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DALL·E generated image of yellow mist over an open pit coal mine, digital art
DALL·E generated image of yellow mist over an open pit coal mine, digital art

Air Quality

What Is An Acceptable Methane Leakage Rate?

The study showing 0.2% leakage natural gas is as bad as coal generation in one scenario could be read as recommending coal instead, which would be a mistake.

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Recently, researchers affiliated with Brown University, the Rocky Mountain Institute (RMI), Harvard, NASA JPL ,and Duke University published a paper on the implications of methane leakage, “Evaluating net life-cycle greenhouse gas emissions intensities from gas and coal at varying methane leakage rates.” The takeaway that’s being promoted in headlines and articles is that natural gas is as bad as coal with as little as 0.2% upstream methane leakage.

“We estimate that a gas system leakage rate as low as 0.2% is on par with coal, assuming 1.5% sulfur coal that is scrubbed at a 90% efficiency with no coal mine methane when considering climate effects over a 20 year timeframe.”

That’s an important claim, so it’s worth looking at. Among other things, when I compared highly differentiated lifecycle assessments of upstream methane emissions in 2021 by Howarth and Jacobson from the USA and a pre-peer-reviewed Bauer et al. paper from Europe, 0.2% was the best case scenario Bauer et al. used, something I considered unfathomably good. I’ve done the math for fugitive emissions’ implications for natural gas generation a few times, and found 0.2% to be problematic, but 1.5% and 3% much more so.

“The global average for methane which is delivered for use is 1.5% upstream methane leakage. In natural gas generation, that turns 400 kg CO2 per MWh into closer to 600 kg CO2e per MWh, about 75% of the best coal plants’ 800 kg, ignoring their fugitive coal bed methane emissions. In the USA with its ~3% upstream emissions, that makes natural gas generation over 700 kg CO2e per MWh, or a bit of a wash compared to the best coal generation, although still better than the 1,200 kg of the worst coal generation.”

There’s a bit of a gap there, which implies that our methodologies differed. And to be transparent, I’m in no way claiming that their methodology is incorrect or that mine is more than napkin math. The paper is peer-reviewed and published in a very credible journal, Environmental Research Letters, which has an impact factor of 6.947. It’s not Nature, but it’s up there in terms of good journals, and its focus is well-aligned with the content of the paper, so peer-reviewers will have had the context to assess the paper. The number of authors — six — isn’t a red flag indicating a weird pile-on or a particle physics paper where global projects lead to papers with 1,000 authors, and even in one case 5,000 authors. The authors themselves include two with h-indices over 80 and one over 40, when 40 is considered the base level for a good mature academic. It’s a solid paper that can be relied upon, even if interpretation might be challenging.

Incidentally, the Bauer et al. paper, On the climate impacts of blue hydrogen production, made it through peer-review and was published in 2022 in the UK’s Royal Society of Chemistry journal, Sustainable Energy & Fuels, another high impact factor publication. It’s on my list to review the final version compared to the pre-peer-reviewed version I assessed. There were some clear places where I suggested peer reviewers should pay attention, and I know Bauer and company saw my comments, so it will be interesting to see if they considered them material.

No, I’m not questioning whether the methane leakage study’s methodology or the findings are valid, I’m trying to figure out the methodology, why its results are so at variance with my understanding and calculations and the implications for some important work I’m engaged with.

What’s that work? Well, I’m involved with an EU-funded initiative to normalize upstream methane emissions leakage methodologies, mitigation strategies, and policies around the world. I’m engaged in the EU-Canadian portion of the effort, which just kicked off this week. I, along with many others, have been increasingly focused on anthropogenic methane emissions as a major heating forcer, and looking at solutions. As I’ve noted, the question isn’t whether we will be burning natural gas for electrical generation and heat for a few decades to come, but how little natural gas generation we can build and how fast we can sunset it.

And upstream emissions from the oil and gas industry and natural gas transmission and distribution networks have been found to be a lot leakier than originally reported. Getting common system boundaries and accounting for emissions is critical. As the paper points out:

“Recent aerial measurement surveys of US oil and gas production basins find wide-ranging natural gas leak rates 0.65% to 66.2%, with similar leakage rates detected worldwide.”

That is, of course, on top of very large anthropogenic biomethane emissions from landfills, ruminant livestock, agricultural waste, and animal dung. Yes, our fossil fuel habit, as well as our food and waste systems, are all big methane emitters. So it’s important to figure out how impactful it is.

Back to the published paper.

“We find that global gas systems that leak over 4.7% of their methane (when considering a 20-year timeframe) or 7.6% (when considering a 100 year timeframe) are on par with life-cycle coal emissions from methane leaking coal mines.”

This less quoted point from the abstract starts to unpack what’s going on. The paper draws system boundaries in multiple ways and assesses them against one another. It also seems more aligned with more usual estimates, but the numbers seem high in the opposite direction.

After all, a basic calculation like the one I did also excludes methane from coal mines (a flaw in my approach), but includes methane leakage for natural gas, and comes up with smaller numbers for methane leakage.

The primary quoted piece, 0.2%, talks about a specific case where the system boundaries are drawn differently. In that case, upstream fugitive emissions from natural gas are included, but equivalent emissions from coal are excluded. What is also included is sulfur aerosols.

What’s the big deal about the aerosols? Well, they mask global warming impacts by reflecting more light back into space. We’ve been geoengineering our climate in two different directions over the past three centuries. The first is the one most of us are more familiar with, where we emit carbon dioxide and other greenhouse gases and the temperature goes up.

