Rethinking how to measure methane’s climate impact
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Why is the 100-year time horizon commonly used for emissions metrics?
Jackson: Carbon dioxide lingers in the atmosphere for thousands of years. Nitrous oxide tends to last a century or so. Methane’s lifetime is closer to a decade. The 100-year timeframe is both a compromise and a convenient round number acknowledging the different lifetimes of greenhouse gases.
Abernethy: Going back to the early Intergovernmental Panel on Climate Change reports, 20, 100 and 500-year time horizons were used as representative examples for which time horizons could be chosen. It seems like 100 years was chosen for the Kyoto Protocol and subsequent climate policy mainly just because it is the middle value of these three.
If international climate change agreements, such as the Paris Agreement, target temperature goals, why haven’t they incorporated into emissions metrics time horizons that specifically account for those goals?
Abernethy: That question led me to do this research, and write this paper. One answer is that there wasn’t previously a way to do this before the development of a scenario database of potential future climate pathways. I think another aspect is that the Paris Agreement goals are sufficiently vague that you have to pick one specific aspect to focus on. I looked at the temperature goal, but there is also a goal to have net zero emissions.
How might a 24-year time horizon alter the way we judge countries’ climate commitments and what the world needs to do to reach goals set by the Paris Agreement?
Jackson: We need to reduce emissions of carbon dioxide in all scenarios, near and far. The more aggressive the temperature goal is, however, the more important potent, shorter-lived greenhouse gases such as methane become. To keep global temperature increases below 1.5 or 2 degrees Celsius above pre-industrial levels – the two Paris Agreement goals – countries need to commit to reducing methane emissions faster. In truth, some countries have yet to make methane commitments at all.
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https://www.eurekalert.org/news-releases/942370paper:
https://iopscience.iop.org/article/10.1088/1748-9326/ac4940Emission 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.
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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
See original link for the full text (lots of formula´s which don´t paste well).
