Methane's Impact on Global Warming

Methane traps over 80 times more heat than carbon dioxide over 20 years and about 30 times more over 100 years. Not good news over time scales that matter. Ironically it’s not all bad news.

The IPCC developed these comparisons (called Global Warming Potential or GWP) to illustrate the potential of various gases to trap heat in the 1990’s. The most commonly used values are for 20 years — GWP₂₀ — and 100 years — GWP₁₀₀. The GWP has been widely adopted and is routinely used when assessing the potential climate impact of projects that emit (or would emit if they’re in the planning stages) planet warming gases.

The IPCC recognized that there’s no perfect way to compare the impact of gases. Limitations of the GWP include:

• The choice of a time frame is arbitrary.
• Values are calculated assuming a single emission (a “pulse”) and do not reflect the behaviour of sustained emissions.
• GWP does not have a direct relationship to temperature.
• Methane only lasts in the atmosphere for about 12 years — it is a short lived gas. The GWP underestimates the impact of these gasses in the short term and overestimates their impact in the long term.

Warming Impacts

The following graph compares the heating caused by 1 ton of methane and 28 tons of carbon dioxide for a single pulse of methane. The two values are chosen because the GWP of methane over 100 years was 28 when the analysis was done. Note that the impact of the carbon dioxide hardly changes over the 100 years — it lasts for a very long time (centuries) in the atmosphere.

Methane — it’s not your grandparents climate polution Environmental Defense Fund,accessed 2023/12/22 at 21:40

When you look at what happens with a constant emission over a period of times things change quite dramatically. About 1/2 of emitted methane breaks down slowly into carbon dioxide in the atmosphere. As the buildup of carbon dioxide increases so too does its impact on warming. So while the impact of the methane drops relatively quickly the impact of the generated carbon dioxide lasts for a very long time.

The following graphs illustrate what happens when methane is emitted continuously over a period of time. From left to right:

• At time t = 0 emissions ramp up from 0 to 4 Mt/yr.
• The concentration only increases for about the first 50 years because methane is broken down into other greenhouse gases. The emissions added equal the amount being broken down. 
• “Forcing” measures the balance between the solar energy absorbed and reflected by earth. If it’s positive then warming happens; negative and the planet cools; 0 and nothing changes. The dotted line is the forcing caused by the methane. The solid line is the total forcing due to methane and the gases to which it breaks down. These gases last for a long time but do not trap as much heat as methane so forcing increases more slowly.
• The warming increases at a slower rate once the concentration of methane stabilizes.

A-step-change-to-sustained-emissions-of-CH4 Retrieved December 20, 2023, from Research Gate

Methane has directly increased the radiative forcing in the lower atmosphere by about 20%. When the contributions of the gases to which it breaks down are included that increases to about 40%. That translates to about 30% of the rise in global temperatures.

Just for comparison here’s the same series of graphs for carbon dioxide. The important thing to note is that the concentration, forcing and warming just keep increasing at a steady rate directly related to the emissions. It takes centuries for carbon dioxide to break down.

A-step-change-to-sustained-emissions-of-CO2 Retrieved December 20, 2023, from Research Gate

The Good News

So what’s the good news? Because methane is a short lived gas if the emissions are reduced the concentration — and therefore the warming — drops quickly.

Here are the same two graphs but this time emissions are constant for the first 50 years and then drop to 0 over the next fifty years. The concentration and forcing due to methane drops quite quickly; warming drops and while the decrease is not quite as dramatic it is still very significant. The concentration and forcing of carbon dioxide drops a bit but warming continues to increase.

Emissions increase and then decrease Retrieved December 20, 2023, from Research Gate

References

Lynch, John, Michelle Cain, Raymond Pierrehumbert, and Myles Allen. “Demonstrating GWP*: A Means of Reporting Warming-Equivalent Emissions That Captures the Contrasting Impacts of Short- and Long-Lived Climate Pollutants.” Environmental Research Letters 15, no. 4 (April 1, 2020): 044023. https://doi.org/10.1088/1748–9326/ab6d7e.

Menon, Shanti. “Methane: It’s Not Your Grandparents’ Climate Pollution,” August 1, 2023. https://vitalsigns.edf.org/story/methane-its-not-your-grandparents-climate-pollution.