Factors to be considered in choosing metrics

1
Factors to be considered in choosing metrics
Workshop on common metrics to calculate the CO2
equivalence of anthropogenic greenhouse gas
emissions by sources and removals by sinks

• 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.
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.