Global Warming Potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere over a specific time period, compared to the same mass of carbon dioxide (CO2). It is essentially a conversion factor that lets scientists and policymakers compare the climate impact of different gases on a level playing field.
Think of it like currency exchange rates for greenhouse gases. Just as you need exchange rates to compare euros, dollars and yen, GWP lets you compare methane, nitrous oxide, fluorinated gases and CO2 in a single unit called CO2 equivalent (CO2eq). This is the unit you see in Ireland’s national emissions figures, expressed as MtCO2eq (megatonnes of CO2 equivalent).
CO2 itself has a GWP of 1, by definition, because it is the reference gas. Every other greenhouse gas is measured against it. The higher the GWP, the more warming that gas causes relative to the same weight of CO2.
🔢 GWP Values for Major Greenhouse Gases
The IPCC Sixth Assessment Report (AR6), published in 2021, provides the most current GWP values. These are the figures now recommended for national reporting and corporate emissions accounting:
🌿 Carbon dioxide (CO2): GWP of 1 over all time periods (the baseline reference)
🌿 Methane (CH4) from biogenic sources: GWP-100 of 27, GWP-20 of 81
🌿 Methane (CH4) from fossil fuel sources: GWP-100 of 30, GWP-20 of 83
🌿 Nitrous oxide (N2O): GWP-100 of 273
🌿 Sulphur hexafluoride (SF6): GWP-100 of 25,200
The critical thing to notice is the difference between 20-year and 100-year values. Methane has a GWP-20 of 81 but a GWP-100 of only 27. This is because methane is a potent but short-lived gas with an atmospheric lifetime of about 12 years. It causes intense warming over the short term but breaks down relatively quickly. CO2, by contrast, persists in the atmosphere for centuries.
⏱️ Why the Time Horizon Matters
The choice of time horizon is not just a technical detail – it has major policy implications. The standard practice is to use GWP-100 (the 100-year value), which is what you see in national inventories and corporate emissions reports. But critics argue this underplays the short-term impact of gases like methane.
Using GWP-20 instead of GWP-100 would make methane emissions look three times worse relative to CO2. For a country like Ireland, where agriculture accounts for nearly half of non-ETS emissions and where biogenic methane from livestock is a dominant source, the choice of metric directly affects how large the agricultural emissions problem appears.
The AR6 report also introduced a distinction between fossil and biogenic methane. Fossil methane gets a slightly higher GWP value (30 vs 27 over 100 years) because when fossil methane oxidises in the atmosphere, it produces CO2 that adds new carbon to the active carbon cycle. Biogenic methane, from sources like cattle or wetlands, produces CO2 from carbon that was recently absorbed from the atmosphere, so it does not add new carbon overall.
🇮🇪 Why GWP Matters in Ireland
Ireland’s emissions profile makes GWP particularly relevant. Agriculture is responsible for 34.3% of national emissions, and the sector is dominated by methane and nitrous oxide rather than CO2. How those gases are weighted against CO2 directly affects Ireland’s reported emissions total and its distance from climate targets.
In 2023, Ireland’s total greenhouse gas emissions were 58.82 Mt CO2eq (excluding LULUCF). Methane was the second largest contributor at 28.9% of the national total, primarily from livestock. The way methane is converted to CO2eq using GWP-100 values determines what proportion of Ireland’s emissions pie chart agriculture occupies.
This is why some agricultural stakeholders have advocated for the use of alternative metrics like GWP* (pronounced “GWP star”), which accounts for the fact that a stable rate of short-lived gas emissions does not cause additional warming in the way that continued CO2 emissions do. Under GWP*, a country that maintains a constant methane output would show lower equivalent emissions than under standard GWP-100. However, GWP* is not currently used in official reporting and remains a subject of scientific and policy debate.
📋 GWP in Corporate and EU Reporting
Businesses reporting their Scope 1, 2 and 3 emissions use GWP-100 values to convert all greenhouse gas emissions into a single CO2eq figure. The EU Taxonomy and the Emissions Trading System also rely on GWP-based calculations. Companies involved in manufacturing, agriculture, waste management or refrigeration need to understand GWP to accurately report their non-CO2 emissions.
The GHG Protocol, which is the most widely used corporate emissions accounting standard, currently allows reporting using either AR5 or AR6 GWP values, though best practice is to adopt the latest AR6 figures.
🏡 What GWP Means for Everyday Life
GWP shows up in practical ways that affect Irish households and businesses:
🌿 Refrigerants: The F-gases used in air conditioning, heat pumps and refrigeration can have GWPs in the thousands. EU regulations are phasing down high-GWP refrigerants, which is why modern heat pump systems increasingly use lower-GWP alternatives like R-290 (propane, GWP of 3) instead of older refrigerants with GWPs of 1,000+.
🌿 Food choices: The high GWP of methane is the reason beef and dairy have a larger carbon footprint per kilogram than most plant-based foods. Understanding GWP helps you interpret food labelling and make informed choices.
🌿 Home energy: Natural gas (primarily methane) has a GWP impact beyond just burning it. Methane leaks throughout the gas supply chain are multiplied by methane’s high GWP, making the total climate impact of gas heating worse than the combustion emissions alone suggest.
❓ Frequently Asked Questions
🌿 What does CO2 equivalent mean?
CO2 equivalent (CO2eq or CO2e) is the standard unit for comparing different greenhouse gases. It represents the amount of CO2 that would produce the same warming effect as a given quantity of another gas. For example, 1 tonne of methane with a GWP-100 of 27 equals 27 tonnes of CO2eq. Ireland’s national emissions of 58.82 MtCO2eq in 2023 are expressed in this unit, combining all greenhouse gases into one comparable figure.
🌿 Why do GWP values change between IPCC reports?
GWP values are refined as climate science improves. Better measurements of how gases absorb infrared radiation, updated atmospheric lifetime estimates and improved understanding of indirect chemical effects all lead to revisions. Methane’s GWP-100 changed from 25 in AR4 (2007) to 28 in AR5 (2013) to 27 in AR6 (2021) for biogenic sources. These changes affect how countries report their emissions totals.
🌿 Does Ireland use GWP-20 or GWP-100 for reporting?
Ireland uses GWP-100 values for its official greenhouse gas inventory, as required by UNFCCC and EU reporting rules. The Paris Agreement mandates the use of 100-year GWP values from AR5, though parties may transition to AR6 values as agreed by the Conference of the Parties. Ireland’s carbon budgets and sectoral emissions ceilings are all based on GWP-100 calculations.
🌿 Is there a better alternative to GWP?
Several alternative metrics exist. Global Temperature Potential (GTP) estimates the actual temperature change caused by a gas rather than cumulative energy absorbed. GWP* better represents the warming behaviour of short-lived gases like methane. The IPCC AR6 report acknowledges that all metrics have limitations and the best choice depends on the policy question being asked. For now, GWP-100 remains the international standard for emissions reporting and climate mitigation policy.
Global Warming Potential is the translation tool that makes climate accounting possible. Without it, there would be no way to add up methane from farms, CO2 from power plants and nitrous oxide from fertilisers into a single national emissions figure. For Ireland, where methane-heavy agriculture dominates the emissions profile, understanding GWP is essential to making sense of the country’s climate challenge.