Average annual near-surface air temperature response to methane, decomposed into short-wave effects only. Credit: Robert Allen/UCR
Potent greenhouse gas impacts: somewhat lower than previously thought.
UC Riverside researchers found that methane not only traps heat in the atmosphere, but also creates cooling clouds that offset 30% of the heat. Methane’s absorption of shortwave energy counterintuitively causes a cooling effect and suppresses the increase in precipitation by 60%. This finding highlights the need to incorporate all known effects of greenhouse gases into climate models.
Most climate models don’t yet account for a new finding from the University of California, Riverside: Methane traps a lot of heat in Earth’s atmosphere, but also creates cooling clouds that offset 30 % heat.
Greenhouse gases like methane create a kind of blanket in the atmosphere, trapping heat from the Earth’s surface, called long-wave energy, and preventing it from radiating out to space. It makes the planet hotter.
“A blanket does not create heat unless it is electric. You feel warm because the blanket inhibits your body’s ability to send its heat out into the air. It’s the same concept,” explained Robert Allen, assistant professor of Earth sciences at UCR.
In addition to absorbing long wave energy, it turns out that methane also absorbs incoming energy from the sun, known as short wave energy. “It should warm the planet,” said Allen, who led the research project. “But counterintuitively, shortwave absorption encourages cloud changes that have a slight cooling effect.”
Mean annual near-surface air temperature response to methane, decomposed into (a) long-wave and short-wave effects; (b) long wave effects only; and (c) shortwave effects only. Credit: Robert Allen/UCR
This effect is detailed in the review
Methane changes this equation. 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,” Allen said. “Shortwave absorption softens the overall warming and rain-increasing effects but does not eradicate them at all.”
The research team discovered these findings by creating 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.
“It’s become a major concern,” said Xueying Zhao, UCR Earth and planetary sciences Ph.D. student and study co-author. “We need to better understand the effects all this methane will bring us by incorporating all known effects into our climate models.”
Kramer echoes the need for further study. “We’re good at measuring the concentration of greenhouse gases like methane in the atmosphere. Now the goal is to say with as much confidence as possible what those numbers mean to us. Work like this gets us toward that goal,” he said.
Reference: “Surface warming and wetting due to methane’s long-wave radiative effects muted by short-wave absorption” by Robert J. Allen, Xueying Zhao, Cynthia A. Randles, Ryan J. Kramer, Bjørn H. Samset and Christopher J. Smith, 16 March 2023, Nature Geoscience.
DOI: 10.1038/s41561-023-01144-z
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