What is global warming potential (GWP)? How CO₂ equivalent is calculated

Gases released into the atmosphere do not affect the climate equally. A kilogram of methane warms far more strongly than a kilogram of carbon dioxide but stays in the atmosphere far less time. Global warming potential (GWP) combines those two differences into one coefficient so that gases can be summed.

The definition

GWP is the ratio of the total warming effect of one kilogram of a gas over a defined time horizon to that of one kilogram of carbon dioxide over the same period. Carbon dioxide’s GWP is 1 by definition. Methane’s is around 28 over a hundred-year horizon, nitrous oxide’s around 265, and some fluorinated gases run into the thousands.

Multiply each gas in an inventory by its own GWP and sum, and you get carbon dioxide equivalent (CO₂e). That number is what we call a carbon footprint.

Why the time horizon changes things

GWP values are always quoted with a time horizon because gases have different atmospheric lifetimes. Methane decays in around twelve years; some carbon dioxide remains for centuries. Over a twenty-year horizon methane’s GWP is around 80 — nearly three times the 28 over a hundred years.

That choice has political consequences. A twenty-year horizon makes methane abatement look far more urgent; a hundred-year horizon foregrounds carbon dioxide. Standard practice is GWP-100, used by EN 15804 and ISO 14067 — but which one was used must always be stated in the report.

The threefold split in EN 15804+A2

The A2 revision of EN 15804 asks for three separate rows instead of one GWP figure, and that matters when reading construction product declarations.

• GWP-fossil — emissions from fossil carbon; the real target of abatement goals
• GWP-biogenic — carbon from biomass; negative when bound into the product, positive when released
• GWP-luluc — emissions from land use and land use change
• GWP-total — the sum of the three; read alone it can mislead

A timber product’s GWP-total can appear negative because of stored biogenic carbon. But that carbon is released at end-of-life and returns as a positive in the C modules. Looking only at the A1-A3 total for wood products is therefore systematically misleading.