AP Syllabus focus:
‘Carbon dioxide has a GWP of 1 and is the reference for comparing gases. CFCs have the highest GWP, followed by nitrous oxide, then methane.’
Greenhouse gases differ in how strongly they warm the planet.
NOAA’s time-series plots show how atmospheric concentrations of CO₂, CH₄, N₂O, and several halogenated gases (including CFCs) have changed over recent decades. The figure highlights that these gases follow different trajectories and are measured in different concentration units (ppm, ppb, ppt). This helps motivate why metrics like GWP are needed to compare climate impacts across gases on a consistent basis. Source
AP Environmental Science uses global warming potential (GWP) to compare gases to carbon dioxide, helping students interpret relative climate impacts and trade-offs.
What “potency” means for greenhouse gases
A greenhouse gas is “more potent” when it produces more warming per unit mass emitted over a chosen time period. Potency depends mainly on:
How strongly the gas absorbs outgoing infrared radiation (radiative efficiency)
How long it persists in the atmosphere (atmospheric lifetime)
How it behaves chemically (including breakdown products that may also warm)
Because different gases persist for different lengths of time, potency is always tied to a time horizon (commonly 20, 100, or 500 years in scientific reporting).
Global warming potential (GWP)
GWP compares the warming impact of 1 kilogram of a gas to the warming impact of 1 kilogram of CO₂ over the same time horizon.
Global warming potential (GWP): A unitless index that compares the cumulative heat trapped by a greenhouse gas to that of CO₂ (set to 1) over a specified time period.
A higher GWP means the gas causes more warming per kilogram than CO₂ over that time horizon.
Why carbon dioxide is the reference (GWP = 1)
The syllabus emphasises that carbon dioxide has a GWP of 1 and is the reference for comparing gases. CO₂ is used as the baseline because:
It is the most commonly discussed anthropogenic greenhouse gas in climate policy
It is long-lived enough that its impacts are meaningful across typical policy time frames
It provides a consistent “yardstick” so different gases can be compared on one scale
What controls a gas’s GWP
GWP is not just “how strong” a gas is at trapping heat; it reflects the combined effect of:
Absorption strength: Some molecules absorb infrared radiation very efficiently.
Lifetime/residence time: Longer-lived gases keep affecting Earth’s energy balance for longer.
Cumulative effect over time: Short-lived gases can have large near-term impacts but smaller long-term impacts than equally strong long-lived gases.
Because of this, the same gas can have different reported GWPs depending on the chosen time horizon.
Potency ranking highlighted by the syllabus
The required comparison is:
CFCs have the highest GWP
Followed by nitrous oxide
Then methane
With CO₂ = 1 as the reference point
This ranking reflects that CFCs (chlorofluorocarbons) are extremely effective infrared absorbers and can persist long enough for very large cumulative warming compared with CO₂. Nitrous oxide is also long-lived and strongly warming. Methane is very effective at trapping heat but is generally shorter-lived than nitrous oxide, which reduces its cumulative warming over longer horizons.
CO₂-equivalent emissions (CO₂e)
To combine different greenhouse gases into a single comparable metric, emissions are often expressed as CO₂-equivalents.
CO₂-equivalent (CO₂e): The amount of CO₂ that would produce the same warming impact as a given mass of another greenhouse gas, using a stated GWP and time horizon.
CO₂e is useful for inventories, targets, and comparing mitigation options across sectors.
= mass of the emitted gas (commonly in kg or metric tons)
= global warming potential for the chosen time horizon (unitless)
When reading CO₂e values, always check which time horizon is being used, because that choice can change the interpretation of which emissions matter most in the near term versus the long term.
How GWP is used (and what it does not tell you)
GWP helps standardise comparisons, but it is not a complete description of climate risk.
What GWP is good for
Comparing relative warming impacts across gases on a common scale
Converting mixed-gas emissions into a single metric (CO₂e)
Supporting policy tools such as emissions trading or sectoral accounting
What GWP does not capture
The timing of warming within the time horizon (front-loaded vs spread out)
Regional differences in impacts (GWP is a global-average measure)
Non-climate effects of a gas (GWP is strictly about heat trapping)
Understanding GWP as an index (not a “temperature forecast”) helps avoid misinterpretations when comparing gases like CFCs, nitrous oxide, methane, and carbon dioxide.
FAQ
Different horizons reflect different policy priorities.
A shorter horizon emphasises near-term warming (important for rapid climate stabilisation), while a longer horizon emphasises persistent, long-lived effects.
CFC molecules can be very effective at absorbing infrared radiation and may persist long enough for large cumulative warming.
In addition, their absorption can occur in “windows” of the infrared spectrum where fewer gases absorb strongly, increasing their relative impact.
No. GWP is an index of relative heat trapping integrated over time, not a direct prediction of future temperature.
Temperature outcomes depend on total emissions, climate sensitivity, feedbacks, and how concentrations change over time.
Uncertainty can come from:
Imperfect knowledge of atmospheric lifetimes and reaction pathways
Overlap in absorption bands among gases
Indirect effects (where a gas influences other warming agents)
These uncertainties are why published values can be updated as science improves.
They often pair $CO_{2}e$ accounting with gas-specific strategies, such as separate targets for short-lived pollutants.
Some frameworks also use multiple metrics (or multiple time horizons) to capture both near-term and long-term climate objectives.
Practice Questions
Define global warming potential (GWP) and state why carbon dioxide is assigned a GWP of 1. (2 marks)
Correct definition of GWP as a relative, unitless comparison to CO₂ over a stated time horizon (1)
CO₂ is the reference/baseline gas and is assigned 1 so other gases can be compared consistently (1)
Explain what it means for a greenhouse gas to be “more potent” and use the syllabus ranking (CFCs highest GWP, then nitrous oxide, then methane; CO₂ = 1) to discuss two reasons why GWPs differ and one implication for reporting emissions as . (6 marks)
Potency linked to greater warming per unit mass over a chosen time horizon (1)
Correct use of the ranking given (CFCs > N₂O > CH₄; CO₂ reference at 1) (1)
Reason 1: differences in radiative efficiency/infrared absorption strength (1)
Reason 2: differences in atmospheric lifetime/residence time affecting cumulative warming (1)
Notes that GWP depends on a specified time horizon (e.g., 20 vs 100 years) (1)
Implication: aggregates gases via , so totals depend on chosen GWP/time horizon and enable comparison across gases (1)
