Methane GWP: a 20-year or 100-year horizon? How to calculate the climate impact of biogas

GWP (Global Warming Potential) is a coefficient that tells you how many times more strongly a given gas warms the climate than the same mass of CO₂ over a specified time horizon. For methane, according to the IPCC Fifth Assessment Report (AR5, 2013), GWP₁₀₀ is about 28 and GWP₂₀ about 84. The difference arises because methane breaks down in the atmosphere within roughly a decade, whereas CO₂ persists for hundreds of years. In a 100-year calculation the impact of methane is "diluted" over time; in a 20-year calculation it is visible in full.

How GWP arises

The GWP value depends on two properties of a gas: how strongly it absorbs radiation (the so-called radiative forcing) and its lifetime in the atmosphere. Methane absorbs infrared radiation many times more strongly than CO₂ per molecule, but it disappears faster: its lifetime is about 12 years (perturbation lifetime). By integrating the impact over 20 or 100 years and dividing by the analogous integral for CO₂, we obtain the respective GWP.

AR5 gives values both with climate-carbon cycle feedbacks (GWP₁₀₀ about 34) and without them (about 28). Emissions reporting usually uses the values without feedbacks.

Which horizon to choose

The choice of horizon is a matter of the purpose of the analysis, not of "truth". For policies aiming to limit warming over the coming decades, rapid methane reductions are important; for long-term climate neutrality, CO₂ is the key. Both perspectives lead to the same practical conclusion in the context of biogas: leaks must not be allowed.

Consequences for biogas plants

Biogas is about half methane, so every leak from a digester, a biogas storage tank, safety valves or an upgrading installation has a climate dimension. At GWP₁₀₀ = 28, 1 tonne of leaked methane corresponds to 28 t CO₂e, and at GWP₂₀ = 84 as much as 84 t CO₂e.

Worked example

Assumptions: a biogas plant produces 4 million m³ of biogas a year (55% CH₄, i.e. 2.2 million m³ of methane), the leak amounts to 2% of the methane, and the density of CH₄ is 0.717 kg/m³. Leak: 44 thousand m³ ≈ 31.5 t CH₄. With GWP₁₀₀ = 28: 883 t CO₂e a year. With GWP₂₀ = 84: 2650 t CO₂e a year. For comparison, burning 31.5 t of methane in a flare would convert it into about 86 t CO₂ (from the reaction CH₄ + 2O₂ → CO₂ + 2H₂O; 16 g CH₄ → 44 g CO₂), which is about 10 times less on the GWP₁₀₀ scale and 30 times less on the GWP₂₀ scale than a leak. This justifies the requirement for an emergency flare and for gas-tightness.

Biogenic CO₂ and methane

The carbon dioxide released when biogas is burned comes from biomass that earlier took it from the atmosphere, so in simplified balances it is treated as neutral. Methane leaking into the atmosphere is not neutral: it turns into CO₂ after a dozen or so years (which is also biogenic), but before that happens it warms the climate. That is why the emissions balance of a biogas plant consists mainly of methane and N₂O, not CO₂.

Practical rules

• Always state the GWP version (e.g. AR4 or AR5) and the time horizon in documents.
• Use the same coefficients when comparing offers and projects.
• Check whether the methodology (e.g. RED) imposes GWP values, because then there is no freedom of choice.
• When presenting results, consider showing both horizons when discussing the impact of leaks.

Other gases in the balance of a biogas plant

Besides methane, the balance of an agricultural biogas plant includes nitrous oxide (N₂O) from digestate applied to fields, whose GWP₁₀₀ according to AR5 is about 265. For N₂O the time horizon matters less, because the gas stays in the atmosphere for over 100 years. For this reason the emissions balance of a biogas plant has to be built from all three gases: CO₂, CH₄ and N₂O, in each case stating the GWP adopted. Omitting any of them leads to understating or overstating the result, and comparisons with other technologies lose their meaning.

Summary

GWP is a tool for comparison, not a physical constant. The values for methane depend on the horizon and the version of the IPCC report, and for biogas plants the practical conclusion is to take care over gas-tightness, measurements and limiting methane losses.