Abstract
Multi-gas climate agreements rely on a methodology (widely referred to as ‘metrics’) to place emissions of different gases on a CO2-equivalent scale. There has been an ongoing debate on the extent to which existing metrics serve current climate policy. Endpoint metrics (such as global temperature change potential GTP) are the most closely related to policy goals based on temperature limits (such as Article 2 of the Paris Agreement). However, for short-lived climate forcers (SLCFs), endpoint metrics vary strongly with time horizon making them difficult to apply in practical situations. We show how combining endpoint metrics for a step change in SLCF emissions with a pulse emission of CO2 leads to an endpoint metric that only varies slowly over time horizons of interest. We therefore suggest that these combined step-pulse metrics (denoted combined global warming potential CGWP and combined global temperature change potential CGTP) can be a useful way to include short and long-lived species in the same basket in policy applications—this assumes a single basket approach is preferred by policy makers. The advantage of a combined step-pulse metric for SLCFs is that for species with a lifetime less than 20 years a single time horizon of around 75 years can cover the range of timescales appropriate to the Paris Agreement. These metrics build on recent work using the traditional global warming potential (GWP) metric in a new way, called GWP*. We show how the GWP* relates to CGWP and CGTP and that it systematically underestimates the temperature effects of SLCFs by up to 20%. These step-pulse metrics are all more appropriate than the conventional GWP for comparing the relative contributions of different species to future temperature targets and for SLCFs they are much less dependent on time horizon than GTP.
Generated Summary
This research paper introduces and explores the use of combined step-pulse climate metrics (CGWP and CGTP) to assess the impact of short-lived climate forcers (SLCFs) like methane, alongside long-lived greenhouse gases (LLGHGs), within the context of climate agreements such as the Paris Agreement. The study builds on existing methodologies, particularly the Global Warming Potential (GWP) metric, and addresses the challenges of applying traditional metrics to climate policy, especially concerning time horizon sensitivity. The core approach involves combining endpoint metrics for a step change in SLCF emissions with a pulse emission of CO2, resulting in metrics that vary less with time horizons, making them more suitable for policy applications. The research investigates the relationship between GWP*, CGWP, and CGTP, assessing their ability to accurately represent temperature effects and their implications for climate targets. The primary objective is to provide a more robust and less time-dependent framework for evaluating the contributions of different species to future temperature targets, aiming for a more consistent and effective approach to climate policy and emission reductions.
Key Findings & Statistics
- The paper references the IPCC 4th Assessment Report (AR4) (Forster et al 2007) noting the many shortcomings in the Global Warming Potential GWP (100).
- IPCC 5th Assessment Report (AR5) presents global warming potentials (GWP) and global temperature change potentials (GTP) for 20-year and 100 year time horizons leading to a range of values (for methane these ranged from 4 to 84).
- The methane perturbation lifetime is 12.4 years.
- The radiative efficiencies for CH4 and CO2 are 4.40 × 10-4 W m-2 ppb-1 and 1.30 × 10-5W m-2 ppb-1 respectively (based on Etminan et al (2016)), CO2 response function is from Joos et al (2013) and temperature response function from Geoffroy et al (2013).
- The indirect effects of methane on ozone and stratospheric water vapour are included following Myhre et al (2013) as 1.82 × 10-4 and 0.54 × 10-4 W m-2 ppb(CH4)-1 respectively.
- Table 1. presents climate metrics for methane (using CO2 as a reference) for 20, 50 and 100 year time horizons.
- For methane, GFP is 48, 5, 0.8, 0.1 for 20, 50, 75, and 100 years.
- For methane, GWP is 99, 57, 42, 34 for 20, 50, 75, and 100 years.
- For methane, GTP is 67, 14, 8, 7 for 20, 50, 75, and 100 years.
- For methane, iGTP is 107, 52, 46, 37 for 20, 50, 75, and 100 years.
- The paper suggests that a step reduction in methane emissions of 1 Gt(CH4) yr-1 was approximately equivalent to an increase in allowed cumulative CO2 emissions of 2900–3300 Gt(CO2).
