Factors to be considered in choosing metrics

1. Workshop on common metrics to calculate the CO2 equivalence of anthropogenic greenhouse gas emissions by sources and removals by sinks Factors to be considered in choosing metrics Shengmin Yu Energy Research Institute of NDRC, China Bonn, April 2012

2. Definition of the metrics for comparing different emissions • “GHG metrics” are used to comparing the effects of various gases and aerosols on climate, as a common scale for the UNFCC Parties to quantify the equivalences among emissions of various gases relative to carbon dioxide, so that emissions of any particular gas can be converted to so-called CO2-equivalent emissions.

3. The basis for comparison is the Cause-effect chain from emissions to climate change and damages. Emissions in Region D Emissions in Region A Emissions in Region B Emissions in Region C Atmospheric Concentrations Radiative Forcing Climate Change ( △T, △SL, etc ) Impacts in Region A Impacts in Region B Impacts in Region C Impacts in Region D Damages in Region A Damages in Region B Damages in Region C Damages in Region D

4. Which effect on climate change are we most concerned about? • The change in atmospheric concentrations of various gases can not be compared directly. • Using the relative average/marginal emission reduction cost as metrics goes too far • “Achieving least cost” is not the dominant principle of the UNFCCC; • Mitigation cost are not of a global nature; • A scenario study is merely one of the many possibilities. Future costs are not predicable. • The change or rate of change in atmospheric energy balance, temperature increase, sea level rise, damages, could all be candidates pertinent to the ultimate objectives and principles of the UNFCCC. • However, to quantify the damages is not a easy thing, especially for trying to achieve consensus among parties.

5. Four more factors have to be considered in defining a “GHG metric” • The accuracy requirements: linearity and less uncertainty; • Measuring the effect at a time point or time-integrated/ averaged? • Choosing time frame of the effect in question; • Selecting the effect due to pulse emissions or sustained emissions?

6. accuracy requirements to the metrics • Linearity: theoretically, the effect chosen as the basis for comparison should response linearly to the amount of emissions of the same gas. This is the foundation for quantify the equivalences among emissions of various gases. • Less uncertainty: the metric values should not strongly influenced by specific model assumptions and uncertainties of model parameters.

7. Measuring the effect at a time point or time-integrated/ averaged? • Merely measuring the effect at one particular chosen time point misses many useful information, since climate change and its impacts occur all the time. • The time-integrated effect can be classified as a type of averaged effect, by putting equal weight on effects at all times up to the chosen time horizon, and no weight on changes that occur thereafter. • Other forms of averaged effect may need to be defined, for example, giving less or more weight to near-term effects.

8. Choosing the time frame • 20 years? 50 years? Or 100 years? • Choosing the time frame may greatly influence results, as the residence time in the atmosphere of different gases varies widely; • Choosing the time frame is a policy choice • for “holding the increase in global average temperature below 2℃above preindustrial levels” throughout the 21th century, 100 years may be an appropriate option. • Currently valid GWP100 uses 100 years

9. Selecting the effect due to pulse emissions or sustained emissions? • pulse emissions or sustained emissions both are a simplification. The real emissions are much more complicated than this. • Currently under the UNFCCC and the KP, Parties calculate and report their annual emissions via national greenhouse gas inventories. • Therefore, selecting the effect due to annual emissions of 1 kg/year may be more relevant to the policy framework. • For short-lived gases , some recommends to measure the effect due to a constant 1 kg/year increase in emission within the chosen time frame.

10. summary • “GHG” metrics are types of conversion factors to express the effect of different gases on climate change in terms of a common accounting unit, tonnes of CO2-equivalent. • More research needs to be done to inform the Parties of the implications of the choice of effect for comparison, the time frame, pulse or sustained emissions, and their uncertainties. • Comparing the time-integrated/averaged effect may be more appropriate than taking into account merely the effect at one time point. • Other forms of averaged effect can be defined, giving different weight to near and long-term effects. • Even though GWP was not designed with a particular policy goal in mind, it is a robust and valuable metric, as “radiative forcing” lies ahead of other impacts in the cause-effect chain, and any other impacts can be derived from it.