How does methane increase global temperature? This article outlines the theory. We also review the best equations, linking atmospheric methane concentrations to radiative forcing, and in turn to global temperatures. These formulae suggest 0.7 W/m2 of radiative forcing and 0.35ยบC of warming has already occurred due to methane, as atmospheric methane has risen from 720 ppb in 1750 to 1,900 ppb in 2021. This is 20-30% of all warming to-date. There are controversies over mathematical scalars. But on reviewing the evidence, we still strongly believe that decarbonizing the global energy system requires replacing coal and ramping natural gas alongside low-carbon energies.
On the Importance of Reaching Net Zero?
There is a danger that writing anything at all about climate science evokes the unbridled wrath of substantially everyone reading. Hence let us start this article by re-iterating something important: Thunder Said Energy is a research firm focused on the best, most practical and most economical opportunities that can deliver an energy transition. This means supplying over 100,000 TWH of useful human energy by 2050, while removing all of the CO2, and avoiding turning our planet into some kind of Waste Land.
Our roadmap to net zero (note below) is the result of iterating between over 1,000 thematic notes, data-files and models in our research. We absolutely want to see the world achieve important energy transition goals and environmental goals. And part of this roadmap includes a greatly stepped up focus on mitigating methane leaks (our best, most comprehensive note on the topic is also linked below).
However, it is also helpful to understand how methane causes warming. As objectively as possible. This helps to ensure that climate action is effective.
It is also useful to construct simple models, linking atmospheric methane concentrations to global temperature. They will not be perfect models. But an imperfect model is often better than no model.
Methane is a powerful greenhouse gas
An overview of the greenhouse effect is written up in a similar post, quantifying how CO2 increases global temperature (note below). We are not going to repeat all of the theory here. But it may be worth reading this prior article for an overview of the key ideas.
Greenhouse gases absorb and then rapidly re-radiate infra-red radiation. This creates a less direct pathway for solar radiation to be reflected back into space. The ability of different gas molecules to absorb and re-radiate infra-red radiation depends on the energy bands of electrons in those molecules, especially the shared electrons in covalent bonds between non-identical molecules with “dipole moments” (this is why H2O, CO2, CH4 and N2O are all greenhouse gases, while N2, O2 and Ar are not).
There are two reasons that methane is up to 200x more effective than CO2 as a greenhouse gas. The first reason is geometry. CH4 molecules are tetrahedral. CO2 molecules are linear. A tetrahedral molecule can generally absorb energy across a greater range of frequencies than a linear molecule.
The second reason is that methane is 200x less concentrated in the atmosphere, at 1,900 parts per billion, versus CO2 at 416 parts per million. We saw in the post below that radiative forcing is a log function of greenhouse gases. In other words, the first 20ppm of CO2 in the atmosphere explains around one-third of all the warming currently being caused by CO2. Each 1ppm increase in atmospheric CO2 has a ‘diminishing impact’, because it is going to absorb incremental radiation in a band that is already depleted by the pre-existing CO2. Thus small increases in methane cause more warming, as methane is currently present in very low concentrations, and thus at a much steeper part of the radiative forcing curve.
The most commonly quoted value we have seen for the instantaneous global warming potential of methane (instantaneous GWP, or GWP0) is 120x. In other words 1 gram of methane has a warming impact of 120 grams of CO2-equivalent. Although the 20, 50 and 100-year warming impacts are lower (see below).
What formula links methane to radiative forcing?
Our energy-climate model is linked below. It contains the maths and the workings linking methane to radiative forcing. It is based on a formula suggested in the past by the IPCC:
Radiative Forcing from Methane (in W/m2) = Alpha x Methane Concentration (in ppb) ^ 0.5 – Small Adjustment Factor for Methane-N2O interaction. Alpha is suggested at 0.036 in the IPCC’s AR5 models, and the adjustment factor for methane-N2O interactions can be ignored if you are seeking an approximation.
This is the formula that we have used in our chart below (more or less). As usual, we can multiply the radiative forcing by a ‘gamma factor’ which calculates global temperature changes from radiative forcing changes. We have seen the IPCC discuss a gamma factor of 0.5, i.e., 1 W/m2 of incremental radiative forcing x 0.5ยบC/[W/m2] gamma factor yields 0.5ยบC of temperature increases. However, there are controversies over the correct values of alpha and gamma.
Interaction Effects: Controversies over Alpha Factors?
The alpha factor linking methane to radiative forcing is suggested at 0.036 in the IPCC’s AR3 – AR5 reports. Plugging 0.036 into our formula above would suggest that increasing methane from 720 ppb in pre-industrial times to 1,900 ppb today would have caused 0.52 W/m2 of incremental radiative forcing. In turn, this would be likely to raise global temperatures by 0.27ยบC.
Helpfully, this tallies with the values you might see in other well-known sources, such as the Wikipedia page on Radiative Forcing.
However, many technical papers, and even the IPCC’s AR5 report, have argued that alpha should be ‘scaled up’ to account for indirect effects and interaction effects.
Tropospheric Ozone. In the troposphere (up to 15km altitude), ozone is a ridiculously powerful greenhouse gas, quantified at around 1,000x more potent than CO2. It is postulated that the breakdown of atmospheric methane produces peroxyl radicals (ROO*, where R is a carbon-based molecule). In turn, these peroxyl radicals react with oxygen atoms in NOx pollutants, yielding O3. And thus methane is assumed to increase tropospheric ozone. Several authors, including the IPCC, have proposed to scale up alpha values by 20% – 80%, to reflect the warming impacts of this additional ozone.
