For now, life on Earth thrives in an atmosphere rich with oxygen, but a stark prediction from scientists suggests this will not always be the case. According to research published in 2021, Earth’s atmosphere is expected to revert to a state similar to that before the Great Oxidation Event (GOE), characterized by high methane and low oxygen levels. This transformation, although projected to occur in about a billion years, could happen swiftly once it begins.
The announcement comes as scientists continue to explore the long-term viability of Earth’s biosphere. Environmental scientist Kazumi Ozaki from Toho University in Japan, who co-authored the study, noted, “For many years, the lifespan of Earth’s biosphere has been discussed based on scientific knowledge about the steadily brightening of the sun and global carbonate-silicate geochemical cycle.” This framework suggests a gradual decline in atmospheric carbon dioxide levels and an increase in global warming over geological timescales.
The Science Behind the Prediction
The researchers utilized sophisticated models to simulate Earth’s biosphere, taking into account the increasing brightness of the Sun and the resultant decrease in carbon dioxide as it is broken down by heat. This reduction in carbon dioxide would lead to fewer photosynthesizing organisms, such as plants, and consequently, less oxygen production.
Chris Reinhard, an Earth scientist from the Georgia Institute of Technology, emphasized the severity of the predicted oxygen drop, stating, “The drop in oxygen is very, very extreme. We’re talking around a million times less oxygen than there is today.”
“The model projects that a deoxygenation of the atmosphere, with atmospheric O2 dropping sharply to levels reminiscent of the Archaean Earth, will most probably be triggered before the inception of moist greenhouse conditions in Earth’s climate system and before the extensive loss of surface water from the atmosphere,” the team explained in their paper.
Implications for Life and Astrobiology
The implications of this research extend beyond Earth’s future; it also affects the search for extraterrestrial life. As more powerful telescopes are developed, scientists are keen to refine the criteria for detecting habitable planets. The study suggests that relying solely on oxygen as a biosignature may not be sufficient, as habitable periods with oxygen-rich atmospheres might be relatively short-lived on a planetary scale.
This research is part of the NASA NExSS (Nexus for Exoplanet System Science) project, which aims to understand the habitability of planets beyond our Solar System. The findings indicate that Earth’s oxygen-rich period could constitute merely 20-30 percent of the planet’s total lifespan, with microbial life potentially persisting long after more complex life forms have vanished.
Looking Ahead
The study’s authors, including Ozaki and Reinhard, underscore the importance of considering alternative biosignatures when searching for life beyond Earth. As Ozaki remarked, “The atmosphere after the great deoxygenation is characterized by an elevated methane, low levels of CO2, and no ozone layer. The Earth system will probably be a world of anaerobic life forms.”
While the prospect of a deoxygenated Earth may seem distant, the research highlights the dynamic nature of planetary atmospheres and the need for a broad perspective in the quest to understand life in the universe. As humanity continues to explore space, these insights will be crucial in identifying and interpreting potential signs of life on other worlds.
The research was published in Nature Geoscience, and an earlier version of this article appeared in March 2021.
