How Green Is My Gas?

Natural gas / methane is seen by some as a gateway or bridge to the end goal of net-zero. However, in part 3 of the BBC's Big Oil vs The World (at 19m25s) Professor Bob Howarth asserts that,

"If the leakage rate of the natural gas is of the neighbourhood of 3%, then it's as bad for the climate as burning coal."

The same programme (15m00s) includes footage shot using an optical gas-imaging camera showing a US gas facility leaking vast amounts of methane into the atmosphere.

Below I explain why I think the quote is wrong, well wrong.

Methane vs Oil vs Coal

First let's check how much greener methane is than other fuels, if we assume that no methane is leaked during production. Let's do this by comparing the amount of CO₂ released by burning to generate the same amount of electricity.

I looked up the carbon intensity of each fuel: that's the amount of CO₂ emitted to produce a kilowatt hour of electricity. Obviously, the carbon intensity varies quite a bit depending on the efficiency of the power station. But I found wildly varying estimates:

  • reports: coal 2.23 pounds CO₂/kWh (1.02 kgCO₂/kWh); petroleum 2.13 pounds CO₂/kWh (0.97 kgCO₂/kWh); gas 0.91 pounds CO₂/kWh (0.41 kgCO₂/kWh)
  • "Fig. 9. GHG emissions factors from the construction and decommissioning of electricity producing facilities" reports "combustion emissions" as solid fuels 900 gCO₂eq/kWh; petroleum 685 gCO₂eq/kWh; natural gas 340 gCO₂eq/kWh
  • "The displayed indicator only illustrates CO₂ emissions generated by the consumption of primary fuel used on power plants located in France. They do not include carbon emissions generated during the construction of power plants or during the mining/processing/transportation of these fuels": 0.986 tCO₂eq/MWh for coal-fired plants; 0.777 tCO₂eq/MWh for oil-fired plants; 0.429 tCO₂eq/MWh for gas-fired plants
  • "Table A.III.2 | Emissions of selected electricity supply technologies" "Direct emissions Min / Median / Max": Coal - PC 670 / 760 / 870 gCO₂eq/kWh; Gas - Combined Cycle 350 / 370 / 490 gCO₂eq/kWh; no oil figure

For simplicity I considered only the carbon intensity of the combustion phase of electricity generation and not the lifetime intensity including mining the fuel, transporting it, building and decommissioning the power plant. I settled on the following central estimates:

Fuel kg CO₂ / kWh
Coal 0.90
Petroleum 0.75
Natural gas 0.40

Observe: electricity generated from coal and oil causes about twice the CO₂ emissions per unit of electricity as from natural gas.

How Much Methane Leaks?

Next, how much methane leaks during the extraction, processing and transportation of these fuels?

The IEA's Global Methane Tracker Report ( estimates that in 2021 methane leaked per fuel:

Fuel Estimated Methane Leaks Total
Coal "42Mt are from coal mine methane" 42Mt
Oil "41Mt" 41Mt
Natural gas "39Mt are from extracting, processing and transporting" + "4Mt leaks from end-use equipment" 43Mt

Note: these are only estimates; no one's measuring this; the real numbers are probably higher.

How Much Production Per Fuel?

Now we need to find out how much of each fuel is produced each year.

BP's Statistical Review of World Energy 2021 tells us that in 2020 world production was as follows:

Fuel World Production And in Usable Units
Coal "159.6 exajoules" (p48) 6,384Mt
Oil "4,165.1 million tonnes" (p19) 4,165Mt
Natural gas "3,853.7bn m³" (p38) 2,832Mt

Unit conversions from

  • 1 billion cubic metres NG = 0.735 million tonnes LNG
  • One exajoule equals approximately = 40 million tonnes of hard coal

Comparing natural gas production to methane leaks (from the IEA above) suggests that 43Mt / 2,832Mt = 1.52% of gas leaks into the atmosphere.

How Much Methane Leaks per kg of Fuel?

Using the figures from "How Much Methane Leaks?" and "How Much Production Per Fuel?" we can work out how much methane leaks per kg of fuel produced.

Fuel kg Methane Leaked per kg Fuel
Coal 42Mt / 6,384Mt = 0.0066
Petroleum 41Mt / 4,165Mt = 0.0098
Natural gas 43Mt / 2,832Mt = 0.0152

How Much Fuel per kWh?

We need to find out how much of each fuel is needed to generate 1kWh of electricity.

So to repeat and expand the first table:

Fuel kg CO₂ / kWh kg Fuel / kWh
Coal 0.90 0.90 / 1.86 = 0.48
Petroleum 0.75 0.75 / 3.15 = 0.24
Natural gas 0.40 0.40 / 2.75 = 0.15

How Much Methane Leaks per kWh?

Let's work out how much methane leaks per kWh of electricity generated from each different fuel by using the figures from "How Much Methane Leaks per kg of Fuel?"

