Revised title: Previous Beliefs on Methane's Impact on Warming Earth may not be as Significant

Methane is a greenhouse gas that has a dual effect on the atmosphere. It heats the Earth's atmosphere 28 times more than carbon dioxide, but its absorption of the sun's radiation high in the atmosphere also changes cloud patterns, leading to a reduction in its overall warming effect of approximately 30%. The findings were reported by researchers in Nature Geoscience on March 16.

The concentration of methane in the atmosphere has increased by approximately 162% since preindustrial times, with humans being responsible for most of the methane entering the atmosphere. The largest sources of anthropogenic methane include fossil fuel use, livestock, rice farming, landfills, and biomass burning. Scientists are concerned that as warming triggers thaw of permafrost in the Arctic regions, this could also lead to increased methane emissions.

Greenhouse gases like methane affect the atmosphere most by absorbing longwave radiation emitted from the planet's surface. However, scientists are discovering that greenhouse gases, including methane, also absorb some of the sun's shortwave radiation. A recent study shows that methane's shortwave absorption has the opposite effect, cooling the atmosphere.

The study suggests that methane's shortwave absorbance affects clouds in different layers of the atmosphere, leading to fewer clouds in the upper layer and more in the lower troposphere. These thicker low-level clouds reflect more of the sun's shortwave radiation back out to space, reducing the amount of radiation that reaches Earth's surface and is converted into longwave radiation. Similarly, fewer upper-level clouds diminish their ability to absorb longwave radiation. Therefore, less of this radiation emitted by the Earth is captured in the atmosphere, and more of it escapes to space without contributing to climate change.

The researchers conducted the study using a computational model of Earth's climate. When they included methane's shortwave absorbance, methane's contribution to warming fell to around 0.16 degrees C, from an estimated 0.2 degrees Celsius caused by longwave absorbance alone. Methane also increases global precipitation, but the researchers found that including shortwave absorbance reduced methane's predicted precipitation effect from 0.3 percent to approximately 0.18 percent.

It will be important to include methane’s shortwave effects in future climate projections, says Daniel Feldman, an atmospheric scientist at the Lawrence Berkeley National Laboratory in California, who was not involved in the study. But he thinks that more work needs to be done to clarify those effects.

The new study analyzed methane’s shortwave impact using only one comprehensive model that included both the atmosphere and ocean, he says. “I would just like to see that sort of analysis done across multiple models,” increasing confidence in the results.