Q.j - Global Warming Potential

j 6 Marks

Write short note on global warming potential.

Answer: Global Warming Potential (GWP)

1. Definition

Global Warming Potential (GWP) is a measure of how much heat (infrared thermal radiation) a greenhouse gas traps in the atmosphere over a specified time period, relative to carbon dioxide (CO2). It allows comparison of the warming impact of different gases on a common scale.

Reference Standard: CO2 is assigned a GWP of 1 as the baseline. All other gases are compared to this reference. A gas with GWP of 28 means 1 kg of that gas has the same warming effect as 28 kg of CO2.

2. Factors Determining GWP

GWP depends on two main characteristics of a greenhouse gas:

Radiative Efficiency: How effectively the gas absorbs infrared radiation per molecule. Depends on molecular structure and which wavelengths it absorbs.
Atmospheric Lifetime: How long the gas persists in the atmosphere before being removed through chemical reactions or absorption by sinks.

GWP = ∫ (Radiative Forcing of Gas × Concentration) dt / ∫ (Radiative Forcing of CO2 × Concentration) dt

(Integrated over specified time horizon, typically 100 years)

3. Time Horizons for GWP

GWP values vary depending on the time period over which warming is measured:

Time Horizon	Use Case	Significance
20-year GWP	Short-term climate policy	Emphasizes short-lived but potent gases (methane); relevant for near-term climate targets
100-year GWP	International climate agreements	Standard metric used in Kyoto Protocol, Paris Agreement, and national inventories
500-year GWP	Long-term climate stabilization	Relevant for understanding cumulative impacts; emphasizes long-lived gases

4. GWP Values of Major Greenhouse Gases

Greenhouse Gas	Formula	GWP (20 yr)	GWP (100 yr)	Atmospheric Lifetime	Main Sources
Carbon Dioxide	CO2	1	1	Variable (100-1000+ yrs)	Fossil fuels, deforestation
Methane	CH4	81	28-36	12 years	Agriculture, landfills, gas leaks
Nitrous Oxide	N2O	273	265-298	121 years	Fertilizers, industry
HFC-134a	CH2FCF3	3,810	1,430	14 years	Refrigeration, AC
HFC-23	CHF3	12,400	14,800	228 years	Refrigerant production
Sulfur Hexafluoride	SF6	18,300	23,500	3,200 years	Electrical switchgear
Nitrogen Trifluoride	NF3	12,800	17,200	740 years	Electronics manufacturing

5. Applications of GWP

A. Carbon Dioxide Equivalent (CO2e)

GWP enables conversion of all greenhouse gas emissions to a common unit for comparison and aggregation:

CO2e = Emission Quantity × GWP

Calculation Examples:

1 tonne CH4 = 1 × 28 = 28 tonnes CO2e (100-year GWP)
1 tonne N2O = 1 × 265 = 265 tonnes CO2e
1 kg SF6 = 1 × 23,500 = 23,500 kg CO2e (= 23.5 tonnes CO2e)

B. Key Applications:

National GHG Inventories: Countries report emissions in CO2e to UNFCCC using standardized GWP values
Climate Policy: Guides emission reduction priorities and target-setting
Carbon Markets: Enables trading of different GHG reductions (1 carbon credit = 1 tonne CO2e)
Corporate Carbon Footprints: Companies report Scope 1, 2, 3 emissions in CO2e
Life Cycle Assessment: Environmental impact assessment of products/processes
Climate Finance: Calculating emission reduction benefits of projects

6. Importance of GWP in Climate Policy

Policy Significance: While CO2 dominates total emissions, high-GWP gases offer significant mitigation opportunities. Reducing 1 tonne of SF6 is equivalent to reducing 23,500 tonnes of CO2 - making targeted interventions highly effective.

Short-lived Climate Pollutants: Methane (GWP-20 = 81) offers rapid climate benefits from emission reductions
F-gas Regulations: Kigali Amendment targets HFC phase-down due to high GWP
Sector Prioritization: GWP helps identify where small reductions yield large climate benefits

7. Limitations of GWP

Time Horizon Sensitivity: Choice of 20, 100, or 500 years significantly affects values and policy conclusions
Simplified Metric: Does not capture all climate impacts (e.g., regional effects, ecosystem responses)
Constant Atmosphere Assumption: Assumes fixed background conditions that change over time
CO2 Uniqueness: CO2's complex removal from atmosphere makes comparison imperfect

Alternative Metrics:

GTP (Global Temperature Potential): Measures temperature change at end of time horizon
GWP*: Modified metric better accounting for short-lived gases

Conclusion

Global Warming Potential is an essential metric for comparing the climate impact of different greenhouse gases and is fundamental to international climate policy. By expressing all emissions in CO2-equivalent terms, GWP enables meaningful aggregation, target-setting, and trading of emission reductions. While CO2 remains the focus due to its abundance, the high GWP of other gases - particularly fluorinated compounds - makes their control equally important for climate mitigation. Understanding GWP helps policymakers and businesses identify where emission reduction efforts can have the greatest climate benefit.

Sources: Module 1 Notes | IPCC AR6 | EPA Understanding Global Warming Potentials | UNFCCC