Surprising: Methane cools even as it heats

Impacts of potent greenhouse gas: a bit lower than previously thought.

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Although greenhouse gases absorb primarily longwave radiation, they also absorb shortwave radiation. Recent studies have highlighted the importance of methane shortwave 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.

In a new study, scientists from UC Riverside discovered that Methane traps a great deal of heat in Earth’s atmosphere but also creates cooling clouds that offset 30% of the heat.

Longwave energy—a type of heat from the Earth’s surface—is trapped in the atmosphere by greenhouse gases like Methane, creating a blanket-like effect that prevents the heat from radiating into space. The planet becomes warmer as a result. Methane appears to absorb incoming solar energy, known as shortwave energy, in addition to longwave energy.

Robert Allen, UCR assistant professor of Earth sciences, said, “This should warm the planet. But counterintuitively, the shortwave absorption encourages changes in clouds with a slight cooling effect.”

Even though Methane often causes an increase in precipitation, this rise is 60% decreased when shortwave energy absorption is considered.

Longwave radiation from the Earth and shortwave radiation from the sun escape from the atmosphere more so than they are taken in by it. The heat produced when water vapor condenses into rain, snow, sleet, or hail provides the atmosphere with the compensation it needs for the energy that was ejected.

Importantly, precipitation acts as a heat source, ensuring the atmosphere maintains energy balance. But, Methane changes this equation by holding energy from the sun and introducing heat the atmosphere no longer needs to get from precipitation.

Moreover, the amount of solar radiation that reaches the surface of the Earth is reduced by Methane’s shortwave absorption. In turn, this lowers the amount of water evaporating. Evaporation and precipitation are typically equal; hence a decline in evaporation causes a decline in precipitation.

Allen said, “This has implications for understanding in more detail how Methane and perhaps other greenhouse gases can impact the climate system. Shortwave absorption softens the warming and rain-increasing effects but does not eradicate them.”

To arrive at these conclusions, the research team made thorough computer simulations of both longwave and shortwave Methane impacts. They intend to conduct more research to discover the effects of various methane concentrations on the climate.

Xueying Zhao, UCR Earth and planetary sciences Ph.D. student and study co-author, said“It’s become a major concern. We need to understand better the effects all this Methane will bring us by incorporating all known effects into our climate models.”

Journal Reference:

  1. Allen, R.J., Zhao, X., Randles, C.A. et al. Surface warming and wetting due to Methane’s longwave radiative effects muted by shortwave absorption. Nat. Geosci. (2023). DOI: 10.1038/s41561-023-01144-z
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