Global temperature goals should determine the time horizons for greenhouse gas emission metrics

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.

Mitigating climate disruption in time: A self-consistent approach for avoiding both near-term and long-term global warming

“…Reporting these pollutants separately allows for consideration not only of potential effects of mitigation measures by source and implications for coemissions but also an assessment of temperature impact on multiple time horizons of interest ( 1 ). With 1.5 °C expected to be crossed in the early 2030s ( 1 , 38 ), Abernethy and Jackson ( 39 ) have advocated for choosing time horizons for GHG aggregation metrics consistent with temperature goals, specifically supporting the use of GWP 20 over the GWP 100 . A similar argument can be made in the context of the urgency to slow warming in the near term ( 2 ).…”

Reply to comment on “How Green is Blue Hydrogen?”

“…In their comment, Romano et al use both 20‐ and 100‐year global warming potentials but fail to emphasize the superiority of the 20‐year time frame for avoiding immediate damage and tipping points for more severe long‐term damage. Along those lines, Abernethy and Jackson 29 in a new 2022 paper have demonstrated that the use of a 100‐year GWP is simply not compatible with reaching the climate goals set by the COP21 Paris accords in 2015.…”

Present global warming: a justifiable and stable metric for evaluating short-lived climate pollutants

“…GWP provides a single number that enables 'apples to apples' evaluation of any SLCP by directly comparing its potency to the potency of carbon dioxide. Additionally, numerous alternative metrics have been proposed, each designed to evaluate different aspects of the climate system [4,[8][9][10][11][12][13][14][15][16][17][18][19]. As reviewed by Balcombe [4], these metrics evaluate different timeframes, physical bases, emission durations, and more.…”

“…PGW leverages the thoughtful analysis used for discount rate selection to give a value that is both justifiable and consistent with other considerations. Temporal weighting values, such as time horizons in GWP and discount rates in PGW, reflect societal preferences and are mostly political selections [15,23]. Today there is some consensus on the political choice of discount rate used in climate policy, and that same chosen discount rate could be used for PGW.…”

“…The metrics are designed to evaluate different aspects of the climate system, and the metrics yield widely varying values. Considering only methane, the values of these metrics deviate from GWP by up to a factor of 1.6 for the same timeframe [4]; for cases in which time horizons are selected to align with temperature goals, these metrics deviate from GWP by a factor of 4.6 [15]. Similar considerations result in only minor changes to PGW.…”