Methane Didn’t Save the Earth From Freezing Up, New Study Says

Scientists had since believed that methane, a potent greenhouse gas, has kept the Earth warm millions of years ago when the sun was dimmer. However, a new study suggests otherwise.

After simulating the ancient environment, the Alternative Earths team of the NASA Astrobiology Institute found that high amounts of sulfate and limited oxygen have kept methane levels low around 1.8 billion to 800 million years ago. Greenhouse gases such as methane played an important role for microscopic ocean dwellers during this time, as the sun was 10 to 15 percent dimmer than it is today, not enough heat to warm a planet. Earth, for instance, needed heat-trapping gases to keep its oceans from freezing up.

The long-held assumption was that methane has kept the Earth warm for most of its first 3.5 billion years of existence when oxygen was absent. This was how scientists made sense of the "Faint Young Sun Paradox." But according to Stephanie Olson, a graduate student at the University California, Riverside, a member of the Alternative Earths team and lead author of the study, methane could not have been responsible for that.

Low concentrations of oxygen during this period of a faint young sun thinned the layer of ozone that shields methane from photochemical destruction. Moreover, the researchers also estimated that high amount of sulfate in the ocean during this period was destructive to methane in two ways: it destroys methane directly and limits its production. These factors have severely limited methane to levels similar to what can be seen today, and according to the researchers, these levels are far too low to keep the entire Earth warm.

"If we detect methane on an exoplanet, it is one of our best candidates as a biosignature, and methane dominates many conversations in the search for life on Mars," Timothy Lyons, a professor of biogeochemistry at University of California, Riverside and co-author of the study, said in a statement. "Yet methane almost certainly would not have been detected by an alien civilization looking at our planet a billion years ago--despite the likelihood of its biological production over most of Earth history."

So which greenhouse gas could have kept the Earth from turning into a giant snowball and could explain the planet's climate history? This will be determined by the researchers in future studies. Meanwhile, the scientists suggest that by knowing the right combination of other warming agents, such as water vapor, nitrous oxide, carbon dioxide, will help assess the habitability of other Earth-like planets in the galaxy.

The researchers detailed their findings in a paper published in the Proceedings of the National Academy of Sciences (PNAS).