Super pollutants are trendy, but we should be careful how we use them
Deep Dives
Explore related topics with these Wikipedia articles, rewritten for enjoyable reading:
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Global warming potential
13 min read
The article explicitly discusses GWP as the framework used to compare methane and CO2, and critiques its limitations. Understanding GWP's technical definition, calculation methods, and controversies would significantly deepen comprehension of the article's core argument about why simple metrics are problematic.
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Atmospheric methane
13 min read
While the article explains methane's behavior as a flow pollutant, the Wikipedia article provides crucial context on methane sources (natural vs anthropogenic), historical atmospheric concentrations, the hydroxyl radical oxidation process mentioned in the article, and current mitigation efforts that inform the policy debate.
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Carbon dioxide in the atmosphere of Earth
13 min read
The article's central distinction between stock and flow pollutants requires understanding CO2's unique persistence in the atmosphere. This Wikipedia article explains the carbon cycle, ocean and land sinks mentioned in the article, and why CO2 warming is effectively permanent on human timescales.
“Super pollutants” – short-lived climate pollutants like methane (CH4) and some refrigerants (halocarbons) – are having a moment. There were numerous sessions on the topic during the recent New York Climate Week, and a number of companies are exploring investments in reducing these emissions as part of their climate goals.
Reducing emissions of short-lived climate pollutants (SLCPs) is, by itself, an unambiguously good thing. Methane in particular is responsible for around a third of all warming to-date from well-mixed greenhouse gases in the atmosphere, and reductions in emissions can have a rapid cooling effect on the planet.
It is when methane (or other SLCPs) are used to offset or neutralize CO2 emissions – to make a claim that the climate effects of CO2 can be counterbalanced by methane – that the problem becomes much, much thornier. As Ray Pierrehumbert explains, “It is useful to reduce methane, but it’s not going to really help us towards net zero. The only real solution to the climate crisis is to get carbon dioxide emissions down to as close to zero as we can.”
Stocks vs flows
The question of how to compare methane and CO2 is one that has long interested me. I wrote a paper a decade ago on how to compare the climate impacts of coal and natural gas (back when talk of a “natural gas bridge” was in vogue), and authored the chapter on methane and other short-lived climate pollutants for Greta Thunberg’s Climate Book.
At its core, the difference in climate impacts between CO2 and methane comes down to the fact that CO2 is a “stock pollutant” and methane is a “flow pollutant”.
CO2 is an extremely stable molecule that accumulates in the atmosphere over time with constant emissions; while a portion of CO2 can be absorbed by land and ocean sinks in the form of organic or inorganic carbon, it does not naturally degrade. The warming that results from CO2 is – to a first order approximation – a largely time-invariant function of cumulative emissions. If CO2 emissions increase, the world warms faster; if they stay constant the world warms at a constant rate; if emissions decline, the world warms more slowly. But even if CO2 emissions get to zero, the world does not meaningfully cool back down for centuries to come; the only way to cool the planet through CO2 is to go net-negative – ...
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