Study shows greenhouse gas methane both heats and cools the Earth

LNew Delhi: A "surprising" new research has discovered that although the powerful greenhouse gas methane traps a lot of heat in the atmosphere, it also produces cooling clouds that counteract 30% of the heat.

Longwave energy, a type of heat from the Earth's surface that cannot radiate into space, is trapped in the atmosphere by greenhouse gases like methane. The world gets hotter as a result.

The study, published in the journal Nature Geoscience, found that in addition to absorbing longwave energy, methane also absorbs incoming energy from the Sun, known as shortwave energy.

"This should warm the planet. But counterintuitively, the shortwave absorption encourages changes in clouds that have a slight cooling effect," said Robert Allen, an assistant professor at the University of California, Riverside (UCR) in the US, who led the study.

Despite the findings, the researchers noted that methane remains a potent contributor to global warming, and efforts to reduce methane emissions are vital for keeping global warming well below 2 degrees Celsius above preindustrial values.

Though methane generally increases the amount of precipitation, accounting for the absorption of shortwave energy suppresses that increase by 60 per cent, the researchers said.

Both types of energy -- longwave (from Earth) and shortwave (from Sun) -- escape from the atmosphere more than they are absorbed into it, they said.

The atmosphere needs compensation for the escaped energy, which it gets from the heat created as water vapour condenses into rain, snow, sleet, or hail, according to the researchers.

"Essentially, precipitation acts as a heat source, making sure the atmosphere maintains a balance of energy," said study co-author Ryan Kramer, a researcher at NASA Goddard Space Flight Center and the University of Maryland, US.

The researchers found that 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.

They explained that 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, according to the researchers.

"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 researcher said.

The team discovered these findings by creating detailed computer models simulating both longwave and shortwave methane effects.

Going forward, the researchers would like to conduct additional experiments to learn how different concentrations of methane would impact the climate.

They noted that methane emissions are also likely to increase as frozen ground underlying the Arctic begins to thaw.

"It's become a major concern. We need to better understand the effects all this methane will bring us by incorporating all known effects into our climate models," said study co-author Xueying Zhao, from UCR.

With PTI inputs