Stratospheric Water Vapor. Water is a greenhouse gas, but it is usually present at relatively low concentrations in the stratosphere (12-50 km altitude). Water vapor prefers to remain in the troposphere, where it is warmer. However, when methane in the stratosphere decomposes, each CH4 molecule yields 2 H2O molecules, which may remain in the stratosphere. Several authors, including the IPCC, have proposed to scale up alpha values by around 15% to reflect the warming impacts of this additional water vapor in the stratosphere.
Short-wave radiation. Visible light has a wavelength of 400-700nm. Infra-red radiation has a wavelength of 700nm – 1mm and is the band that is mainly considered in radiative forcing calculations. However, recent research also notes that methane can absorb short-wave radiation, with wavelengths extended down to as little as 100-200nm. Some authors have suggested that the radiative forcing of methane could be around 25% higher than is stated in the IPCC (2013) assessment when short-wave radiation is considered. This impact is not currently in IPCC numbers.
Aerosol interactions. Recent research has also alleged that methane lowers the prevalence of climate-cooling aerosols in the atmosphere, and this may increase the warming impacts of CH4 by as much as 40%. This impact is not currently in IPCC numbers.
Hydrogen interactions. Even more recent research has drawn a link between methane and hydrogen GWPs, suggesting an effective GWP of 11x for H2, which is moderated by methane (note below).
N2O interactions. The IPCC formula for radiative forcing of methane suggests a small negative adjustment due to interaction effects with N2O, another greenhouse gas, which has been rising in atmospheric concentration (from 285ppb in 1765 to 320ppb today). The reason is that both N2O and CH4 seem to share an absorption peak at 7-8 microns. Hence it is necessary to avoid double-counting the increased absorption at this wavelength. The downwards adjustment due to this interaction effect is currently around 0.08 W/m2.
The overall impact of these interaction effects could be argued to at least double the instantaneous climate impacts of methane. On this more strict vilification of the methane molecule, rising atmospheric methane would already have caused at least a 1.0 W/m2 increase in radiative forcing, equivalent to 0.5ยบC of total temperature increases since 1750 due to methane alone.
Uncertainty is high which softens methane alarmism?
Our sense from reviewing technical papers is that uncertainty is much higher when trying to quantify the climate impacts of methane than when trying to quantify the climate impacts of CO2.
The first justification for this claim is a direct one. When discussing its alpha factors, the IPCC has itself acknowledged an “uncertainty level” of 10% for the scalars used in assessing the warming impacts of CO2. By contrast, it notes 14% uncertainty around the direct impacts of methane, 55% on the interaction with tropospheric ozone, 71% on the interaction with stratospheric water vapor. These are quite high uncertainty levels.
A second point is that methane degrades in the atmosphere, with an average estimated life of 11.2 years, as methane molecules react with hydroxyl radicals. This is why the IPCC has stated that methane has a 10-year GWP of 104.2x CO2, 20-year GWP of 84x CO2, 50-year GWP of 48x CO2 and a 100-year GWP of 28.5x CO2.
There is further uncertainty around the numbers, as methane that enters the atmosphere may not stay in the atmosphere. The lifetime of methane in additional sinks is estimated at 120 years for bacterial uptake in soils, 150-years for stratospheric loss and 200 years for chlorine loss mechanisms. And these sources and sinks are continuously exchanging methane with the atmosphere.
Next, you might have shared our sense, when reading about the interaction effects above, that the mechanisms were complex and vaguely specified. This is because they are. I am not saying this to be some kind of climate sceptic. I am simply observing that if you search google scholar for “methane, ozone, interaction, warming”, and then read the first 3-5 papers that come up, you will find yourself painfully aware of climate complexity. It would be helpful if the mechanisms could be spelled out more clearly. And without moralistic overtures about natural gas being inherently evil, which sometimes simply makes it sound as though a research paper has strayed away from the scientific ideal of objectivity.
Finally, the biggest reason to question the upper estimates of methane’s climate impact is that they do not match the data. There is little doubt that the Earth is warming. The latest data suggest 1.2-1.3C of total warming since pre-industrial times (chart below). Our best guesses, based on our very simple models point to 1.0ยบC of warming caused by CO2, 0.35ยบC caused by CH4 and around <0.2ยบC caused by other greenhouse gases. If you are a mathematical genius, you may have noticed that 1.0 + 0.35 + <0.2 adds up to 1.5C, which is more warming than has been observed. And this is not including any attribution for other factors, such as changing solar intensity or ocean currents. So this may all suggest that our alpha and gamma factors are, if anything, too high. In turn, this may mute the most alarmist fears over the stated alpha factors for methane being materially too low.
Conclusions for gas in the energy transition
How does methane increase global temperature? Of course we need to mitigate methane leaks as part of the energy transition, across agriculture, energy and landfills; using practical and economical methods to decarbonize the entire global energy system. Methane is causing around 20-30% of all the incremental radiative forcing, on the models that we have considered here. If atmospheric methane doubles again to 3,800 ppb, it will cause another 0.2-0.4ยบC of warming, as can be stress-tested in our model here.
However, we still believe that natural gas should grow, indeed it should grow 2.5x, as part of the world’s lowest cost roadmap to net zero. The reason is that while we are ramping renewables by over 5x, this is still not enough to offset all demand for fossil energy. And thus where fossil energy remains, pragmatically, over 15GTpa of global CO2 abatement can be achieved by displacing unchecked future coal consumption with gas instead.
Combusting natural gas emits 40-60% less CO2 than combusting coal, for the same amount of energy, which is the primary motivation for coal-to-gas switching (note below). But moreover, methane leaks into the atmosphere from the coal industry are actually higher than methane leaks from the gas industry, both on an absolute basis and per unit of energy, and this is based on objective data from the IEA (note below).