The release of methane into the atmosphere is the second biggest cause of human-induced global warming after carbon dioxide. However, our understanding of the effects of this important gas remains incomplete. A major step forward has come from studying its impact not on Earth but in the atmospheres of Jupiter and Titan.
Atmospheric models can never be perfect, although they are improving all the time. In 2016, scientists recognized that estimates of future warming were too low because they ignored methane's effects at shorter wavelengths. Methane is known to absorb radiation at many wavelengths in the ultraviolet, but there are so many of these absorption bands that their strength hasn't even been measured for inclusion in spectroscopic databases.
Shortwave absorption means methane captures radiation emitted directly from the Sun, as well as the longer wavelength reflections from the Earth that cause the greenhouse effect. This previously neglected factor accounts for some 15 percent of methane’s impact, and therefore about 3 percent of global warming as a whole. This figure will grow if, as expected, we are slower to control methane emissions than other gasses.
Professor William Collins of the University of California, Berkeley was keen to refine the estimates of the shortwave effects further. However, it's hard to detect absorption from such a rare gas in Earth's atmosphere. Moreover, Collins notes in Science Advances, methane is a particularly difficult gas to study, with unusually complex spectroscopic lines, and its behavior in the atmosphere is known to differ from lab results.
Methane is at least 1,000 times as common in the atmospheres of the outer Solar System, however, so Collins and co-authors looked to observations from missions to other worlds to reduce the uncertainties in previous studies.
Collins reports that incorporating observations of radiation absorption from the atmospheres of Titan and Jupiter doesn't greatly change the most recent estimates of methane's overall climate effect. However, this extra data increases our confidence in existing models. It also allows us to understand the way the absorption of radiation methane varies with cloud conditions and the amount of light reflected back from the ground – when factors like these cause photons to be scattered more frequently, the opportunity for absorption by a methane molecule increases.
Applying this information allowed Collins to map the effects of methane compared to pre-Industrial levels, geographically, revealing a remarkable hotspot over the Sahara, where methane is having an exceptional impact.
The Trump administration has redirected NASA funding from observations of the Earth that might help us understand climate change into the exploration of other planets. However, since missions to Venus helped reveal the dangers of a runaway greenhouse effect, the things we learn about other worlds keep revealing inconvenient truths about our own.