Abstract
Anthropogenic global warming at a given time is largely determined by the cumulative total emissions (or stock) of long-lived climate pollutants (LLCPs), predominantly carbon dioxide (CO2), and the emission rates (or flow) of short-lived climate pollutants (SLCPs) immediately prior to that time. Under the United Nations Framework Convention on Climate Change (UNFCCC), reporting of greenhouse gas emissions has been standardised in terms of CO2-equivalent (CO2-e) emissions using Global Warming Potentials (GWP) over 100-years, but the conventional usage of GWP does not adequately capture the different behaviours of LLCPs and SLCPs, or their impact on global mean surface temperature. An alternative usage of GWP, denoted GWP*, overcomes this problem by equating an increase in the emission rate of an SLCP with a one-off “pulse” emission of CO2. We show that this approach, while an improvement on the conventional usage, slightly underestimates the impact of recent increases in SLCP emissions on current rates of warming because the climate does not respond instantaneously to radiative forcing. We resolve this with a modification of the GWP* definition, which incorporates a term for each of the short-timescale and long-timescale climate responses to changes in radiative forcing. The amended version allows “CO2-warming-equivalent” (CO2-we) emissions to be calculated directly from reported emissions. Thus SLCPs can be incorporated directly into carbon budgets consistent with long-term temperature goals, because every unit of CO2-we emitted generates approximately the same amount of warming, whether it is emitted as a SLCP or a LLCP. This is not the case for conventionally derived CO2-e.
Generated Summary
This research article introduces an improved method for calculating the warming-equivalent emissions of short-lived climate pollutants (SLCPs). The study builds upon the concept of Global Warming Potential (GWP*), which offers an alternative to the conventional use of GWP, particularly in addressing the varying behaviors and impacts of long-lived and short-lived climate pollutants. The core of the study is the modification of the GWP* definition to better capture the delayed temperature response to past increases in SLCP emissions. The approach involves equating a change in the emission rate of an SLCP with a one-off “pulse” emission of CO2. The authors introduce a term for both short- and long-timescale climate responses to changes in radiative forcing. The study uses historical emissions data and RCP scenarios to estimate the best-fit values for the rate and stock contributions, which are key to the revised GWP* definition. The goal is to provide a more accurate and policy-relevant metric, where every unit of CO2-we emitted generates approximately the same amount of warming, applicable whether it is emitted as a SLCP or a LLCP, aligning with the goals of the Paris Agreement.
Key Findings & Statistics
- The study references the UNFCCC, which requires the use of 100-year Global Warming Potential (GWP100) to account for all gases as carbon dioxide equivalent (CO2-e) quantities.
- The research highlights that GWP100 has received criticism and is not suitable to appraise temperature-related goals.
- The study uses the methane radiative forcing from the RCP database with a factor of 1.65 applied to account for the secondary effects of ozone and stratospheric water vapour.
- The coefficients to weight the long and short term effects are found for each scenario using a linear regression model of the equation ΔT = a.CGWP* + b.CGWP100, where CGWP100 is the cumulative CO2-e emission of methane defined conventionally using GWP100, and CGWP* is the cumulative CO2-e emission of methane defined using GWP* from Allen, et al.12 with At = 20 years.
- The study found that r = 0.75 and s = 0.25 are the mean values based on the three RCPs.
- The study shows a linear relationship between cumulative CO2 emissions and warming.
- The study determined that the exponential decline of 0.3% per year corresponds to a time constant of about 300 years.
- The study mentions that, for representative values, (ECS = 2.75 °C, TCR = 1.6 °C, d2 = 239 years, after Millar, et al.³¹) this indicates a decline rate, (ECS-TCR)/(d2 × TCR), of 0.3% per year, corresponding to a time-scale of 333 years.
- The study states that the rate of decline of methane emissions required to reduce the rate of methane-induced warming to zero has not been explored systematically with complex models.
Other Important Findings
- The original GWP* definition underestimated the impact of recent increases in SLCP emissions on current rates of warming.
- The amended version allows “CO2-warming-equivalent” (CO2-we) emissions to be calculated directly from reported emissions.
- SLCPs can be incorporated directly into carbon budgets consistent with long-term temperature goals.
- The rate-based equivalence for SLCPs overcomes the problems inherent in GWP, in not adequately distinguishing their largely non-cumulative behaviour.
- The multi-century component of the thermal response of the climate system, together with carbon cycle feedbacks, act to prolong the warming impact of SLCP emissions.
- The study demonstrates a method that unambiguously links aggregated greenhouse gas emissions with their warming outcomes on decade to century timescales.
- The study found the mean values based on three RCPs and are found to provide a good fit for all three.
- The benefits of GWP* are most apparent when SLCP emission rates are declining.
Limitations Noted in the Document
- GWP100 is criticized for its inability to appraise temperature-related goals.
- The study notes the use of a small correction to account for the delayed temperature response to past increases in SLCP emissions, but this contribution was not pursued in detail due to its small impact in the scenarios considered.
- The study’s empirical method to determine the values of r and s is based on a combination of historical and future scenario emissions.
- The study uses a simple climate model (FaIR) with prescribed TCR, which, while crucial, would likely be similar with different radiative forcing.
- The coefficients and therefore the rate of decline are based on a combination of historical and future scenario emissions.
- The study acknowledges the uncertainty in how the real climate would evolve, for example including feedback.
Conclusion
The study successfully introduces a revised definition of GWP* that provides a more accurate representation of the warming potential of SLCPs. By accounting for both the short-lived nature of SLCPs and the long-timescale adjustment of the climate system, the study offers a single metric for converting SLCP emissions to CO2-equivalent emissions, thereby preserving a clear connection to global warming. This work underscores the shortcomings of conventional GWP metrics, particularly when evaluating the impacts of SLCPs. The revised GWP* allows for a more accurate representation of the impacts of methane and other SLCPs, which has direct implications for the ability to meet the Paris Agreement goals. The method provides a practical tool for policymakers and researchers to assess the contributions of various climate forcing agents and aggregate them into a global total, ultimately estimating the resulting warming. The study concludes that while linearization is key, the approach provides a close match to a linear relationship for the scenarios. By incorporating both timescales of climate response, the revised GWP* offers a more nuanced approach to evaluating climate impacts, especially in scenarios involving declining SLCP emissions, which is when traditional GWP methods can be misleading. The authors highlight that the use of GWP* can be instrumental in aligning emissions scenarios with temperature goals. The development of this metric represents a significant advancement in climate science, offering a more refined and accurate tool for assessing the impacts of SLCPs and supporting the development of effective mitigation strategies.