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By Karina Shah

One billion years from now, Earth’s atmosphere will contain very little oxygen, making it uninhabitable for complex aerobic life.

Today, oxygen makes up around 21 per cent of Earth’s atmosphere. Its oxygen-rich nature is ideal for large and complex organisms, like humans, that require the gas to survive. But early in Earth’s history, oxygen levels were much lower – and they are likely to be low again in the distant future.

Kazumi Ozaki at Toho University in Funabashi, Japan, and Chris Reinhard at the Georgia Institute of Technology in Atlanta modelled Earth’s climatic, biological and geological systems to predict how atmospheric conditions on Earth will change.

The researchers say that Earth’s atmosphere will maintain high levels of oxygen for the next billion years before dramatically returning to low levels reminiscent of those that existed prior to what is known as the Great Oxidation Event of about 2.4 billion years ago.

“We find that the Earth’s oxygenated atmosphere will not be a permanent feature,” says Ozaki.

One central reason for the shift is that, as our sun ages, it will become hotter and release more energy. The researchers calculate that this will lead to a decrease in the amount of carbon dioxide in the atmosphere as CO2 absorbs heat and then breaks down.

Ozaki and Reinhard estimate that in a billion years, carbon dioxide levels will become so low that photosynthesising organisms – including plants – will be unable to survive and produce oxygen. The mass extinction of these photosynthetic organisms will be the primary cause of the huge reduction in oxygen.

“The drop in oxygen is very, very extreme – we’re talking around a million times less oxygen than there is today,” says Reinhard.

The researchers also estimate there will be a coinciding increase in methane to levels as high as 10,000 times the amount in the atmosphere today.

Once the changes in Earth’s atmosphere begin to occur, they will progress rapidly: the team’s calculations suggest that the atmosphere could lose its oxygen over the course of just 10,000 years or so.

“The biosphere cannot adapt to such a dramatic shift in environmental change,” says Ozaki.

Afterwards, life on Earth will be exclusively microbial, says Reinhard. “A world where many of the anaerobic and primitive bacteria are currently hiding in the shadows will, again, take over.”

Terrestrial life will cease to exist, as will aquatic life. The ozone layer – which is made up of oxygen – will deplete, exposing Earth and its oceans to high levels of ultraviolet light and heat from the burning sun.

The research was conducted as part of a NASA project into planet habitability, and the predictions have implications for searching for life on other planets. Oxygen-containing biosignatures are typically used to identify habitable planets.

“Oxygen, in its many forms, is a very important biosignature since it is intertwined with life so fully on Earth,” says Natalie Allen at Johns Hopkins University in Maryland. But the new prediction shows that oxygen presence is variable and may not be permanent on a habitable planet.

“It suggests that even for planets around other stars that are very similar to Earth, large amounts of oxygen may not be detected in their atmosphere, even if they can support, or have supported, complex life,” says Kevin Ortiz Ceballos at the University of Puerto Rico. Not detecting oxygen around planets doesn’t mean that they are uninhabitable, he says.

Ozaki and Reinhard suggest that other biosignatures could be used to search for alien life instead of oxygen. For instance, hydrocarbon hazes in a planet’s atmosphere might provide a more long-lasting signature of extraterrestrial life.

Journal reference: Nature Geoscience, DOI: 10.1038/s41561-021-00693-5