Don't worry; it's not expected to happen for another billion years or so. But when Earth's atmosphere does indeed revert back to one that's rich in methane and low in oxygen, it's going to happen fairly rapidly, according to a 2021 study co-authored by Chris Reinhard, associate professor in the School of Earth and Atmospheric Sciences. This shift will take the planet back to something like the state it was in before what's known as the Great Oxidation Event (GOE) around 2.4 billion years ago. What's more, the researchers behind the study say that atmospheric oxygen is unlikely to be a permanent feature of habitable worlds in general, which has implications for our efforts to detect signs of life further out in the Universe.  (This study was also covered at Financial Express, MSN, ScienceAlert, India Today and WION.)