much higher heat-trapping ability. On a weight basis, methane has more than 20 times the global warming potential of carbon dioxide.
There are two ways to measure methane and other greenhouse gas emissions.
There’s the traditional ‘bottom-up’ method, where data comes from modelling and sampling – usually reported from those doing the emitting. The other is the ‘top-down’ approach, where satellites observe and record levels of methane or other pollution in the atmosphere above certain locations.
Depending on who you ask, one or the other of these methods can’t be trusted. The controversy is at the centre of an allegation that the Hail Creek Mine, about 80 kilometres south-west of Mackay in Queensland’s Bowen Basin, is emitting 10 times more methane than it’s telling authorities.
To understand this, let’s start with how bottom-up reporting works.
To get our emissions under control we first need to know how much we’re actually producing. Creating an ‘inventory’ of our emissions might seems straightforward but it can be quite difficult to do.
According to the CSIRO, in Australia, sources of this emissions information include livestock population counts, Bureau of Statistics demographic data and National Greenhouse and Energy Reporting from the oil and gas companies.
Collecting this data is a good starting point, but it’s not foolproof. Although there are rules in place to make sure the numbers are as correct as possible, they’re regularly lower than top-down emissions reviews suggest they’d be.
According to research published in Science back in 2018, oil and gas emissions were underestimated by the United States Environmental Protection Agency by about 60%.
This has important implications for how much we need to be lowering our carbon emissions over the next few years to be able to limit the worst effects of climate change.
Methane (CH4) is a particular problem. It’s the second most abundant greenhouse gas in our atmosphere after carbon dioxide (CO2), and it’s 25 times as potent at trapping heat, although it doesn’t stay in the atmosphere for as long. It comes from sources including wetlands, cow burps, and mining.
Gas like methane can escape from coal seams during coal mining – if this isn’t noted in bottom-up emissions it’s called ‘leaking’. In coal mining, the methane and other gas is usually released from degasification or ventilation systems. Gases are a by-product of the process of coal formation – where organic material, in the absence of oxygen, is heated and compressed over time. Some coal-mining regions can be particularly methane rich.
Stopping methane emissions would take us a long way towards limiting the climate crisis but we need to actually know how much of it is being released into the atmosphere.
This is where the top-down approach comes in. Instead of relying on methane emissions reported from the source, new satellites like the Copernicus Sentinel-5 Precursor can monitor the atmosphere and show in quite high resolution – each pixel covers 7 x 5.5 km2 – where polluting gases like ozone and methane are escaping from. This allows researchers to not only measure the methane of a whole country, but also measure the gases released from local areas such as mines.
“Satellite observations of methane are improving rapidly in both accuracy and spatial resolution, Using TROPOMI [the TROPOspheric Monitoring Instrument that’s aboard the Sentinel 5 satellite] it is possible to tell if a single mine or wetland has very large emissions, but newer satellites are making it possible to see even more detail and this will allow more accurate quantification and estimation of smaller emission sources. This is a very rapidly developing area” says Dr Cathy Trudinger, a lead researcher in atmospheric chemistry at CSIRO.
The problem is that these top-down satellite surveys consistently show much more methane and other greenhouse gases than the bottom-up approach.
The researchers also looked at two other plumes spotted in the area. One originates from the underground mines of Broadmeadow, Moranbah North, and Grosvenor, while the other comes from the Grasstree and Oaky North underground mines.
But 40% of the quantified emissions from the whole study came from a single surface mine – Hail Creek.
Glencore – a large international mining company that owns the Hail Creek mine – has rejected the findings, suggesting that the research includes inaccurate information.
But independent researchers are pretty confident that this is likely methane emissions, and not due to aerosols like dust or smoke or issues with albedo – a measure of reflection of light back from the Earth that the satellite uses to get a result.
“The plumes in Figure 1 seem to move around with the wind, so presumably this would rule out albedo problems due to fixed features like soil type, I couldn’t really say how accurate they are … [but] the results seem likely to me and would certainly warrant further investigation” says CSIRO’s Trudinger.
Dr Nicholas Deutscher – a scientist with the University of Wollongong whose work specialises in validating satellites with ground-based measurements (including TROPOMI) – agrees.
“There’s enough corroborating evidence there to make me think that they’ve picked up the sources,” he says. “In terms of how good the quantification is I think they’ve actually done a pretty rigorous job on it. And there’s a couple of things that make me think they’ve even erred on the side of caution perhaps.”
Of course, this is science, and we can’t be 100% sure, at least not just yet.
Deutscher suggests using ground-based measurements to double-check the validity of the results.
“What we really need is to independently verify these findings by some other method,” he says. “So, getting out there on the ground and making measurements within that plume near the surface, or making our type of ground-based measurements that validate the satellite measurements over that range.”
This is not just an issue at Hail Creek. Although we have pretty good estimates of methane emissions worldwide, and countrywide emissions due to satellite top-down data, we’re only now seeing the closer picture of which mines seem to be underreporting.
“CSIRO led a top-down study with two monitoring stations in the coal seam gas producing Surat Basin in Queensland and found that methane emissions were 33% larger than those from the inventory in that region,” says Trudinger.
This is also partly due to the way monitoring works. Government monitoring requires companies to provide these reports that can easily miss emissions like methane leaks. For example, you might measure how much coal you burn times the amount of methane produced per piece of coal, with sampling to check your working. However, as top-down satellite measurements suggest, this isn’t quite picking up all the details.
“It’s not necessarily nefarious. It comes down to what the reporting requirements are as well,” says Deutscher. “It’s really hard to quantify these different emissions because they change in space and time. That’s the fundamental problem with bottom-up estimates, you make a lot of assumptions about this process being the same all the time or just perfectly linearly dependent. That’s not always the case.”
Unfortunately, as far as we know, there doesn’t seem to be an easy way to limit these methane emissions without closing the whole mine.
“If coal mining companies continue to argue that fugitive methane management is too hard for open-cut coal mines, then the obvious and material solution is to cease opening new mines,” stated Naomi Hogan, strategic projects lead with Australasian Centre for Corporate Responsibility Inc “And for the known very gassy mines to seriously consider early closure options.”
Although Glencore has announced it will reduce emissions by 15% by 2026, and 50% by 2035, it hasn’t responded to questions about how this will take place. It’s noteworthy that in 2019 that Glencore had been bankrolling a coordinated campaign to prop up coal and push an anti-renewables message.
In the meantime, emission satellites zooming in on these coal mines are giving us more data.
Deutscher believes this – and papers like the one in Environmental Science & Technology – are going to prompt mining companies into cleaning up their reporting act.
“They may come out and deny it, but you can bet that behind the scenes they are out there to do all they can to make sure it’s not real,” he says.