Fuel kg Fuel / kWh kg Methane Leaked / kWh
Coal 0.48 0.48 x 0.0066 = 0.0032
Petroleum 0.24 0.24 x 0.0098 = 0.0023
Natural gas 0.15 0.15 x 0.0152 = 0.0022

What's the Total Warming per Fuel?

Let's add the warming effect from the leaked methane to "kg CO₂ / kWh" figures from "Methane vs Oil vs Coal".

By mass methane has 27.9 times the warming effect of CO₂ (see when averaged over 100 years.

Fuel kg CO₂ / kWh kg CO₂e from Leaked Methane / kWh Total kg CO₂e / kWh
Coal 0.90 0.0032 x 27.9 = 0.089 0.99
Petroleum 0.75 0.0023 x 27.9 = 0.065 0.82
Natural gas 0.40 0.0022 x 27.9 = 0.062 0.46

Despite its very leaky production, methane is so much more efficient than coal for electricity generation, that it has a far lower total climate-warming footprint.

If the shocking leaks from methane production could be staunched, its lower climate-warming footprint could be reduced even further by 0.062 / 0.46 = 13%.

What If the IEA Are Underestimating Methane Leaks?

As per section "How Much Production Per Fuel?" the IEA estimates that 1.5% of gas leaks into the atmosphere. What if 3% leaked, as hypothesised in part 3 of the BBC's Big Oil vs The World?

In that case we could add an extra row to the table above:

Fuel kg CO₂ / kWh kg CO₂e from Leaked Methane / kWh Total kg CO₂e / kWh
Natural gas (3% leaked) 0.40 0.0022 x 3 / 1.52 x 27.9 = 0.122 0.52

This is still less than half the CO₂e / kWh from coal.

How Much Methane Leaks Before Natural Gas Is Worse Than Coal?

The answer is that if 14.5% of the methane leaks it is as bad for electricity generation as coal:

Fuel kg CO₂ / kWh kg CO₂e from Leaked Methane / kWh Total kg CO₂e / kWh
Natural gas (14.5% leaked) 0.40 0.0022 x 14.5 / 1.52 x 27.9 = 0.59 0.99

Why Choose The Warming Effect of Methane Over 100 Years?

A molecule of methane warms the atmosphere 87 times more strongly than a molecule of CO₂ but disappears from the atmosphere much faster than CO₂: methane breaks down in years/decades; CO₂ hangs around for centuries/millennia. I chose the average warming of methane over 100 years because we've been releasing methane steadily for decades and unfortunately probably will continue doing so for decades. The methane concentrations in the atmosphere are probably rising only slowly: new methane is being leaked only a bit faster than old methane is decomposing.

For interest, what would the comparison look like if we used the average warming effect of methane over 20 years? I think this is how Bob Howarth finds burning methane to be as climate-warming as burning coal.

Suppose by mass methane has 80 times the warming effect of CO₂ (see when averaged over 20 years. We end up with:

Fuel kg CO₂ / kWh kg CO₂e from Leaked Methane / kWh Total kg CO₂e / kWh
Coal 0.90 0.0032 x 80 = 0.25 1.15
Petroleum 0.75 0.0023 x 80 = 0.19 0.94
Natural gas (1.5% leaked) 0.40 0.0022 x 80 = 0.18 0.58
Natural gas (3% leaked) 0.40 0.0022 x 3 / 1.52 x 80 = 0.35 0.75
Natural gas (6.5% leaked) 0.40 0.0022 x 6.5 / 1.52 x 80 = 0.75 1.15

Burning methane is still producing a lot less total climate-warming than burning coal, unless we're leaking in the neighbourhood of 6.5% of the methane.

How Inefficient Is LNG? (an Aside)

Given that the UK imports a large proportion of its natural gas by ship as liquified natural gas (LNG), a friend asked me whether this means that the carbon-footprint of natural gas is being underestimated? says that LNG plants are massive energy consumers. This is due to the fact that the energy required for liquefying one kilogram of natural gas is around 1,188 kJ.
(this text is visible when I google "lng liquefaction efficiency" but restricted on the page itself) says "The heat value of a fuel is the amount of heat released during its combustion ... Natural gas 42-55 MJ/kg".

Around 2.5% of the energy stored in natural gas is required to cool/compress it.

However, electricity generation from natural gas is inherently inefficient (obviously less so than from coal). As per section "How Much Fuel per kWh?" it takes 0.15kg methane to generate 1kWh electricity. 1kWh = 3.6MJ. So 1kg natural gas generates 3.6 / 0.15 = 24MJ electricity.

Around 5% of the electrical energy generated from natural gas is required to cool/compress it into LNG to transport it by ship.


Good question, Mark Abbas. Thanks for your feedback, Alex Zeffertt.

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