Evolution of IPCC Assessment Reports and Global Warming Potentials
The Intergovernmental Panel on Climate Change (IPCC) has released six major Assessment Reports since 1990. Each report reflects advances in scientific understanding of climate change and greenhouse gas effects, leading to updates in Global Warming Potential (GWP) values.
Timeline of IPCC Assessment Reports from 1990 to 2021
Evolution of GWP Values
| Greenhouse Gas | AR4 GWP₁₀₀ | AR5 GWP₁₀₀ | AR6 GWP₁₀₀ | Trend |
| Carbon dioxide (CO₂) | 1 | 1 | 1 | → |
| Methane (CH₄) | 25 | 28 | 29.8 | ↑ |
| Nitrous oxide (N₂O) | 298 | 265 | 273 | ↓ |
| HFC-134a | 1,430 | 1,300 | 1,530 | ↑ |
Key Changes Across Assessment Reports
First Assessment Report (FAR, 1990)
- Established the first set of GWP values
- Limited coverage of greenhouse gases
- Simple atmospheric lifetime calculations
Second Assessment Report (SAR, 1995)
- Expanded list of greenhouse gases
- Improved atmospheric chemistry models
- Introduction of uncertainty ranges
Third Assessment Report (TAR, 2001)
- Updated carbon cycle models
- Refined indirect effects
- More comprehensive uncertainty analysis
Fourth Assessment Report (AR4, 2007)
- Updated methane GWP significantly
- Improved understanding of atmospheric chemistry
- Better accounting of indirect effects
Fifth Assessment Report (AR5, 2013)
- Introduction of climate-carbon feedbacks
- Updated methane oxidation chemistry
- Revised N₂O lifetime estimates
Sixth Assessment Report (AR6, 2021)
- Updated metrics including GWP*
- Improved understanding of short-lived climate forcers
- More comprehensive treatment of indirect effects
- Enhanced uncertainty characterization
Major Scientific Advances
Key scientific developments that influenced GWP updates:
- Carbon Cycle Understanding: Improved models of carbon uptake and release
- Atmospheric Chemistry: Better understanding of chemical interactions and lifetimes
- Radiative Forcing: More accurate calculations of warming effects
- Feedback Mechanisms: Inclusion of climate-carbon feedbacks
- Indirect Effects: Better quantification of secondary impacts
Implications for GHG Accounting
The evolution of GWP values has important implications for greenhouse gas accounting:
- Historical comparisons need to account for GWP changes
- Inventory updates may be needed when new values are adopted
- Documentation should specify which AR values are used
- Different reporting schemes may require different AR values
Best Practice: When reporting greenhouse gas emissions, always clearly state which IPCC Assessment Report's GWP values are being used, as this choice can significantly affect the final results.
