expedition in 1871 from the Queensland government steamship Governor Blackall. The expedition was initiated by the Royal Society of Victoria and was the first major scientific expedition undertaken by the fledgling Australian colonies. The SS Governor Blackall left Sydney on 28th November 1871 to observe the total eclipse of the sun from a point near Cape York on 12th December, returning to Sydney on Christmas Day. The expedition was thought at the time to have been a failure – clouds and rain obscured the sun on the day of the eclipse.

But the expedition inadvertently collected a rare gem of a dataset – the first high-quality water temperature data on the GBR on both northward, and the southward, journeys along almost the entire length of the reef. It was a major scientific expedition with scientists from the Melbourne and Sydney Observatories, and included not only astronomers, but also a diarist and biologists interested in studying wildlife.

Widely reported in newspapers of the day, it was an expedition that would follow the footsteps of James Cook whose mission in 1770 was to do astronomic observations in Tahiti before exploring Australia and, inadvertently, discovering the Great Barrier Reef.

The expedition carried scientific instruments that were very advanced for the time. Thus, it is safe to assume that they used a well-calibrated thermometer to measure water temperature. A similar scientific temperature transect along the length of the reef would not be carried out again for the best part of a century.

The question is – were the water temperatures measured in 1871 within the range of variability of temperatures that one would measure today?

To answer this question, data for the 1871 transects were compared to data along the GBR that has been regularly collected by the Australian Institute of Marine Science (AIMS) over the last 30 or so years. The biggest challenge was taking account of the fact that the AIMS data is not collected at exactly the same locations as for the 1871 data. Thus, a degree of spatial interpolation is required to give some comparability between the modern data and that of the 1871 expedition. In addition, it is important to determine the natural variability of water temperature (typically a couple of degrees) from one year to the next – at least for the modern data.

Does the eclipse transect data fall within this range of variability or is it colder? If those members of the 1871 science expedition sailed the coast in 2025 – would they have noticed any appreciable difference?

In short, the answer is no. The 1871 data falls within the range of variability that has occurred from year to year over the last few decades.

It could be argued that perhaps the summer of 1871 was particularly hot for that period, thus minimising any temperature change since 1871. With data for only one year, this cannot be discounted entirely, but data shows it was unlikely. The ‘official’ Hadley Centre temperature data for the region, compiled from very sparse ship traffic, do not indicate that 1871 was especially warm.

If those eminent gentlemen who undertook that expedition in 1871 did the trip again, they would be unlikely to notice any difference in temperature, except that their cabin would now be air-conditioned.

In a similar vein, unlike many Australian ecosystems on land, the GBR has also changed very little. None of the three thousand individual coral reefs has been lost. All still have excellent coral. Morris Island Reef, where those scientists gloomily sheltered from the rain in December 1871, still has excellent coral.

Other data from AIMS shows that in each of the last three years there has been more coral on the GBR than at any time since records began in 1985.

The coral itself contains an embedded record of conditions on the GBR since well before the 1871 expedition. The large block-type corals are like trees in that they have annual growth rings. Corals living in hotter water grow faster, and produce thicker growth rings. Because some corals live for centuries, analysing coral growth rings gives an indication of water temperature going back well before British settlement. Such data also shows minimal changes in growth rate and therefore temperature.

Of course, this begs the question – why are we even worrying about gently rising water temperature which will inevitably lead to faster growing corals? The most diverse, fastest growing coral on earth live in the warmest ocean water on earth.

Cool and warm years come and go and the 1871 data suggest temperature along the GBR has remained within its range of natural variation for at least the last 150 years. This, together with record high amounts of coral, is good evidence that the catastrophism associated with the Great Barrier Reef is misplaced.

As for the ‘failed’ Australian Eclipse Expedition, it is apparent that the data they collected on water temperature is far more useful than anything they might have recorded if the clouds had cleared on that remote island in 1871.