Transcript 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
Factors to be considered in
choosing metrics
Shengmin Yu
Energy Research Institute of NDRC, China
Bonn, April 2012
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.
The basis for comparison is the Causeeffect chain from emissions to climate
change and damages.
Emissions in
Region A
Emissions in
Region B
Emissions in
Region C
Emissions in
Region D
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
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.
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?
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.
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.
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
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.
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.