Abstract
Emission metrics, a crucial tool in setting effective exchange rates between greenhouse gases, currently require an arbitrary choice of time horizon. Here, we propose a novel framework to calculate the time horizon that aligns with scenarios achieving a specific temperature goal. We analyze the Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5 °C Scenario Database to find that time horizons aligning with the 1.5 °C and 2 °C global warming goals of the Paris Agreement are 24 [90% prediction interval: 7, 41] and 58 [90% PI: 41, 74] years, respectively. We then use these time horizons to quantify time-dependent emission metrics for methane. We find that the Global Warming Potential (GWP) values that align with the 1.5 °C and 2 °C goals are GWP1.5 °C = 75 [90% PI: 54, 107] and GWP2 °C = 42 [90% PI: 35, 54]. For the Global Temperature change Potential (GTP) they are GTP1.5 °C = 41 [90% PI: 16, 102] and GTP2°C = 9 [90% PI: 7, 16]. The most commonly used time horizon, 100 years, underestimates methane’s GWP and GTP by 34% and 38%, respectively, relative to the values we calculate that align with the 2 °C goal and by 63% and 87%, respectively, relative to the 1.5 °C goal. To best align emission metrics with the Paris Agreement 1.5 °C goal, we recommend a 24 year time horizon, using 2045 as the endpoint time, with its associated GWP1.5 °C = 75 and GTP1.5 °C = 41.
Generated Summary
This research article explores the relationship between global temperature goals and the time horizons used in greenhouse gas emission metrics. The study proposes a novel framework for determining time horizons that align with specific temperature targets outlined in the Paris Agreement. The methodology involves analyzing data from the Intergovernmental Panel on Climate Change (IPCC) Special Report on Global Warming of 1.5°C scenario database to ascertain time horizons that correspond to the 1.5°C and 2°C global warming goals. The authors then use these time horizons to evaluate time-dependent emission metrics for methane. The study aims to improve the accuracy of emission metrics by aligning them with the goals of the Paris Agreement, thereby providing a more effective tool for policymakers and decision-makers in climate change mitigation strategies. The research focuses on how global temperature goals should determine the time horizons for greenhouse gas emission metrics.
Key Findings & Statistics
- The time horizons aligning with the 1.5°C and 2°C global warming goals of the Paris Agreement are 24 [90% prediction interval: 7, 41] and 58 [90% PI: 41, 74] years, respectively.
- The Global Warming Potential (GWP) values that align with the 1.5 °C and 2 °C goals are GWP1.5 °C = 75 [90% PI: 54, 107] and GWP2 °C = 42 [90% PI: 35, 54].
- For the Global Temperature change Potential (GTP) they are GTP1.5 °C = 41 [90% PI: 16, 102] and GTP2°C = 9 [90% PI: 7, 16].
- The most commonly used time horizon, 100 years, underestimates methane’s GWP and GTP by 34% and 38%, respectively, relative to the values we calculate that align with the 2 °C goal and by 63% and 87%, respectively, relative to the 1.5 °C goal.
- The relationship between peak temperature and the time the peak temperature is reached for all 213 scenarios and all 10 IAMs are as follows: tpeak = aTpeak + b, where a = 68 ± 4 years/°C and b = 1943 ± 7 years
- Our estimates for when the temperature goals of the Paris Agreement will be reached are 2045 [90% PI: 2028, 2062] for 1.5 °C and 2079 [90% PI: 2062, 2095] for 2 °C.
- These results correspond to time horizons of 24 and 58 years, respectively, from present day (2021).
- The 100 year time horizon most commonly used by the IPCC lies far beyond these 24 and 58 year time horizons that align with their specific temperature goals.
- When the 100 year time horizon is applied to a 2 °C goal, instead of our proposed 58 year time horizon, methane emission metrics are significantly underestimated relative to those we find that align with the 2 °C goal: 28 vs 42 for GWP (34% lower) and 5.4 vs 8.7 for GTP (38% lower).
- The 100 year time horizon underestimates emission metrics that align with the 1.5 °C goal even more drastically: by 63% for GWP and 87% for GTP.
Other Important Findings
- The study emphasizes the importance of aligning emission metrics with specific temperature goals, especially those of the Paris Agreement.
- The research finds that the commonly used 100-year time horizon significantly underestimates the impact of methane, especially when compared to the 1.5°C goal.
- The framework suggests that emission metrics should be time-dependent and adjusted based on the proximity to the peak temperature.
- The study acknowledges that the choice of emission metrics should be based on the outcome of interest and the goals of the analysis, such as radiative forcing targets.
Limitations Noted in the Document
- The relationship between peak temperature and the time it occurs is not motivated by an underlying physical mechanism, and a quadratic or logarithmic function changes the predicted time of peak warming by less than 2 years for all temperature goals relevant to the Paris Agreement (1.5 °C-2 °C).
- The set of scenarios in SR1.5 is not exhaustive and does not include scenarios with temperature peaks higher than 2.3 °C.
- The SR1.5 scenarios end in 2100, potentially biasing the prediction interval downwards (earlier).
- The framework is primarily designed for times before peak temperature and may not be applicable after the peak.
- The study recognizes that the application of its results in climate policy is complicated by the use of time-dependent emission metrics.
Conclusion
The central argument of the study is that global temperature goals, as set by the Paris Agreement, should dictate the time horizons used in greenhouse gas emission metrics. The research demonstrates that the commonly used 100-year time horizon significantly underestimates the impact of methane emissions, especially when aiming to meet the 1.5°C goal. “To best align emission metrics with the Paris Agreement 1.5 °C goal, we recommend a 24 year time horizon, using 2045 as the endpoint time, with its associated GWP1.5 °C = 75 and GTP1.5 °C = 41.” The study suggests a shift towards shorter time horizons to accurately reflect the influence of greenhouse gases on achieving specific temperature goals. “We emphasize the importance of increasing and achieving NDCs such that global temperature stabilization becomes feasible. Only then, after substantial simultaneous reductions of all greenhouse gas emissions, can our framework be used to directly compare carbon dioxide with short-lived climate forcers such as methane.” The study’s findings underscore the need for dynamic carbon pricing and the regular updating of emission metrics to align with evolving climate goals. The authors advocate for a consistent approach to emission metrics that supports the achievement of the Paris Agreement targets. “While such a dynamic approach may have inhibited the early adoption of time-dependent emission metrics, and may prove politically difficult to implement, we believe that dynamic carbon pricing already signals an ability for government agencies to use time-dependent emission metrics for policy applications.”