Study indicates reduced warming effect of methane

A study has shown that the short-wave interactions between methane and the atmosphere reduce its warming impact by around 30%.

The study (Surface warming and wetting due to methane’s long-wave radiative effects muted by short-wave absorption), which appeared in Nature Geoscience, said that “Although greenhouse gases absorb mostly long-wave radiation, they also absorb short-wave radiation.” This reduces methane warming effect.

“Many studies have highlighted the importance of methane short-wave absorption, which enhances its stratospherically adjusted radiative forcing by up to ~ 15%. The corresponding climate impacts, however, have been only indirectly evaluated and thus remain largely unquantified.”

They found that methane absorbs short-wave radiation from the sun, which counteracts methane’s surface warming effect.

Methane’s ability to absorb short-wave radiation also affects clouds, which creates cooling.

“Using targeted climate model simulations,” the scientists say, “we have shown that methane short wave absorption and the associated adjustments act to reduce its ERF [effective radiative forcing] by about 20% and mute its warming and wetting effects in coupled simulations by up to 30% and 60%, respectively.”

Therefore methane short-wave absorption counteracts ~30% of the surface warming associated with its long-wave radiative effects – and an even larger impact occurs for precipitation, as methane short-wave absorption offsets ~60% of the precipitation increase relative to its long-wave radiative effects.

“The short-wave-induced cooling is due largely to cloud rapid adjustments, including increased low-level clouds, which enhance the reflection of incoming short-wave radiation, and decreased high-level clouds, which enhance outgoing long-wave radiation. This leads to corresponding changes in temperature and relative humidity.”

This paper adds to our emerging understanding of methane’s role in global warming.

And in their recent, Sixth Assessment Report, the IPCC accepted that the common metric for assessing global warming potential overstates the impact of stable ongoing emissions, by short-lived gases such as methane, by a factor of 3 to 4.

Professor Dave Frame, who was part of the Oxford Martin research team that developed GWP*, said on Twitter “[This] looks a significant result, if it holds up in other models/future studies. To me it’s all the more reason to treat the gases separately.”

The original paper, Surface warming and wetting due to methane’s long-wave radiative effects muted by short-wave absorption, can be found here

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