For now, complex life flourishes on our planet thanks to its supply of oxygen. But Earth’s atmosphere wasn’t always like it is today, and scientists predict that in the future, it will revert back to one that’s rich in methane and low in oxygen.
Breathe easy though. This won’t happen for probably another billion years or so. But when the change comes, it’s going to happen fairly rapidly, according to research published in 2021.
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.
“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,” environmental scientist Kazumi Ozaki from Toho University in Japan said when the study was published.
“One of the corollaries of such a theoretical framework is a continuous decline in atmospheric CO2 levels and global warming on geological timescales.”
The researchers 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.
“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.
At that point it’ll be the end of the road for human beings and most other life forms that rely on oxygen to get through the day, so let’s hope we figure out how to get off the planet at some point within the next billion years.
To reach their conclusions, the researchers ran detailed models of Earth’s biosphere, factoring in changes in the brightness of the Sun and the corresponding drop in carbon dioxide levels, as the gas gets broken down by increasing levels of heat.
Less carbon dioxide means fewer photosynthesizing organisms such as plants, which would result in less oxygen.
Scientists have previously predicted that increased radiation from the Sun would wipe ocean waters off the face of our planet within about 2 billion years, but the model here – based on an average of just under 400,000 simulations – says the reduction in oxygen is going to 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. “We’re talking around a million times less oxygen than there is today.”
What makes the study particularly relevant to the present day is our search for habitable planets outside of the Solar System.
Increasingly powerful telescopes are coming online, and scientists want to be able to know what they should be looking for in the reams of data these instruments are collecting.
It’s possible that we need to be hunting for other biosignatures besides oxygen to have the best chance of spotting life, the researchers say. Their study was part of the NASA NExSS (Nexus for Exoplanet System Science) project, which is investigating the habitability of planets other than our own.
According to the calculations run by Ozaki and Reinhard, the oxygen-rich habitable history of Earth could end up lasting for just 20-30 percent of the planet’s lifespan as a whole – and microbial life will carry on existing 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 was published in Nature Geoscience.
An earlier version of this article was published in March 2021.