Methane emissions: choosing the right climate metric and time horizon

Abstract

Methane is a more potent greenhouse gas (GHG) than CO2, but it has a shorter atmospheric lifespan, thus its relative climate impact reduces significantly over time. Different GHGs are often conflated into a single metric to compare technologies and supply chains, such as the global warming potential (GWP). However, the use of GWP is criticised, regarding: (1) the need to select a timeframe; (2) its physical basis on radiative forcing; and (3) the fact that it measures the average forcing of a pulse over time rather than a sustained emission at a specific end-point in time. Many alternative metrics have been proposed which tackle different aspects of these limitations and this paper assesses them by their key attributes and limitations, with respect to methane emissions. A case study application of various metrics is produced and recommendations are made for the use of climate metrics for different categories of applications. Across metrics, CO2 equivalences for methane range from 4-199 gCO2eq./gCH4, although most estimates fall between 20 and 80 gCO2eq./gCH4. Therefore the selection of metric and time horizon for technology evaluations is likely to change the rank order of preference, as demonstrated herein with the use of natural gas as a shipping fuel versus alternatives. It is not advisable or conservative to use only a short time horizon, e.g. 20 years, which disregards the long-term impacts of CO2 emissions and is thus detrimental to achieving eventual climate stabilisation. Recommendations are made for the use of metrics in 3 categories of applications. Short-term emissions estimates of facilities or regions should be transparent and use a single metric and include the separated contribution from each GHG. Multi-year technology assessments should use both short and long term static metrics (e.g. GWP) to test robustness of results. Longer term energy assessments or decarbonisation pathways must use both short and long-term metrics and where this has a large impact on results, climate models should be incorporated. Dynamic metrics offer insight into the timing of emissions, but may be of only marginal benefit given uncertainties in methodological assumptions.

Timelines for mitigating the methane impacts of using natural gas for carbon dioxide abatement

“…Although most commitments under the Paris Agreement use the GWP(100) to compare CO 2 and non-CO 2 GHGs in CO 2 -equivalent terms, alternative metrics are a subject of active debate in policy and environmental impact research [60,70,84]. For example, recent research has called for reporting CO 2 -equivalent emissions using multiple metrics to better represent the effects of different time horizons, physical and economic impact indicators, and modeling uncertainties [70,71]. Here we show how multiple metrics can be used to evaluate the benefits and drawbacks of policies that assign differing levels of importance to mitigating CH 4 .…”

“…decision makers to understand the impacts of choosing this over other metrics. This importance is reflected in an active debate about which metrics to use in policy [70,71]. Because of this we consider in our analysis not just one but a set of metrics.…”

The Use of Artificial Neural Networks to Analyze Greenhouse Gas and Air Pollutant Emissions from the Mining and Quarrying Sector in the European Union

“…The most adverse greenhouse gas emissions are CO 2 , CH 4, and N 2 O. Given the amount of greenhouse gas emissions from the mining and quarrying sector, methane ranks second behind carbon dioxide, but its impact is 21-34 times greater than CO 2 [59,60]. and the state of individual sectors of the economy.…”

LNG and Cruise Ships, an Easy Way to Fulfil Regulations—Versus the Need for Reducing GHG Emissions

“…HFO <3.5% S (heavy fuel oil) [13,27] 8.9 8.9 6 Chryssakis and Stahl (2013) Reading the table, the first observation is that interpreting and explaining all the different values could have been a study on its own. However, there are some clear trends and gaps among the reported figures.…”

“…Unlike CO2, which stays in the atmosphere for hundreds of years, the impact of CH4 emissions is largest during the first decades. With the urgent need to reduce global GHG emissions [4], there is a strong argument for focusing also on the shortterm impact of the individual GHG gases over the next 20 years [6], as illustrated in the left side of Figure 1 based on Intergovernmental Panel on Climate Change (IPCC) report 2013 [7]. Basically, the difference between using GWP20 instead of GWP100 is that the impact of CH 4 increases by 100%, from 20% to 40% of the total GHG's emitted.…”