Abstract
The 2015 Paris Agreement sets out that rapid reductions in greenhouse gas (GHG) emissions are needed to keep global warming to safe levels. A new approach (known as GWP*) has been suggested to compare contributions of long- and short-lived GHGs, providing a close link between cumulative CO2-equivalent emissions and total warming. However, comparison factors for non-CO2 GHGs under the GWP* metric depend on past emissions, and hence raise questions of equity and fairness when applied at any but the global level. The use of GWP* would put most developing countries at a disadvantage compared to developed countries, because when using GWP* countries with high historical emissions of short-lived GHGs are exempted from accounting for avoidable future warming that is caused by sustaining these emissions. We show that when various established equity or fairness criteria are applied to GWP* (defined here as eGWP*), perceived national non-CO2 emissions vary by more than an order of magnitude, particularly in countries with high methane emissions like New Zealand. We show that national emission estimates that use GWP* are very sensitive to arbitrary choices made by countries and therewith facilitate the creation of loopholes when CO2-equivalent emissions based on the GWP* concept are traded between countries that use different approaches. In light of such equity-dependent accounting differences, GHG metrics like GWP* should only be used at the global level. A common, transparent and equity-neutral accounting metric is vital for the Paris Agreement’s effectiveness and its environmental integrity.
Generated Summary
This research article critiques the application of the Global Warming Potential (GWP*) metric for assessing greenhouse gas (GHG) emissions at the country level. The study explores the implications of using GWP* for non-CO2 GHGs, particularly short-lived climate forcers (SLCFs) like methane, and its impact on equity and fairness in the context of the Paris Agreement. The authors investigate how the GWP* metric, which links cumulative CO2-equivalent emissions to total warming, can lead to unintentional unfairness when applied to different countries due to variations in historical emissions. The research employs a methodological approach, contrasting the standard GWP-100 metric with GWP* and alternative equity-based approaches (eGWP*). These alternative approaches account for historical emissions and future emission levels. The scope includes assessing the sensitivity of national emission estimates to arbitrary choices in GWP* calculations and evaluating the potential for creating loopholes in emissions trading.
Key Findings & Statistics
- The study utilizes data from the PRIMAP-hist database for historical emissions.
- GWP-100 is the standard emission metric to aggregate GHG emissions for its assessment and presentation of emissions pathways.
- GWP* is defined as a new approach to compare contributions of long- and short-lived GHGs, providing a close link between cumulative CO2-equivalent emissions and total warming.
- The authors use GWPs for different GHGs as reported in AR5 (Myhre et al 2013) and use the PRIMAP-hist database (Version 2.0) (Gütschow et al 2016) for historical emissions
- The choice of metric (GWP-100 or grandfathering GWP*) fundamentally alters the ranking of countries in terms of their accounted (perceived) per capita CH4 emissions.
- The study also mentions that if New Zealand decreases its CH4 emissions by 50% in 2035 relative to 2015, this would equate to a perceived ‘additional’ CO2 budget of about 2.5 times New Zealand’s annual CO2 emissions in the year 2015.
Other Important Findings
- The use of GWP* can disadvantage developing countries compared to developed countries.
- The GWP* metric is sensitive to arbitrary choices made by countries, potentially creating loopholes in emissions trading.
- National emission estimates using GWP* vary significantly based on the equity or fairness criteria applied.
- Applying the GWP* metric at the country level introduces a preferential treatment of countries with high historical methane emissions.
- Reducing methane emissions would result in less warming, not cooling.
- Alternative equity concepts exist to distribute emissions more fairly.
- The study highlights how the application of GWP* methods that more directly reflect on the equity dimension of emissions metrics, like approaches that start from a per capita equity baseline instead of historical national emissions, lead to a strongly altered picture.
Limitations Noted in the Document
- The arbitrariness in the choice of the time interval (∆T) in GWP* calculations is a limitation, potentially leading to significantly altered results depending on the approach (per capita vs. zero reference).
- The study focuses primarily on CH4 and acknowledges that the implications for other SLCFs would be qualitatively similar, but smaller in magnitude.
- The study’s findings are specific to the context of national climate policy and global climate targets.
- The article is limited by the value judgements and considerations of equity and fairness that are needed to be made explicit, which can introduce subjectivity.
- The study does not explore all possible equity concepts.
Conclusion
The article concludes that the GWP* metric, while having scientific merits at the global level, is not suitable for application in national climate policy due to its inherent biases and equity concerns. The authors stress the importance of a transparent and equity-neutral accounting metric for effective GHG emission reductions under the Paris Agreement. The study’s primary argument revolves around the grandfathering effect of GWP*, which favors countries with high historical emissions. The authors propose alternative equity-based eGWP* metrics that offer a more equitable distribution of emissions. The article advocates for a shift towards methodologies that reflect on the equity dimension of emissions metrics, like approaches that start from a per capita equity baseline instead of historical national emissions, which leads to a strongly altered picture. The choice of which equity approach is applied in a national context is at the discretion of individual national governments. The article emphasizes that the application of eGWP* methods could lead to significant alterations in the assessment of emissions, with potential implications for national CO2 emission budgets. The researchers suggest that in order to achieve the required stringent emission reductions of all GHGs, transparent and robust national accounting is key and each tonne of GHGs of any kind in any country needs to be accounted for in the same way.