Global Warming Potential (GWP)

What is Global Warming Potential (GWP)?

Global Warming Potential is not an emission; it is a metric and a conversion factor. GWP expresses how much heat a given mass of a greenhouse gas traps in the atmosphere over a chosen time period, relative to the same mass of carbon dioxide. Unlike a direct emission figure (tonnes of methane released), GWP is a multiplier or ratio. When you multiply a gas mass by its GWP value, the result is kg CO2e (carbon dioxide equivalent). This conversion makes emissions from different gases additive and comparable in a single figure, the foundation of all carbon accounting in construction.

The key insight is that GWP encodes two choices: which gas you measure and which time horizon you use. A 100-year horizon is conventional in building standards and EPDs, but this choice is not neutral. It reflects a judgment about which generation's climate impacts matter most.

How does the time horizon shape which gases matter most?

Methane illustrates this starkly. Methane is a potent greenhouse gas but decays in the atmosphere over roughly 12 years. Over 100 years, much of it has left the atmosphere by year 50, so its GWP100 is modest. Over 20 years, before decay occurs, methane's GWP20 is five to ten times higher. This is not a minor technical detail. A 20-year horizon prioritizes near-term warming and makes short-lived gases look worse; a 100-year horizon smooths impact across time and favors slow-decay gases like CO2. In construction and building standards, the 100-year horizon is embedded in most EPDs and the EU's Energy Performance of Buildings Directive (EPBD).

Why are multiple greenhouse gases relevant in construction?

Most construction processes emit primarily CO2 from fuel combustion, cement chemistry, and waste decomposition. However, three other gases appear regularly in building supply chains. Nitrous oxide (N2O) arises from agricultural-based materials and some manufacturing. Methane (CH4) appears in some insulation and from waste. Fluorinated gases serve as refrigerants in heat pumps and chillers.

Refrigerants are a critical GWP link in mechanical design, often overlooked in embodied carbon discussions. Older refrigerants (HFCs) carried GWPs of 1,000–4,000 or higher. Modern alternatives (HFOs) are engineered to GWPs below 100, often below 1, driven by EU F-gas regulation and climate commitments. A 20 kW heat pump with a 3 kg charge of old HFC refrigerant (GWP ~1,100) carries roughly 3,300 kg CO2e of refrigerant GWP impact over its lifetime. With a modern HFO (GWP ~4), that drops to about 12 kg CO2e. Over a 15–25 year system lifespan, if leakage is controlled and maintenance is diligent, the choice of refrigerant is as important as many material decisions. A poor seal or lax maintenance loses 2–3% of refrigerant per year, compounding the impact. Heat pump specification sheets must state refrigerant type and charge volume so architects can calculate this impact themselves and factor it into whole-building embodied carbon.

How is GWP calculated and why does the source matter?

GWP values come from radiative forcing models developed by climate scientists and published by the Intergovernmental Panel on Climate Change (IPCC) in peer-reviewed Assessment Reports, updated roughly every 5–7 years. Each report includes updated data on atmospheric lifetime, radiative forcing, and feedback mechanisms, so GWP values can shift between reports. Methane's GWP100 has been refined multiple times as atmospheric chemistry data improved.

When reading an Environmental Product Declaration (EPD) or life-cycle assessment (LCA) for a material, the GWP figure comes from an LCA database or scientific literature, typically citing an IPCC Assessment Report. A responsible EPD states which report (AR4, AR5, AR6) underpins its figures. If none is stated, you cannot be certain whether the data reflects current science. A 10–20% change in a gas's GWP value can meaningfully shift the ranking of material options in design.

How does GWP translate into a building's embodied carbon?

A building's embodied carbon is calculated by summing GWP-weighted emissions from all materials and processes in production, transport, assembly, and end-of-life phases. For most buildings, the dominant contributor is CO2 from manufacturing energy and cement chemistry. However, other gases can be significant in specific components: a heat pump's refrigerant leakage, blowing agents in certain insulation foams (if older stocks are in use), and process N2O from agricultural-based materials.

This is why specifying a low-GWP refrigerant is not a footnote but a real embodied carbon decision. The embodied carbon of a heat pump includes not just the materials and manufacturing but the refrigerant's GWP and potential leakage. Over the system's lifetime, modern low-GWP refrigerants with good maintenance yield substantially lower total impact than older formulations.

Why is GWP only one impact category among several?

Environmental Product Declarations report multiple impact categories beyond GWP: acidification potential, eutrophication potential, ozone depletion potential, water consumption, resource depletion. A material optimized solely for low GWP can shift burden elsewhere. A concrete formulation that lowers GWP by using supplementary cementitious materials (SCMs) might increase water extraction. A paint with ultra-low-GWP solvents might carry higher toxicity risk during application. EPDs and life-cycle assessments require reading the full profile, not just the single GWP number.

What should architects and builders verify about GWP in practice?

First, read the full EPD, not just the headline GWP number on the front page. Verify which IPCC Assessment Report underpins the figures, and note the timeframe of data. For refrigerants, confirm the type, charge volume, and sealed-system leak rate with the manufacturer, then calculate the lifecycle refrigerant GWP impact. For insulation, check whether older CFC or HCFC blowing agents are present; modern foams use low-GWP gases or water, but older stocks may not. For concrete, specify GWP-optimized formulations with supplementary cementitious materials. Most importantly, remember that GWP is one impact indicator among many in an EPD. A material with lower GWP but significantly higher cost, poor durability, or difficult maintenance can shift overall sustainability burden elsewhere. The goal is not to minimize GWP in isolation but to reduce whole-life environmental impact across all impact categories while designing buildings that are durable and genuinely sustainable.