Emissions estimation concepts
To quantify and compare the climate impacts of various emissions,it is necessary to choose a climate parameter by which to measure the effects; that is, RF, temperature response, and so forth. Thus, various choices are needed for the steps down the cause–effect chain from emissions to climate change and impacts. The cause–effect chain from emissions to climate change and impacts showing how metrics can be defined to estimate responses to emissions (left) and for development of multi-component mitigation (right). The relevance of the various effects increases downwards but at the same time the uncertainty also increases. The dotted line on the left indicates that effects and impacts can be estimated directly from emissions, while the arrows on the right side indicate how these estimates can be used in development of strategies for reducing emissions.
Each step in the cause effect chain requires a modelling framework. For assessments and evaluation one may—as an alternative to models that explicitly include physical processes resulting in forcing and responses—apply simpler measures or metrics that are based on results from complex models. Metrics are used to quantify the contributions to climate change of emissions of different substances and can thus act as ‘exchange rates’ in multi-component policies or comparisons of emissions from regions/countries or sources/sectors.
Metrics can be given in absolute terms (e.g., K kg–1) or in relative terms by normalizing to a reference gas — usually CO2. To transform the effects of different emissions to a common scale — often called ‘CO2 equivalent emissions’—the emission (Ei) of component i can be multiplied with the adopted normalized metric (Mi): Mi × Ei = CO2-eqi. Ideally, the climate effects of the calculated CO2 equivalent emissions should be the same regardless of the mix of components emitted. However, different components have different physical properties, and a metric that establishes equivalence with regard to one effect cannot guarantee equivalence with regard to other effects and over extended time periods, for example, Lauder et al. (2013), O’Neill (2000), Smith and Wigley (2000), Fuglestvedt et al. (2003).
Metrics can include physical metrics as well as more comprehensive metrics that account for both physical and economic dimensions.
The need to define a standard metric for different gas leads to the introduction of the Global Warming Potential (GWP). (in french : Pouvoir de réchauffement Global / PRG).
- Time frames: One can apply a backward-looking (i.e., historical) or a forward-looking perspective on the responses to emissions. In the forward-looking case one may use pulses of emissions, sustained emissions or emission scenarios. All choices of emission perturbations are somewhat artificial and idealized, and different choices serve different purposes. One may use the level (e.g., degrees Celsius) or rate of change (e.g., degrees Celsius per decade). Furthermore, the effects of emissions may be estimated at a particular time or be integrated over time up to a chosen time horizon. Alternatively, discounting of future effects may be introduced (i.e., a weighting of effects over time).
- Type of effect or end-point: Radiative forcing, temperature change or sea level change, for example, could be examined. Metrics may also include eco/biological or socioeconomic damages. The choice of climate impact parameters is related to which aspects of climate change are considered relevant for interpretation of ‘dangerous anthropogenic interference with the climate system’ (UNFCCC Article 2).
- Spatial dimension for emission and response: Equal-mass emissions of NTCFs from different regions can induce varying global mean climate responses, and the climate response also has a regional component irrespective of the regional variation in emissions. Thus, metrics may be given for region of emission as well as region of response.
Bottom line : Some of the choices involved in metrics are scientific (e.g., type of model, and how processes are included or parameterized in the models). Choices of time frames and climate impact are policy-related and cannot be based on science alone, but scientific studies can be used to analyse different approaches and policy choices.
See the folowing article to deep dive into the GWP
Source : IPCC - WG1AR5 - Chapter 8 Anthropogenic and Natural Radiative Forcing (2018)