Abstract
The atmospheric lifetime and radiative impacts of different climate pollutants can both differ markedly, so metrics that equate emissions using a single scaling factor, such as the 100-year Global Warming Potential (GWP100), can be misleading. An alternative approach is to report emissions as ‘warming-equivalents’ that result in similar warming impacts without requiring a like-for-like weighting per emission. GWP*, an alternative application of GWPs where the CO2-equivalence of short-lived climate pollutant emissions is predominantly determined by changes in their emission rate, provides a straightforward means of generating warming-equivalent emissions. In this letter we illustrate the contrasting climate impacts resulting from emissions of methane, a short-lived greenhouse gas, and CO2, and compare GWP100 and GWP* CO2-equivalents for a number of simple emissions scenarios. We demonstrate that GWP* provides a useful indication of warming, while conventional application of GWP 100 falls short in many scenarios and particularly when methane emissions are stable or declining, with important implications for how we consider ‘zero emission’ or ‘climate neutral’ targets for sectors emitting different compositions of gases. We then illustrate how GWP* can provide an improved means of assessing alternative mitigation strategies. GWP* allows warming-equivalent emissions to be calculated directly from CO2-equivalent emissions reported using GWP 100, consistent with the Paris Rulebook agreed by the UNFCCC, on condition that short-lived and cumulative climate pollutants are aggregated separately, which is essential for transparency. It provides a direct link between emissions and anticipated warming impacts, supporting stocktakes of progress towards a long-term temperature goal and compatible with cumulative emissions budgets.
Generated Summary
This journal article introduces GWP* (Global Warming Potential star) as a refined method for reporting emissions, specifically designed to capture the contrasting impacts of short- and long-lived climate pollutants. The study employs a climate-carbon-cycle model, FaIR, to simulate the effects of various emission scenarios, primarily focusing on methane and carbon dioxide. The research compares GWP* with the conventional GWP100 metric, highlighting the limitations of GWP100 in accurately reflecting the impact of short-lived pollutants like methane. The core methodology involves analyzing the warming-equivalent emissions generated by different scenarios, focusing on how GWP* provides a more accurate representation of the relationship between emissions and temperature change compared to the standard GWP100. The study explores scenarios with stable, declining, and increasing methane emissions, demonstrating how GWP* can better inform mitigation strategies and assess the impact of different emission pathways, emphasizing the need for a more nuanced approach to climate policy that considers the specific characteristics of various greenhouse gases.
Key Findings & Statistics
- The study uses the FaIR (Finite-Amplitude Impulse Response) v1.3 climate-carbon-cycle model to generate changes in atmospheric concentration, radiative forcing, and temperature for a number of methane and/or CO2 emissions scenarios.
- The study specifies a time-period (At) of 20 years over which to assess the change in SLCP emission rates (∆ESLCP), and scaling the CO2-w.e. per year of this period (i.e. /∆t).
- The 100-year variant of the Global Warming Potential (GWP100) has been formally adopted in international climate policy.
- GWP* was further refined in Allen et al [17], showing that specifying a time-period (At) of 20 years over which to assess the change in SLCP emission rates (∆ESLCP), and scaling the CO2-w.e. per year of this period (i.e. /∆t) provides a good fit for modelled warming, and a means of annualising the reported emissions.
- The equation for CO2-w.e.(SLCP) is: ECO2 w.e.(SLCP) = (r × ∆ESLCP / ∆t × H) + s × ESLCP × GWPH, where r represents the weighting given to the impacts of changing the rate of SLCP emissions, and s the weighting given to the impacts of the current emissions rate.
- Using the values suggested above, under all scenarios except near-constant emissions the equation is dominated by (0.75 × 100 = 75) the rate-based component, with much less weight (0.25) assigned to the stock component.
- A default annual emission of 4 Mt methane, close to the average UK methane emissions between 1990 and 2016 [23], was selected to represent a significant, policy-relevant methane emission that would not be so large as to greatly perturb the RCP4.5 background conditions. For reference, this is just over 1% of the 353 Mt total anthropogenic methane emissions estimated for 2012 [24].