- The difference between the 20 year metric and the 100 year metric is still large (roughly a factor of 1.5 for methane compared to the factor of 10 for GTP(20) versus GTP (100)), but the differences between 50 years and 100 years are much smaller (table 2)-factors of 1.2-1.3 for the combined metrics for SLCFs with a lifetime of around 20 years or less rather than the factors up to 2
- The CGTPCH₄ is around 7% smaller than the CGWPCH4 on the longer time horizons (figure 2).
Other Important Findings
- The research finds that traditional climate metrics like GWP are criticized for over- or under-stating the importance of different species on climate, particularly for short-lived climate forcers (SLCFs).
- The Paris Agreement specifies two clear scientific goals: to limit temperature increases (Article 2), and to achieve balance between sources and sinks of greenhouse gases (Article 4).
- Endpoint metrics (such as global temperature change potential GTP) are the most closely related to policy goals based on temperature limits (such as Article 2 of the Paris Agreement).
- The advantage of a combined step-pulse metric for SLCFs is that for species with a lifetime less than 20 years a single time horizon of around 75 years can cover the range of timescales appropriate to the Paris Agreement.
- The GWP* approach and the CGWP and CGTP developed here differ in their precise values, they are structurally and conceptually similar.
- The combined step-pulse metrics (CGWP and CGTP) provide a useful way to compare changes in emission rates of SLCFs (lifetimes less than around 50 years) with cumulative emission changes in LLGHGs so that they may be included within common baskets in climate agreements.
- These are endpoint climate metrics which are closely tied to long-term climate goals such as the Paris Agreement.
- The paper emphasizes that the calculation of the combined metrics proposed here need no additional inputs, or assumptions, than are already used to generate the GWP and GTP values in Myhre et al (2013); hence tabulated values can be easily constructed.
- The time variation in the combined metrics for short-lived species is mostly due to the decrease in CO2 concentration following a pulse.
Limitations Noted in the Document
- The authors acknowledge that the step/pulse equivalence in GWP* is only approximate.
- The choice of time horizon for the metrics, particularly for SLCFs, can impact their values, making them less straightforward for policy applications.
- The study focuses primarily on methane, and its findings may not be directly applicable to all SLCFs or other greenhouse gases without further analysis.
- The analysis relies on specific climate models and data, and the results may be subject to uncertainties associated with these inputs.
- The study does not incorporate carbon cycle-temperature responses for non-CO2 species.
Conclusion
The study underscores the limitations of the traditional GWP in accurately representing the climate impacts of short-lived climate forcers (SLCFs) within the context of long-term climate goals. The core argument of the study is that combined step-pulse metrics, specifically CGWP and CGTP, offer a more robust and less time-dependent approach to evaluating the contributions of different species to future temperature targets. These metrics are designed to address the shortcomings of existing metrics, such as the Global Warming Potential (GWP), which can misrepresent the impacts of SLCFs due to their reliance on a fixed time horizon and their inability to accurately reflect the dynamics of climate change. The key advantage of using combined step-pulse metrics lies in their ability to reduce the sensitivity to the time horizon, making them more suitable for policy applications. The study’s findings suggest that the CGTP, in particular, can be a valuable tool for policymakers, as it provides a more consistent and effective way to compare the contributions of different species to future temperature targets. This allows for a more accurate assessment of emission reduction strategies. The authors emphasize that the approach does not require any additional inputs or assumptions beyond those already used to generate the GWP and GTP values. The study suggests that the CGTP(75) can be used as a suitable metric for SLCFs, covering the timescales relevant to the goals of the Paris Agreement. This work contributes to the ongoing discussion about how best to measure and compare the effects of different greenhouse gases, highlighting the need for metrics that align with long-term climate objectives. The study highlights that although the GWP* approach and the CGWP and CGTP developed here differ in their precise values, they are structurally and conceptually similar. The time variation in the combined metrics for short-lived species is mostly due to the decrease in CO2 concentration following a pulse. The difference between the 20 year metric and the 100 year metric is still large, but the differences between 50 years and 100 years are much smaller. These metrics are endpoint climate metrics which are closely tied to long-term climate goals such as the Paris Agreement.