Methane Leakage Measurement

The U.S. government's official methane inventory says natural gas is relatively clean. Satellites overhead disagree. Both numbers are the result of careful methodology, which is exactly the problem.

The official EPA methane inventory is built "bottom-up": engineers count every valve, compressor, and pipeline segment in the country, apply standard emission factors, and sum them up. Satellites and aircraft measure methane concentrations in the atmosphere and work backwards to infer total emissions, a "top-down" approach. Top-down estimates consistently run 25-75% higher than the EPA bottom-up inventory. The discrepancy is not a rounding error. It reshapes the entire lifecycle comparison between natural gas and coal.

The reason for the gap matters as much as the gap itself. Roughly 5% of production sites, called "super-emitters," account for approximately 50% of total methane emissions from the oil and gas sector. These are malfunctioning equipment, deliberate venting, and operational upsets. Bottom-up inventories use average emission factors and miss rare catastrophic events. Top-down measurements catch the actual atmosphere.

Why averages fail here

Imagine a gas field with 1,000 well pads. The EPA methodology assigns each pad a standard leak rate, say 0.1% of throughput. Total estimated leakage: 0.1% of total production.

But what if 50 of those pads have a failed valve releasing 20 times the standard rate?

Those 50 pads alone would contribute as much methane as the other 950 combined, but bottom-up accounting treats all 1,000 identically. A satellite flying over would see the anomalously high atmospheric methane concentration directly. The bottom-up number would be half the real one.

This is the super-emitter problem: the distribution is not a bell curve. It has a fat right tail, and that tail decides whether gas beats coal on climate.

Methane monitoring policy, such as EPA's methane rules for oil and gas, directly targets super-emitters with leak detection and repair requirements. The measurement dispute is not academic: it determines whether natural gas retirements or coal retirements should be the higher climate priority.

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