Global warming potential (GWP)

“The conversion factor is a choice about time as well as a statement about chemistry. ”

Global warming potential is the metric most commonly used to convert greenhouse gases into carbon dioxide equivalent. It compares the cumulative radiative effect of a pulse emission of a gas with the effect of the same mass of carbon dioxide over a selected period, usually 100 years.

The time horizon changes the result. Methane exerts strong warming in the near term but remains in the atmosphere for a much shorter period than carbon dioxide. Its GWP over twenty years is therefore much higher than its GWP over one hundred years. Nitrous oxide and many fluorinated gases have different lifetimes again.

GWP100 became the dominant convention because inventories and policy frameworks need a stable common metric. Consistency is valuable. Yet the number can be misread as a timeless physical property. It is not. It is a modelled comparison shaped by scientific assumptions, carbon-cycle responses and the chosen period.

The values also change as climate science improves. The IPCC updates estimates across assessment reports. An inventory calculated using one assessment’s GWP values may not be directly comparable with another unless the difference is disclosed or the historical series is recalculated.

The metric’s limitations do not make it arbitrary. GWP is grounded in atmospheric science and supports multi-gas accounting. The problem begins when one aggregate number is expected to answer every policy and management question. A 100-year metric may be suitable for consistent inventories, while near-term methane strategy may require additional information about absolute methane emissions and shorter-term warming.

Agriculture illustrates the challenge. A farm or food company may report a CO2e footprint dominated by methane. Reducing methane can deliver relatively rapid climate benefits, while eliminating carbon dioxide remains essential for stabilising long-term warming. Treating the gases as interchangeable quantities can obscure the different actions and timelines required.

For practitioners, metric choice should follow purpose. Use the required GWP values for compliance and comparability. Disclose the assessment report and time horizon. Where the emissions profile contains material short-lived gases, supplement the standard total with gas-specific information rather than changing metrics opportunistically to produce a preferred narrative.

GWP makes unlike gases commensurable for accounting. It does not erase their different roles in climate change.

Practical application

Lock the GWP source and time horizon in the inventory methodology. Maintain emissions by gas, not only as CO2e. Recalculate base years when required for consistency and provide supplementary methane or short-lived-climate-pollutant metrics where decision-relevant.

Why it matters

Targets and footprints can shift materially when different GWP values are used. Transparent metric choices are essential for comparability and for understanding which gases require which interventions.

Common misconception

GWP is a fixed conversion rate like currency exchange. It is a scientifically assessed metric that depends on the gas, time horizon and assessment framework.

Connections

CO2e is the resulting common unit. Radiative Forcing is part of the physical basis. Methane and Short-lived Climate Pollutants require interpretation beyond a single aggregate figure.

A question worth asking

Would your emissions priorities change if decision-makers saw the gases separately before seeing the CO2e total?

Selected references

• IPCC, Sixth Assessment Report, Working Group I, emissions metrics. • GHG Protocol, required GWP values for inventories. • UNFCCC, common metrics for greenhouse-gas reporting.