Generated Summary
The document is a Twitter thread by Andy Reisinger discussing the findings of the IPCC AR6 WGIII report on GHG emission metrics. It explores various aspects of GHG emission metrics, including GWP, GTP, and GWP*, and their implications for climate change mitigation. The thread covers a range of topics, such as the cost-benefit and cost-effectiveness of different metrics, sectoral versus global perspectives, and the impact of different metrics on net-zero emissions targets. The methodology involves summarizing the key findings from the IPCC report, presenting visual aids and figures, and discussing the practical implications of using different GHG metrics in climate policy. The scope includes an overview of GHG emission metrics, their application in different scenarios, and their impact on achieving climate goals, focusing on the findings related to mitigation strategies.
Key Findings & Statistics
- GWP100 is consistent with a cost-benefit perspective, while GTP is not. The time horizon can be linked to the discount rate. GWP100 ≈ 3% discount rate, while GWP20 implies > 10% discount rate.
- GWP100 for CH₄ is consistent with a ~3-5% discount rate across a range of damage functions, while GWP20 implies a discount rate > 10%.
- Mitigation pathways to limit warming to 2°C (>67%) and lower, informed by GWP100, are close to least global cost (within a few percent).
- Using dynamic GTP could reduce global costs by up to a few percent, depending on temperature limit, policy foresight, and flexibility in abatement choices over time.
- GWP100 produces close to global least-cost pathways because its values aren’t too far away from dynamic GTP for <2°C pathways and because much of the CH4 abatement is assumed to be available at low costs, so using GWP100 already realizes most of the abatement potential.
Other Important Findings
- Comprehensive mitigation policy relies on consideration of all anthropogenic forcing agents, which differ widely in their atmospheric lifetimes and impacts on the climate system. GHG emission metrics provide simplified information about the effects that emissions of different GHGs have on global temperature or other aspects of climate, usually expressed relative to the effect of emitting CO2.
- Key issues explored by WGIII relate to cost-benefit and cost-effectiveness of metrics, sectoral vs global perspectives, differences between GWP100 and GWP*, relationship between metrics and targets, and how global pathways look under different metrics.
- Cost-benefit means weighting each emission based on the damages that emission will cause. GWP is conceptually consistent with the cost-benefit approach to mitigation. GTP as an end-point metric does not reflect a cost-benefit framework.
- Cost-effectiveness means weighting each emission based on its contribution to temperature when the target or peak is reached. Dynamic GTP is conceptually consistent with the cost-effectiveness approach to mitigation.
- Dynamic GTP means the time horizon changes over time. E.g. for 1.5°C, temperature peaks around 2050, so today’s emissions would use GTP27, 2030 emissions use GTP20, 2040 emissions use GTP10. Cost-effective mitigation relies on escalating weight given to short-lived gases.
- Whether GWP or GTP is ‘better’ depends on whether we want our mitigation choices to help achieve a pre-determined temperature limit at least cost (dynamic GTP), or to reflect the damages we would avoid by avoiding an emission (GWP).
- Sectoral LCA often has no single policy objective, so do sensitivity tests. Global least-cost doesn’t mean sectoral least-cost, so preferences will differ! Step-change metrics (eg GWP*) need to be used with care to make sense.
- Use of different metrics and time horizons can lead to divergent conclusions in Lifecycle Assessments (LCA). LCA often does not have a single or clear policy objective.
- Historical CO2 and CH4 emissions have driven most warming to date. The warming from past CO2 emissions will persist for centuries, while warming from past CH4 emissions will disappear again naturally over the coming decades.
- Future CO2 and CH4 emissions will cause further warming, on top of warming from historical emissions. This further ‘marginal’ warming from future CH4 is substantial and comparable to that from CO2 in deep mitigation pathways.
- GWP/GTP are always greater than zero. They tell us that any future CH4 emission will make the Earth warmer than it would be without that emission, and that this further (‘marginal’) contribution to global warming could be avoided if we avoided that emission.
- GWP* can be less than zero, if CH4 emissions are dropping enough for warming due to those emissions to decline. GWP* doesn’t tell us how much a given rate of CH4 emissions contributes to global warming; just whether we’re emitting and warming more or less than we used to.
- Using different metrics gives different numbers when reporting total CO2-eq emissions, and hence also affects when emissions are reported to reach net-zero. This is shown for 4 global emission pathways. Same actual emissions, very different reported CO2-eq emissions.
- If we wanted to change the GHG emission metric, we need to reconsider the emissions target (such as ‘net zero’ by a given date) at the same time.
Limitations Noted in the Document
- All emission metrics have limitations and uncertainties.
- The most suitable metric for any given climate policy application, depends on judgments about the specific context, policy objectives and the way in which a metric would be used.
- Step-change metrics are not well suited for product/event LCA, but have some uses for long-term effects such as lifetime dietary change.
- WGIII literature often does not report individual gases, only GWP.
- Assessment of GHG emission metrics is largely based on economic principles.
Conclusion
The document provides a detailed analysis of GHG emission metrics, as discussed in the IPCC AR6 WGIII report, with a focus on their impact on climate change mitigation strategies. It highlights the importance of understanding the differences between metrics like GWP, GTP, and GWP*, as they lead to different outcomes and interpretations of climate change impacts. The core message is that the choice of metric affects not only how we measure emissions but also how we set and achieve climate goals. The thread underscores the need for careful consideration of the context, policy objectives, and specific application when choosing a GHG metric. The implications of using different metrics are far-reaching, as they affect the assessment of mitigation options and the setting of emission targets. The discussion emphasizes that no single metric is universally superior, and the most suitable metric depends on the specific climate policy goals and the nature of the emissions being analyzed. Understanding these metrics is crucial for making informed decisions about how to reduce global emissions, monitor climate change, and evaluate the effectiveness of mitigation strategies. In essence, the thread emphasizes that to effectively address climate change, it is critical to understand the limitations and appropriate uses of each metric, and to consider the specific objectives when setting and achieving emission reduction targets. The document further states that choosing the correct metric is essential for making consistent and informed choices about how to value emissions and design effective strategies for tackling climate change.