- A fixed GWP value was used, as though in practice this should be updated as greenhouse gas concentrations and hence the unit forcing per emission change in the future, most current climate policy is also based on fixed, present-date GWP values
- The TCRE (Transient Climate Response to cumulative carbon Emissions), can link cumulative CO2 emissions and temperature.
- The TCRE was obtained by identifying the warming response to cumulative CO2 emissions for RCP4.5 within the same FaIR set-up. By expressing short-lived GHGs as a CO2-w.e. consistent with a ‘cumulative carbon’ framework, GWP* thus provides a means of directly linking emissions of different gases to their resulting temperature impact through the TCRE.
Other Important Findings
- GWP* provides a means of generating warming-equivalent emissions, offering a more accurate representation of the relationship between emissions and temperature change compared to the standard GWP100.
- GWP* can better inform mitigation strategies and assess the impact of different emission pathways, emphasizing the need for a more nuanced approach to climate policy.
- Sustained emissions of SLCPs result in a similar impact to a one-off release of a fixed amount of CO2.
- GWP* requires only two values, which are already calculated and reported within the UNFCCC.
- The study demonstrates a number of methane and CO2 emissions scenarios in a simple climate model to display their contrasting dynamics.
- The differing warming legacies once emissions of either gas are removed, as shown in figure 2, also applies to one-off emission pulses.
- For CO2, modelling an equivalent (using GWP100) emissions scenario, we see that sustained emissions do not result in stabilising concentrations.
- GWP* CO2-w.e. is much less than GWP 100 CO2-e (×0.25, following (2) with the suggested s), and cumulative CO2-w.e. (×TCRE) a much better match for the modelled warming.
- The climatically optimum strategy is to reduce emissions of any greenhouse gases as early as possible.
- The study shows how these emissions scenarios translate to equivalents derived using either GWP100 or GWP* noting some of the shortcomings of GWP 100, and how these are overcome by GWP*.
Limitations Noted in the Document
- GWP*, as a simple metric reliant on assumptions of linearity inherent in the GWP, cannot capture the full complexities of the climate response that alternative approaches may be able to, such as CO2-forcing-equivalents, the CGTP and CGWP, or comprehensive modelling of the climate system.
- The study focuses exclusively on methane.
- The match between cumulative CO2-w.e. emissions (×TCRE) and warming is not exact, and in this case the underestimation of warming may reflect, among other dynamics, an increase in the radiative efficiency of methane over this period expected as wider global methane concentrations fall under RCP4.5.
- GWP100 would understate the total warming impact of sustained emissions before this point, and increasingly overstate the impacts after it.
- There will remain applications where more complex methods are preferred, but the ease of calculating GWP* will likely prove a significant advantage for many purposes.
Conclusion
The study underscores the limitations of the widely used GWP100 metric in accurately representing the climate impact of short-lived pollutants, particularly methane. The research emphasizes the importance of GWP* as a more accurate tool for assessing the effects of emissions, providing a direct link between emissions and anticipated warming impacts. The findings reveal that the choice of metric significantly influences the assessment of mitigation strategies, with GWP* offering a more nuanced understanding of the dynamics of different greenhouse gases. The study highlights that relying on GWP100 could lead to unfair, inefficient, and potentially dangerous climate policies. The research advocates for a shift towards GWP* to enhance the transparency of climate policies and ensure they reflect the true warming potential of various emissions. By providing a more accurate representation of the warming impacts of emissions, the study suggests that GWP* can aid in more effective and equitable climate change mitigation strategies. The authors emphasize that the choice of metric significantly impacts how we understand the climate impact of methane and other short-lived pollutants, stressing that a more accurate assessment is essential for the design of effective climate change policies. The study underscores the need for a more nuanced approach to climate policy that considers the specific characteristics of various greenhouse gases. GWP* provides a straightforward means of generating warming-equivalent emissions, offering a more accurate representation of the relationship between emissions and temperature change compared to the standard GWP100, by providing a means of directly linking emissions of different gases to their resulting temperature impact through the TCRE.