Greenhouse Gases by Source

CO2 gets the headlines, but methane warms the planet 80 times more intensely per molecule over 20 years. Whether methane or CO2 matters more for policy depends entirely on which time horizon you choose, and that choice is a policy decision disguised as a technical one.

Three gases dominate human-caused warming. CO2 (roughly three-quarters of global GHG emissions by CO2-equivalent) comes primarily from burning fossil fuels and land-use change. It persists in the atmosphere for centuries. Methane (CH4) (most of the remainder) comes from livestock, rice paddies, landfills, and fossil fuel extraction. It is 80x more potent than CO2 over 20 years but breaks down in about 12 years, dropping to 30x over 100 years. Nitrous oxide (N2O) (a few percent) comes mainly from agriculture (fertilizer) and is 273x more potent than CO2 over 100 years.

The Global Warming Potential (GWP) converts different gases to a common unit (CO2-equivalent) over a chosen time horizon. GWP-100 (the standard) emphasizes long-lived gases like CO2. GWP-20 emphasizes short-lived but potent gases like methane. The choice between GWP-20 and GWP-100 changes the relative importance of methane by nearly 3x, making it a policy-relevant decision, not just an accounting convention.

Worked Example

A natural gas leak releases 1 tonne of methane per year.

• Convert to CO2-equivalent using GWP-100. 1 tonne CH4 x 30 = 30 tonnes CO2-eq.
• Convert using GWP-20. 1 tonne CH4 x 80 = 80 tonnes CO2-eq.

Why does the choice of time horizon matter for natural gas policy?

At GWP-100, methane leaks look manageable. At GWP-20, the same leaks may erase much of the CO2 advantage gas has over coal. Analysts arguing gas is a "bridge fuel" often use GWP-100; those arguing against it often use GWP-20. Same physics, different framing.

Every climate policy implicitly chooses a time horizon. Knowing which one, and why, is essential for evaluating any emissions comparison.

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