Methane is a greenhouse gas with dual personalities. It heats Earth’s atmosphere 28 times as potently as carbon dioxide, gram for gram. But its absorption of the sun’s radiation high in the atmosphere also alters cloud patterns — casting a bit of shadow on its warming effect.

So rather than adding even more thermal energy to the atmosphere, as previously thought, methane’s solar absorption sets off a cascade of events that reduces its overall warming effect by about 30 percent, researchers report March 16 in Nature Geoscience.

“These are really interesting and important results,” says Rachael Byrom, a climate scientist at the CICERO Center for International Climate Research in Oslo who wasn’t involved in the new study. Nonetheless, she says, “methane still remains a really key gas that we need to target in emissions reductions.”

Humans are responsible for most of the methane entering the atmosphere, where it worsens global warming. Concentrations of the potent greenhouse gas have risen about 162 percent since preindustrial times, according to the U.S. National Oceanic and Atmospheric Administration.

The largest sources of anthropogenic methane include fossil fuel use, livestock, rice farming, landfills and biomass burning (SN: 9/29/22; SN: 7/14/20). Scientists fear that as warming triggers thaw of permafrost in the Arctic regions, this could also lead to increased methane emissions, as microbes in the soil consume dead plant material and release the gas (SN: 9/25/19).

Rising methane

The concentration of methane (CH₄) in the atmosphere, measured in parts per billion, continues to increase. The gas absorbs radiation, worsening global warming.

Global monthly average of atmospheric methane, 1983–2021

Greenhouse gases like methane exert their strongest effects by absorbing infrared “longwave” radiation emitted from the planet’s surface. Earth emits this longwave radiation when it is struck by “shortwave” radiation coming directly from the sun. Most studies of greenhouse gases focus on longwave absorption.

But scientists are learning that greenhouse gases, including methane, also absorb some of the sun’s shortwave radiation. Recent estimates suggested that methane might contribute up to 15 percent more thermal energy to the atmosphere than previously thought, due to this additional shortwave absorption.

However, the new study reveals that methane’s shortwave absorption has the opposite effect. This finding is based on a detailed analysis of the gas’s absorption at various wavelengths.

The result is “counterintuitive,” says climate scientist Robert Allen of the University of California, Riverside. It happens because of the way that methane’s shortwave absorbance affects clouds in different layers of the atmosphere, Allen and colleagues’ sim