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 GWP100 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 GWP100, 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 research article, published in Environmental Research Letters, introduces and assesses the application of GWP* (Global Warming Potential star) as a means of reporting warming-equivalent emissions. The study employs a climate-carbon-cycle model (FaIR) to simulate various methane and CO2 emission scenarios, evaluating their impacts on atmospheric concentrations, radiative forcing, and temperature. The methodology involves comparing CO2-equivalents derived from both the traditional GWP100 and GWP*, with a focus on how GWP* better captures the contrasting impacts of short-lived (e.g., methane) and long-lived climate pollutants (e.g., CO2). The scope of the study includes a range of emission scenarios to illustrate the dynamics of different climate pollutants and to highlight the shortcomings of GWP100 in specific contexts. The study further explores the implications of GWP* for assessing mitigation strategies, particularly in relation to achieving climate targets.
Key Findings & Statistics
- The article references the Global Warming Potential (GWP), with the 100-year variant (GWP100) being formally adopted in international climate policy.
- Emissions of a given climate pollutant (E) can be converted to a CO2-e emission (Eco₂-e) quantity by multiplying by the appropriate GWP conversion factor, for the specified time-horizon (H): ECO₂-e = EX GWPH.
- The 100-year variant of the Global Warming Potential (GWP100) has been formally adopted in international climate policy.
- The study uses a default annual emission of 4 Mt methane.
- 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.
- The 100-year variant of the Global Warming Potential (GWP100) has been formally adopted in international climate policy.
- The 100-year variant of the Global Warming Potential (GWP100) has been formally adopted in international climate policy.
- The 100-year variant of the Global Warming Potential (GWP100) has been formally adopted in international climate policy.
- The 100-year variant of the Global Warming Potential (GWP100) has been formally adopted in international climate policy.
- Using GWP* over the first 20 years the methane emissions are represented by very high CO2-w.e. emissions.
- For methane, concentrations increase rapidly but then plateau after a few decades.
- For CO2, sustained emissions do not result in stabilising concentrations.
- The 100-year variant of the Global Warming Potential (GWP100) has been formally adopted in international climate policy.
- The 100-year variant of the Global Warming Potential (GWP100) has been formally adopted in international climate policy.
- GWP100, or indeed any pulse-based metric treating long- and short-lived climate pollutants in the same way, cannot capture these contrasting dynamics.
- Using GWP100, it is only possible to equate any ongoing methane emissions at all to a positive CO2 emission.
- If we sought to offset the continued warming from long-term stable methane emissions by annually sequestering an appropriate amount of CO2 derived using the GWP100, we would overstate the impacts of this methane and result in cooling, rather than temperature stabilisation.
- If we considered an emissions-accounting framework that did reflect total warming contribution, and could describe responsibilities to reverse warming rather than just encourage no further temperature increases, the baseline for CO2 emitters would not just be their current or recent annual CO2 emission rates, but total cumulative CO2 emissions to date, and they would have a responsibility to sequester carbon and reverse historic contributions.
- In this context, it is worth noting that annual fossil fuel CO2 emissions were the highest ever in 2019 and predicted to continue increasing in 2020.
Other Important Findings
- The study demonstrates that GWP* provides a useful indication of warming, while conventional application of GWP100 falls short in many scenarios, particularly when methane emissions are stable or declining.
- GWP* offers an improved means of assessing alternative mitigation strategies. It allows warming-equivalent emissions to be calculated directly from CO2-equivalent emissions reported using GWP100, consistent with the Paris Rulebook.
- The study reveals that the relationship between aggregate CO2-e emissions calculated using GWP100 and global warming itself is ambiguous, primarily because GWP100 does not sufficiently differentiate between the impacts of short- and long-lived climate pollutants.
- Sustained emissions of an SLCP result in a similar impact to a one-off release of a fixed amount of CO2.
- GWP* can be derived, relating a change in the rate of emissions of SLCPs to a fixed quantity of CO2.
- GWP*, as the means of defining equivalence is better associated with temperature change contribution, can be considered a ‘CO2 warming equivalent (CO2-w.e.)’, in contrast to a per-emission CO2-e.
- The article further refined in Allen et al [17], showing that specifying a time-period (∆t) 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.
- For CO2, reducing the emission rate merely acts to slow the rate of continued increase in concentration until emissions reach nearly 0, at which point atmospheric concentrations slowly decline.
- 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-induced warming depends on cumulative emissions to date, but for SLCPs such as methane, their warming contribution is predominantly a function of recent emission rates.
- For the most part, the actual warming from methane, than the stable and declining emissions scenarios.
- This new means of deriving CO2-equivalence is a significant reframing of how we report and conceptualise emissions of SLCPs.
- The most widely used CO2 equivalence metric is the Global Warming Potential (GWP), defined as the integrated change in radiative forcing (the perturbation of the Earth’s atmospheric energy balance, which leads to warming) over a specified time-period following an emission pulse of a given climate pollutant, relative to the same quantity of CO2.
- The scenarios presented show that the climatically optimum strategy is to reduce emissions of any greenhouse gases as early as possible.
- Stopping both gases early results in rapid cooling from the cessation of methane emissions, and also prevents the ongoing temperature increases that would occur from sustained CO2 emissions.
Limitations Noted in the Document
- The relationship between aggregate CO2-e emissions calculated using GWP100 and global warming itself is ambiguous.
- The study acknowledges that, as GWP* is a simple metric, it cannot capture the full complexities of the climate response.
- The article does not include variations in solar and volcanic forcing to focus on the impacts of the emissions scenarios alone.
- The accuracy of GWP* is further improved by 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 study acknowledges that the TCRE was obtained by identifying the warming response to cumulative CO2 emissions for RCP4.5 within the same FaIR set-up.
Conclusion
The study underscores the limitations of using GWP100 to direct climate change mitigation strategies. The article argues that GWP100 can be unfair, inefficient, and dangerous because it does not provide a clear link between emissions and climate change contributions. It can lead to an overestimation of the action needed to offset long-term methane emissions while simultaneously undervaluing the short-term benefits of reducing methane emissions. The study emphasizes that the choice of metric has crucial implications for policy and the accurate representation of climate impacts. The research advocates for the use of GWP* to accurately represent the warming impacts of emissions, as the metric provides a more direct link between emissions and anticipated warming impacts, supporting progress toward long-term temperature goals. The article concludes that a focus on methane emissions is not just a matter of prioritizing short- over long-term impacts; it involves a trade-off. Reducing methane emissions offers an immediate benefit, but it does not substitute the need for reducing CO2 emissions. The climatically optimal approach is to reduce emissions of all greenhouse gases as early as possible. While acknowledging that more complex methods may be preferred, the study suggests that the ease of calculating GWP* will likely prove a significant advantage for many purposes, and the scale of contemporary methane emissions is such that this element could equate to a significant increase in total allowable CO2 emissions for warming of 1.5° C if action is taken on methane earlier rather than later.