Global Warming Potential (GWP) is a metric designed to compare the climate impact of different greenhouse gases against a single standard. This comparison is necessary because gases vary widely in their chemical properties and their ability to trap heat in the atmosphere. The GWP calculation provides a single number that reflects the total warming effect of a specific gas emission over a defined period of time. It allows scientists and policymakers to aggregate the emissions of multiple gases into a single, understandable unit, which is fundamental for tracking and managing global efforts to combat climate change.
Setting the Baseline: Why Carbon Dioxide Equals One
The Global Warming Potential is a relative measurement scale, meaning all other gases are measured against a chosen reference. Carbon dioxide (\(\text{CO}_2\)) was selected as the reference gas due to its status as the most prevalent greenhouse gas and the primary driver of anthropogenic climate change since the Industrial Revolution. By definition, \(\text{CO}_2\) is assigned a GWP value of exactly 1 for all time horizons. This choice establishes the standard against which the warming contribution of all other gases can be benchmarked.
Setting \(\text{CO}_2\)‘s warming effect to unity provides a stable and universally accepted comparison point. The GWP of any other gas is then expressed as a multiple of the warming caused by the same mass of \(\text{CO}_2\). This ratio simplifies the complex physics of atmospheric warming into a single, interpretable number.
The Core Inputs: Radiative Efficiency and Atmospheric Lifetime
Calculating the Global Warming Potential of a gas requires integrating two fundamental physical properties that define its warming effect. The first property is the gas’s radiative efficiency, which quantifies its ability to absorb infrared radiation per unit of concentration. Some gases, such as sulfur hexafluoride (\(\text{SF}_6\)), are thousands of times more effective at absorbing heat than a molecule of \(\text{CO}_2\). Radiative efficiency measures how potent a gas is at trapping outgoing longwave radiation.
The second property is the gas’s atmospheric lifetime, which is the average time a pulse of the gas remains in the atmosphere before removal through chemical reactions or other processes. A longer atmospheric lifetime means the gas will continue to exert its warming influence over a more extended period. The GWP calculation mathematically integrates the combined effect of both high radiative efficiency and atmospheric lifetime over a chosen time period.
For instance, methane (\(\text{CH}_4\)) is a potent, short-lived gas with an atmospheric lifetime of about 12 years, but it absorbs much more energy than \(\text{CO}_2\). Conversely, perfluorocarbons (PFCs) have extremely long lifetimes, sometimes lasting tens of thousands of years, meaning they exert a warming influence long after their initial emission. A gas with high efficiency and a long life will have a very high GWP, while one with high efficiency but a short life will have a GWP that varies significantly depending on the time horizon chosen.
Time Horizons: Integrating the Warming Effect
The resulting Global Warming Potential value for any gas is not static because the calculation must integrate the warming effect over a specific duration, known as the time horizon. This time horizon represents the period over which the total cumulative radiative forcing of the gas is evaluated. The two most commonly used time horizons in climate policy and reporting are 20 years (GWP20) and 100 years (GWP100).
The choice of time horizon significantly affects the final GWP number, particularly for gases with short atmospheric lifetimes. Gases like methane, which has a short life, have a much higher GWP20 value—around 81.2—than their GWP100 value, which is approximately 27.9. This is because the GWP20 captures the intense, initial warming effect of the gas before it has largely decayed out of the atmosphere.
For policy purposes, the choice of 100 years is the most common standard used by international bodies, as it balances the short-term effects with the longer-term climate impacts. Conversely, very long-lived gases, such as the industrial gas tetrafluoromethane (\(\text{CF}_4\)) with a lifetime of 50,000 years, show a GWP that is less sensitive to the time horizon. The selection of the time horizon is a methodological choice that prioritizes either near-term mitigation urgency or long-term climate stability.
Translating GWP into Carbon Dioxide Equivalents
The end goal of the GWP calculation is to provide a standardized metric known as the carbon dioxide equivalent (\(\text{CO}_2\)e). This metric allows all emissions of various greenhouse gases to be converted into a single unit for easy comparison and aggregation. To obtain the \(\text{CO}_2\) equivalent, the mass of a specific gas emitted is multiplied by its calculated GWP value.
For example, if 1 ton of a gas with a GWP100 of 25 is emitted, it is reported as 25 tons of \(\text{CO}_2\)e. This standardization is required for national greenhouse gas inventories and forms the basis for emissions targets within international agreements, such as the Kyoto Protocol. The Intergovernmental Panel on Climate Change (IPCC) periodically updates and publishes the GWP values based on the latest scientific advancements in atmospheric chemistry and climate modeling. These standardized values ensure that all countries and organizations are using a consistent scientific basis for their reporting.