Observations of Jupiter and Saturn’s largest moon Titan have been used to boost our understanding of how methane could contribute to future climate change. The study was done by William Collins of the Lawrence Berkeley Laboratory in the US and colleagues. They have identified several different scenarios in which methane heats the Earth by absorbing sunlight. The researchers say that their findings should be included in future reports by the Intergovernmental Panel on Climate Change (IPCC).

Carbon dioxide, methane and some other molecules in the atmosphere act as greenhouse gases by absorbing infrared radiation coming off the Earth and re-emitting in a downwards direction. This prevents significant amounts of heat from being radiated into space. Methane is known to be a potent greenhouse gas and while there is much less of it in the atmosphere than carbon dioxide, methane levels have been rising steadily for decades.

As well as absorbing infrared radiation from the Earth, methane also absorbs some shorter-wavelength solar radiation before it reaches Earth’s surface. “Think of the energy flow in the climate system like an economy: you have an input which is sunlight and an output which is heat,” explains Collins. “Methane is warming the climate system in two ways: first by absorbing heat and secondly by absorbing sunlight.”

Future mitigation models

Collins says he first identified the effect of methane’s shortwave absorption on Earth’s climate in 2006. Shortly afterwards, the physics was incorporated into the detailed IPCC models of Earth’s present and future climate. It was omitted, however, from what Collins describes as “the handy, back-of-the-envelope formulas” used to model various scenarios to mitigate climate change. In 2016, however, climate scientist Keith Shine of the University of Reading in the UK and colleagues showed that these shortwave effects are significantly stronger than previously thought, concluding they needed to be incorporated into future mitigation models.

There was a catch, however, because many of methane’s short wavelength absorption lines have never been resolved in laboratory studies. “If you look at the outer planets with very methane-rich atmospheres, it is obvious that methane is absorbing sunlight in the visible all the way out to violet,” says Collins. “All that absorption has been omitted from climate models to date. The first thing we asked was, is that omission important?”

I realized that we could use Jupiter and [Saturn’s largest moon] Titan as natural laboratories William Collins

Collins – who trained as an astronomer – knew astronomical data could provide the answer. “I realized that we could use Jupiter and [Saturn’s largest moon] Titan as natural laboratories for the absorption,” he explains. “If you find a planet that’s incredibly rich in methane, you can have the planet pass between a satellite and the Sun and watch the transmission from the Sun to the satellite change as you pass through different layers in the atmosphere.” This allowed the researchers to deduce upper limits for the absorption of methane at poorly-studied wavelengths and conclude that, at the relatively low concentrations in Earth’s atmosphere, it could be ignored: “The laboratory measurements we currently have are good enough for looking at climate change on Earth,” says Collins.

Newfound confidence

With this newfound confidence, the researchers then modelled how methane’s absorption on Earth varies with the seasons, between years and at different locations. They reached several intriguing conclusions. High-altitude clouds, for example, reduce methane’s net absorption by reflecting light back into space before it reaches the methane. Conversely, low altitude clouds increase absorption by methane by reflecting sunlight back up into the methane, giving it a second chance to be absorbed. Similarly, they concluded, methane has a particularly strong effect over desert regions such as the Sahara, where the ground tends to reflect more light. Overall, the researchers conclude that their findings are robust enough to be incorporated into the IPCC Sixth Assessment Report, due in 2021.

Shine, who was not involved in the current research, agrees that the work is significant: “Most calculations of the greenhouse effects of gases have tended to ignore the effects of solar radiation directly absorbed by the gases,” he says. “It’s only in the last couple of years that they’ve been properly quantified and shown to be quite a significant contributor. This [research] goes beyond what was done before and shows the geographical distribution of the extra heating due to methane: it’s a very thorough and useful.” The astronomical observations are “a real novelty”, he says, “it’s just a slight shame that the result of that novelty wasn’t to change the results by a huge amount”.

The research is described in Science Advances.