Greenhouse gases such as methane absorb primarily long-wave radiation, trapping heat from Earth’s surface and preventing it from radiating out into space—thus making the planet hotter. However, greenhouse gases also absorb short-wave radiation incoming from the sun. A new study by researchers from the University of California, Riverside and colleagues in the US and Europe has now found that methane short-wave absorption counteracts ~30% of the surface warming associated with its long-wave radiative effects.

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. AN open-access paper on their work is published in Nature Geoscience.

Although greenhouse gases absorb primarily long-wave radiation, they also absorb short-wave radiation. Recent 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. Here we present a systematic, unambiguous analysis using one model and separate simulations with and without methane short-wave absorption.

… The methane 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. The cloud responses, in turn, are related to the profile of atmospheric solar heating and corresponding changes in temperature and relative humidity.

Despite our findings, methane remains a potent contributor to global warming, and efforts to reduce methane emissions are vital for keeping global warming well below 2 °C above preindustrial values.

—Allen et al.

Annual mean near-surface air temperature response to methane, decomposed into (a) longwave and shortwave effects; (b) longwave effects only; and (c) shortwave effects only. (Robert Allen/UCR)

Both types of energy—longwave (from Earth) and shortwave (from sun)—escape from the atmosphere more than they are absorbed into it. The atmosphere needs compensation for the escaped energy, which it gets from heat created as water vapor condenses into rain, snow, sleet, or hail.

Essentially, precipitation acts as a heat source, making sure the atmosphere maintains a balance of energy.

—co-author Ryan Kramer, a researcher at NASA Goddard Space Flight Center and the University of Maryland, Baltimore County

By holding on to energy from the sun, methane is introducing heat the atmosphere no longer needs to get from precipitation.

Additionally, methane shortwave absorption decreases the amount of solar radiation reaching Earth’s surface. This in turn reduces the amount of water that evaporates. Generally, precipitation and evaporation are equal, so a decrease in evaporation leads to a decrease in precipitation.

This has implications for understanding in more detail how methane and perhaps other greenhouses gases can impact the climate system. Shortwave absorption softens the overall warming and rain-increasing effects but does not eradicate them at all.

—Robert Allen, corresponding and lead author

The research team created detailed computer models simulating both longwave and shortwave methane effects. Going forward, they would like to conduct additional experiments to learn how different concentrations of methane would impact the climate. 

Scientific interest in methane has increased in recent years as levels of emissions have increased. Much comes from industrial sources, as well as from agricultural activities and landfill. Methane emissions are also likely to increase as frozen ground underlying the Arctic begins to thaw.

Resources

• Allen, R.J., Zhao, X., Randles, C.A. et al. (2023) “Surface warming and wetting due to methane’s long-wave radiative effects muted by short-wave absorption.” Nat. Geosci. doi: 10.1038/s41561-023-01144-z