Stable Climate Metrics For Emissions Of Short And Long-lived Species—combining Steps And Pulses

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 article published in Environmental Research Letters explores the use of stable climate metrics for emissions of short and long-lived species, combining step and pulse emissions. The study focuses on the limitations of traditional climate metrics, such as the Global Warming Potential (GWP), in accurately reflecting the impacts of different greenhouse gases, particularly short-lived climate forcers (SLCFs). The authors propose and evaluate the effectiveness of combined step-pulse metrics (CGWP and CGTP) to provide a more accurate and less time-horizon dependent method for comparing the relative contributions of different species to future temperature targets. The research employs a methodology that combines endpoint metrics for step changes in SLCF emissions with pulse emissions of CO2, which results in metrics that vary less with time horizons of interest. The study is based on the concepts of relating the impacts of a step change in SLCF emissions with a pulse emission of CO2, originally introduced in previous studies.

Key Findings & Statistics

• The study uses the following radiative efficiencies for CH4 and CO2: 4.40 × 10-4 W m-2 ppb-1 and 1.30 × 10-5 W m-2 ppb-1 respectively, with a methane perturbation lifetime of 12.4 years.
• The study incorporates the indirect effects of methane on ozone and stratospheric water vapor, following Myhre et al. (2013) and using values of 1.82 × 10-4 and 0.54 × 10-4 W m-2 ppb(CH4)-1 respectively.
• Table 1 of the study presents climate metrics for methane, using CO2 as a reference, for 20, 50, 75, and 100-year time horizons. The table includes values for GFP, GWP, GTP, and iGTP for these time horizons.
• The GWP and GTP values are provided for CH4, with specific values for different time horizons, to illustrate the variability of these metrics. The GWP value for methane is 99 for a 20-year time horizon, 57 for 50 years, 42 for 75 years, and 34 for 100 years. GTP for methane is 67 for a 20-year time horizon, 14 for 50 years, 8 for 75 years, and 7 for 100 years.
• The combined metrics, CGWP and CGTP, are moderately flat for HFC-32 and CH4 but rise steadily for CFC-11 and N2O.
• The more usual pulse/pulse metrics of GFPP and GTPP decrease significantly with time for HFC-32 and CH4, but are steadier for N2O.
• 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)).
• Table 2 provides a comparison of pulse metrics with combined metrics for the 4 gases relative to CO2, showing values for GFP, GTP, CGWP, and CGTP at different time horizons (50, 75, and 100 years). For CH4: at 50 years, GFP is 5.2, and CGWP is 3600, and CGTP is 3300.
• For the combined metrics, the difference between the radiative forcing and temperature approaches is not large; the CGTPCH4 is around 7% smaller than the CGWPCH4 on the longer time horizons.

Other Important Findings

• The study highlights that endpoint metrics (such as global temperature change potential – GTP) are most closely related to policy goals based on temperature limits, but for short-lived climate forcers (SLCFs), these metrics vary strongly with time horizon.
• The research builds on prior work, specifically the use of GWP* (Allen et al., 2016), to create a metric that approximates the relative temperature impacts of a step change in SLCF emissions with a pulse emission of CO2.
• The authors emphasize that the choice of metric depends on the climate goal. For instance, the Paris Agreement specifies two clear scientific goals: to limit temperature increases and to achieve a balance between sources and sinks of greenhouse gases.
• The study indicates 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 research demonstrates that the combined step-pulse metrics (CGWP and CGTP) are less dependent on time horizon than GTP.
• The paper also notes that the combined step-pulse metrics (CGWP and CGTP) provide a useful way to compare changes in emission rates of SLCFs with cumulative emission changes in LLGHGs.
• 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 study acknowledges the limitations associated with the use of metrics, specifically the variability of endpoint metrics for SLCFs with time horizon.
• The research recognizes that the approximations in Allen et al. (2016, 2018) lead to an underestimation of the contribution of SLCFs to temperature change.
• The paper notes that the choice of a 100-year time horizon for the GWP, as applied in the Kyoto Protocol, was arbitrary rather than based on scientific reasoning.
• The study does not incorporate carbon cycle-temperature responses for non-CO2 species.
• The authors emphasize that the calculation of the combined metrics proposed here needs no additional inputs or assumptions than are already used to generate the GWP and GTP values in Myhre et al. (2013).

Conclusion

The study concludes that the combined step-pulse metrics (CGWP and CGTP) provide a more accurate and less time-horizon dependent method for comparing the relative contributions of different species to future temperature targets. The authors suggest that the CGTP(75) can be used as a suitable metric for SLCFs, covering the timescales relevant to the goals of the Paris Agreement. They highlight that the choice of metric depends on the climate goal and that for any temperature-based target, CO2 emissions need to fall to zero, while SLCFs need to stabilize. The research also emphasizes the importance of the step-pulse approach (such as CGTP or GWP*) as a significant improvement in the comparison of emissions of SLCFs with long-lived greenhouse gases. The findings suggest that the time variation in the combined metrics for short-lived species is mostly due to the decrease in CO2 concentration following a pulse. The authors note that the use of GWP is clearly not suitable for comparing the contributions of short and long-lived climate agents towards the Paris temperature goals. The study’s findings are relevant for climate policy and the development of emission metrics that can be used to inform national targets and policies. The research also emphasizes the need for the scientific community to continuously improve and refine climate metrics to better reflect the impacts of different greenhouse gases. In essence, this research offers a more precise method for evaluating the effects of SLCFs on the climate, therefore providing a framework for constructing more effective strategies to meet climate objectives. By utilizing the combined step-pulse metrics, the researchers aim to offer a more dependable and stable approach to the evaluation of short-lived climate forcers within the broader landscape of climate policy.

DOI

10.1088/1748-9326/ab6039