What Is CO2e?
CO2e (carbon dioxide equivalent) is a unit that converts all greenhouse gases into the equivalent warming effect of CO₂. It lets us add up methane, nitrous oxide, and other gases into a single number, making it easier to measure and compare climate impact—whether for a flight, a meal, or an entire year.
Why CO2e is used instead of just CO2
Carbon dioxide (CO₂) is the most common greenhouse gas, but it's not the only one. Methane, nitrous oxide, and fluorinated gases also trap heat in the atmosphere—often much more powerfully, per molecule, than CO₂. If we only counted CO₂, we'd underestimate the climate impact of agriculture, landfills, and industry.
CO2e solves this by expressing every gas in terms of how much CO₂ would cause the same warming over a given period. One unit of methane might equal 28 units of CO2e over 100 years—so we can compare emissions across different sources fairly.
Methane vs CO2 comparison
Methane (CH₄) is about 28–34 times more potent than CO₂ over 100 years, depending on the time horizon used. That means 1 tonne of methane has roughly the same warming effect as 28–34 tonnes of CO₂. Methane also breaks down in the atmosphere within about 12 years, while CO₂ can persist for centuries.
Major methane sources include livestock (especially ruminants like cattle), rice paddies, landfills, and fossil fuel extraction. Because methane is so potent, reducing it can have a big, fast impact on slowing warming—but CO2e helps us see how methane fits into the bigger picture alongside CO₂.
How CO2e is calculated
Scientists use Global Warming Potential (GWP) to convert each gas into CO2e. GWP compares how much heat a gas traps over a fixed period (usually 100 years) relative to CO₂, which has a GWP of 1. For example, nitrous oxide (N₂O) has a GWP of about 265, so 1 kg of N₂O ≈ 265 kg CO2e.
To get a total CO2e for an activity or product, each gas emitted is multiplied by its GWP and then summed. Carbon footprint calculators use these conversion factors to turn your lifestyle choices—flights, diet, heating, etc.—into a single tCO2e (tonnes of CO2e) number per year.
Why it matters for climate targets
Climate science tells us we need to limit total warming to 1.5–2°C above pre-industrial levels. That means cutting global greenhouse gas emissions to net zero by around mid-century. CO2e is the common yardstick used in international agreements (like the Paris Agreement), national pledges, and corporate targets.
On a personal level, per-capita targets—often around 2–4 tonnes CO2e per year by 2030—are expressed in CO2e because they include all gases from transport, food, energy, and consumption. Understanding CO2e helps you interpret your own footprint and see how it compares to what the planet can sustain.
Calculate your CO2e footprint
Ready to see your annual emissions in CO2e? Our calculators estimate your carbon footprint in about 2 minutes.