But with burning lots of higher sulfur fuels, something we used to do a lot more of, sulfur aerosols come with the fuel. That includes mostly coal and marine fuels, but also crappier variants of diesel and the like. And so we’ve been masking the degree of heating we’ve been forcing because of air pollution.

That’s the premise of what I consider to be the bad idea of solar geoengineering, where we intentionally release tons of sulfates into the upper atmosphere to increase the reflection of sunlight. I consider it a bad idea because it treats exactly one symptom of global warming, not the cause, and would significantly increase the likelihood that we’d just keep burning fossil fuels long past the time we should have stopped. As atmospheric CO2 would keep increasing, so would CO2 uptake in the oceans, which would strip away carbonates required for shellfish shell formation. Lots of bad would likely come of that. I’m a strong supporter of global adoption of the Oxford Principles on geoengineering, where rigorous global governance and scrutiny is involved at every step of the way, including experimentation.

But the paper treats the sulfates as a poorly explored variable in contrasting coal and natural gas electrical generation, which is fair. Instead of just methane and carbon dioxide, which force heating, it includes sulfur aerosols, which reduce heating. So what are the negative global warming potentials (GWP) of sulfur aerosols, specifically sulfur dioxide? As a reminder, everything GWP starts with CO2 being equal to one, a positive forcing. Because carbon dioxide lasts hundreds of years in the atmosphere, it has the same GWP at 20 years as 100, but other greenhouse gases last different lengths of time.

Methane from fossil fuels, for example, has a GWP of 82.5 at 20 years and only 30 at 100 years because it only persists in the atmosphere for around 12 years.

The basic forcing with sulfur aerosols are -156 over 20 years and -40 over 100 years, per the supplementary material document that’s part of the report. Three notes about this. First, the error bars are large, but there’s no doubt that sulfur aerosols reflect more light so reduce heating. Second, they don’t persist a long time in the atmosphere, although they do more so near the poles, so the 100-year GWP is a lot lower than the 20-year GWP. Third, they persist in the atmosphere for such a short period of time — days — that some consider negative GWPs to be an inappropriate measure.

And there’s another factor, which is that sulfur aerosols can promote cloud formation of specific types which also reflect sunlight, once again reducing heating forcing from carbon dioxide and other greenhouse gases. The study uses -367 for 20 years and -298 for 100 years when cloud formation is included. That’s an interesting one, because heating models were updated a couple of years ago based on empirical assessments of specific types of clouds, which found that they were less reflective than had been assumed, and so heating models were adjusted appropriately. I’m insufficiently deep to assess whether the paper uses the new ranges for cooling or the older ones.

The primary scenario that’s being quoted is moderately high-sulfur coal with only 90% removal of the sulfur. Coal has a range of 0.5% to 5% sulfur by dry weight, so 1.5% is a relatively conservative choice.

By the way, removal of sulfur is considered a good thing, because sulfur is the primary driver of acid rain which kills forests and lakes, and which saw treaties in Europe and North America to significantly reduce it in 1985 and 1991 respectively. It’s also fairly bad for human health, creating quite bad air pollution. I certainly remember the yellowish haze over outlying fields on bad air days near Toronto prior to the shutting down of all coal generation in the province.

So there it is. The 0.2% is natural gas upstream methane emissions heating forcing plus the carbon dioxide heating forcing when it’s used, while the coal excludes the upstream methane emissions, but includes both the carbon dioxide heating forcing and sulfur dioxide cooling forcing.

Is this an argument for burning coal for electricity? No, but it’s an exploration of why burning natural gas isn’t particularly better. Do I think it’s a great stat to promote? Not particularly. I think it weakens their argument to exclude upstream coal methane.

It does make clear that upstream methane emissions from coal mining should be included in coal generation statistics. It did beg a question I hadn’t looked at before, which is how bad that problem is. And apparently it’s very bad.

“Coal mining emits 52.3 million tonnes of methane per year, rivaling oil (39 million tonnes) and gas (45 million tonnes), and comparable to the climate impact of the CO2 emissions of all coal plants in China.”

That’s the first sentence of a Global Energy Monitor report from 2022 on the subject. And remember, methane has a much higher GWP than CO2 — 82.5 on 20 years, and 30 on 100 years — so that 52.3 million tons is the equivalent of 4.3 billion tons of carbon dioxide in the short term. That’s over 10% of global carbon dioxide emissions, for context.

Also for context, we burn about 8 billion tons of coal a year. A ton of coal turns into about 3.3 tons of carbon dioxide. But we should be adding about another half ton of CO2e to that from coal mine methane. The sulfur in our atmosphere really isn’t a win, so while it’s interesting and important to understand, getting rid of it will be a blessing too, although a mixed one.

The study showing 0.2% leakage natural gas is as bad as coal generation in one scenario could be read as recommending coal instead, which would be a mistake. Instead it should be read as saying both have to go as quickly as possible.

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is a member of the Advisory Boards of electric aviation startup FLIMAX, Chief Strategist at TFIE Strategy and co-founder of distnc technologies. He hosts the Redefining Energy - Tech podcast ( , a part of the award-winning Redefining Energy team. He spends his time projecting scenarios for decarbonization 40-80 years into the future, and assisting executives, Boards and investors to pick wisely today. Whether it's refueling aviation, grid storage, vehicle-to-grid, or hydrogen demand, his work is based on fundamentals of physics, economics and human nature, and informed by the decarbonization requirements and innovations of multiple domains. His leadership positions in North America, Asia and Latin America enhanced his global point of view. He publishes regularly in multiple outlets on innovation, business, technology and policy. He is available for Board, strategy advisor and speaking engagements.


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