In an entertaining and somewhat chaotic episode of Q&A (Monday 15th August) pitching science superstar Brian Cox against climate contrarian and global conspiracy theorist and now senator Malcolm Roberts, the question of cause and effect and empirical data was raised repeatedly in regard to climate change.

Watching I pondered on a question - what would it take me to change my mind? After all, I should dearly love to be convinced that climate was not changing, or if it were, it were not due to our unrelenting emissions of CO2 and other greenhouse gases. That would make things just so much easier, all round.

So what would make me change my mind?

There are two elements to this question. The first is the observational basis, and the question of empirical data. The second relates to cause and effect, and the question of the greenhouse effect.
Here I will focus on the the first.

On the second, I will only add that the history of our planet is not easily reconciled without recourse to a strong greenhouse effect. If you have any doubt then you simply need to read my former colleague Ian Plimer.
As I have pointed out before, in his 2001 award-winning book “A Short History of Planet Earth”, Ian has numerous references to the greenhouse effect especially in relation to what all young geologists learn as the faint young sun paradox:

“The early sun had a luminosity of some 30 per cent less than now and, over time, luminosity has increased in a steady state.”

“The low luminosity of the early sun was such that the Earth’s average surface temperature would have been below 0C from 4500 to 2000 million years ago. But there is evidence of running water and oceans as far back as 3800 million years ago.”

The question is, what kept the early Earth from freezing over?

Plimer goes on to explain: “This paradox is solved if the Earth had an enhanced greenhouse with an atmosphere of a lot of carbon dioxide and methane.”

As one of the grand priests of climate contrarians, I doubt that Malcolm would consier Ian is to be part of the cabal of global climate change conspiracists, though that would be ironic.

As a geologist, I need to be able to reconcile the geological record of a watery planet from time immemorial, with the faint young sun hypothesis. As Ian points out so eloquently, with nothing else on the menu, the greenhouse effect is all we have.

If the menu changes, then I will reconsider.

How about the empirical data?

Along with Brian Cox, I find it implausible that an organisation like NASA, with a record of putting a man on the moon, could or would fabricate data to the extent Malcolm Roberts insinuates. It sounds such palpable nonense, it is something you might expect from an anti-vaxer.

However, a clear message from the Q&A episode is there is no way to convince Malcolm Roberts that the meteorological temperature data has not been manipulated to achieve a predetermined outcome. So he simply is not going to accept those data as being empircal.

However, the relevant data does not just include the records taken by meteorological authorities. It also includes the the record presereved beneath our feet in the temperature logs from many thousands of boreholes across all inhabited continents. And the importance of those logs is that they are reproducible. In fact Malcolm can go out an re-measure them himself, if he needs convincing they are “emprical”.

The idea that the subsurface is an effective palaeo-thermometer is a simple one that we use in our every day life, or used to at least prior to refrigeration, as it is provides the logic for the cellar.

When we peturb the temperature at the surface of the earth, for example as the air temperature rises during the day, it sends a heat pulse downwards into the earth. The distance the pulse travels is related to its duration. As the day turns to night and the surface cools, a cooling pulse will follow, lagging behind, but eventually cancelling, the daily heating. The diurnal surface temperature peturbations produce a wave like train of heating and cooling that can felt with diminishing amplitude down to a skin depth less than a metre beneath the surface before all information is cancelled out, and the extremes of both day and night are lost.

Surface temperatures also change on a seasonal basis from summer to winter and back again, and those temperatures propagate even further to depths of around 10 metres before completely cancelling [1].

On even longer cycles the temperature anomalies propagate much further, and may reach down to a kilometre or more. For example, we know that over the last million years the temperature on the earth has cycled in an out of numerous ice ages, on a cycle of about 100,000 years. Cycles on that timescale can propagate more than one kilometre into the earth, as we see in deep boreholes, such as the Blanche borehole near the giant Olympic Dam mine in South Australia. From our analysis of the Blanche temperature logs we infer a surface temperature amplitude of around 8°C over the glacial cycle.

So what do we see in the depth range of 20-100 metres that is sensitive to the last 100 years, and most relevant to the question fo changing climate?

The image below shows the temperature log from a borehole that we purpose drilled in Gippsland as part of AuScope AGOS program.

Temperature log in the upper 70 metres of the Tynong AGOS borehole drilled and cored to a depth of 500 metres. The temperature profile shows various stages. Above the groundwater level at 10 m, in the vadose zone, the temperatures in the borehole rapidly equilibrate on seasonal timescales to surface temperature changes due to infiltration of groundwater. In the winter when this temperature log was obtained, the temperatures in this zone trend towards the ambient temperature much lower than 12°C. In summer, they rise to over 20°C. Beneath the vadose zone the temperature in the borehole responds to the conduction of heat influenced by two dominant factors, the changing surface temperature on time-scales of decades to many hundred of years, and the heat flow from the deeper hot interior of th earth. During a rapid surface warming cycle lasting more than several decades the normal temperature gradient in which temperatures increase with depth can be reversed, so that we get a characteristic rollover (with a minimum here seen at about 30 depth). This shallow rollover is characteristic of all the boreholes we have explored, and has been reported in many thousands of borehoes from around the world. The temperature logs shown here were obtained by Kate Gordon, as a student at the University of Melbourne.

In geophysics we use the techniques of inversion to identify caustive signals, and their uncertainties, in records such as the Tynong borehole log, as well as in the estimation of the value of buried ore bodies and hydrocarbon resources. This image shows the result of the inversion of the Tynong temperature log for surface temperature change over the last 700 years, with uncertainties at the 95% confidence interval. Not suprisingly as we go back in time the uncertainties become larger. However, the inversion, which is based on Fourier’s law of heat conduction, shows that we can be confident that the Tynong AGOS borehole temperature record is responding to a long-term heating cycle of 0.3-1.3°C over the last century at the 95% conficnece level. The inversion shown here was performed by Kate Gordon.

If there were just one borehole that showed this record, it would not mean much. However the characteristic shallow rollover is present in all the boreholes we have explored, and has been reported in many thousands of borehoes from all around the world.

The physics that can explains these observations dates back to Joseph Fourier, over 200 years ago, so its not exactly new or even contentious. The only way we can sensibly interpret such empirical evidence is that ground beneath our feet, down to a depth of around 50 metres or so is now heating from above. In effect the solid earth below is now absorbing heat from the atmosphere above, counter to the normal process of loosing heat to it. However, if Malcolm can bring to the table an alternative physics to explain these observations, while not falling foul of all the other empirical observations that Fourier’s law of heat conduction admits, then I am happy to consider, and put it to the test.

Perhaps the hyper-skeptical Malcolm thinks that somehow the global cabal of climate scientists has got into all these thousands of boreholes with an electrical heater to propagate the heat signal that artificially simulates surface heating. More fool me.

But, if he does, then I am perfectly happy to arrange to drill a new hole and, along with him, measure the temperature profile, making sure we don’t let those pesky climate scientists get the hole with their heating coils before we have done so. And I’ll bet him we can reproduce the signal from Tynong shown above.

But I’ll only do it on the condition that Malcolm agrees, that when we do, he will publicly acknowledge the empirical evidence of a warming world entirely consist with NASA’s surface temperature record.

Malcolm, are you on?

Disclosure

Mike Sandiford receives funding from the Australian Research Council to investigate the thermal structure of the Australian crust.

Mike Sandiford, Professor of Geology, University of Melbourne

This article was originally published on The Conversation. Read the original article.