For now, life is flourishing on our oxygen-rich planet, but Earth wasn’t always this way – and scientists predict that the atmosphere will revert to one rich in methane and low in oxygen in the future.
This is unlikely to happen for another billion years or so. But, according to a study from earlier this year, when the change comes, it will happen quickly.
This shift will return the planet to a state similar to what it was before the Great Oxidation Event (GOE) around 2.4 billion years ago.
Furthermore, the authors of the new study believe 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.
“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,” wrote the researchers in their published paper.
At that point, it will be the end of the road for humans and most other life forms that rely on oxygen to survive, so let’s hope we figure out how to leave the planet within the next billion years.
To arrive at their conclusions, the researchers ran detailed models of Earth’s biosphere, taking into account changes in the brightness of the Sun and the corresponding drop in carbon dioxide levels as the gas is broken down by increasing levels of heat. Less carbon dioxide means fewer photosynthesizing organisms like plants, which means less oxygen.
Scientists previously predicted that increased solar radiation would wipe out ocean waters in about 2 billion years, but the new model – based on an average of just under 400,000 simulations – predicts that a decrease in oxygen will kill off life first.
“The drop in oxygen is very, very extreme,” Earth scientist Chris Reinhard, from the Georgia Institute of Technology, told New Scientist earlier this year. “We’re talking around a million times less oxygen than there is today.”
The study is especially relevant today because of our search for habitable planets outside of our Solar System.
Scientists want to know what to look for in the massive amounts of data collected by these instruments as more powerful telescopes become available.
According to the scientists, we may need to look for other biosignatures besides oxygen to have the best chance of finding life. Their study is part of the NASA NExSS (Nexus for Exoplanet System Science) project, which is looking into the habitability of planets other than Earth.
According to calculations done by Reinhard and environmental scientist Kazumi Ozaki of Toho University in Japan, Earth’s oxygen-rich habitable history may end up lasting only 20-30 percent of the planet’s overall lifespan – and microbial life will continue to exist long after we are gone.
“The atmosphere after the great deoxygenation is characterized by an elevated methane, low-levels of CO2, and no ozone layer,” said Ozaki. “The Earth system will probably be a world of anaerobic life forms.”
The research has been published in Nature Geoscience.