A hotter sun and changing geological processes will cause Earth’s atmosphere to lose 99% of its oxygen in about 1.1 billion years, according to new research putting a cap on the planet’s oxygen-breathing biosphere and providing new insights into the search for habitable exoplanets.

The atmospheric oxygen will quickly dissipate as Earth eventually becomes inhospitable to oxygen-producing plant life by heating up and losing atmospheric carbon dioxide, according to the paper published Monday in Nature Geoscience. Derived from climate and biogeochemical modeling, the NASA-supported research is the first to calculate when the end of Earth’s oxygen-rich atmosphere will occur.

Despite being essential for a vast swath of living creatures today, oxygen gas was almost nonexistent in the Earth’s atmosphere until a dramatic transition roughly 2.4 billion years ago known as the Great Oxidation Event. Photosynthesizing bacteria emerged and began producing oxygen as a waste product, eventually producing large enough amounts to change the air’s composition — and triggering the planet’s first known extinction event.

The influx of highly reactive oxygen was toxic to much of the existing anaerobic life and also created carbon dioxide by reacting with the highly concentrated methane in the atmosphere. Because carbon dioxide is a less potent greenhouse gas than methane, the change sent the world into a global ice age.

Kazumi Ozaki, the study’s lead author and a professor of environmental science at Toho University in Tokyo, uncovered a comparably jarring shift taking place in the far future. Searching for the end of Earth’s high-oxygen period, he created a model that captured interactions between the oceans, atmosphere, mantle, sun and biosphere, which includes all living things.

“The question about the future lifespan of oxygenated atmosphere is related to the future lifespan of the biosphere,” Ozaki said.

He ran more than 400,000 simulations starting at 600 million years ago and sampled the nearly 4,800 model runs that approximately aligned with modern atmospheric conditions.

The modeling showed that as the sun becomes 10% more luminous in the next billion years, the more intense sunlight will accelerate geological cycles and lead to a steep fall in carbon dioxide concentration in the atmosphere, gradually transforming it into carbonate rocks. The loss of the greenhouse gas will starve plants, which will no longer replenish oxygen through photosynthesis.

Christopher Reinhard, a professor of biogeochemistry at the Georgia Institute of Technology and the study’s other author, noted that the plant and oxygen declines would be accelerated by the harsh temperatures as well as the loss of the atmosphere’s ozone layer, which protects life from most of the harmful ultraviolet radiation in sunlight and would dissipate without enough oxygen in the air.

The loss of oxygen was a “robust” statistical result, Ozaki said, and is expected to occur rapidly on a geological time scale. The simulations predicted that oxygen levels in the atmosphere will fall from 10% of modern concentrations in approximately 1.05 billion years to 1% about 30 million years later.

What will be left is a “hazy” atmosphere with increased concentrations of methane, according to the research, resembling conditions found when the Earth was young. Ozaki said he is interested in further exploring the evolution of the environment in this post-oxygen period — which could only support life that looks very different than it does now.

“Our model is a first step to understand the long-term evolution of the whole planet,” Ozaki said, adding that these are "very interesting conclusions, not only for geochemists but for astronomers or astrobiologists who are looking for the sign of life in the universe.”

Scientists first measured an exoplanet’s atmosphere in 2001 and have even detected clouds around some of these planetary bodies outside the solar system. As methods improve, understanding the evolution of habitable planets such as Earth, including the time frame for deoxygenation, can provide better interpretations of atmosphere measurements, including the history and stability of a potentially life-bearing planet.

Reinhard expects a similar evolution to take place on other habitable planets and said he’d like to investigate how it may change depending on different planetary conditions. 

“We think it's ... something that's pretty basic to how habitable planets work, and it’s a behavior that should probably be pretty widespread, both on Earth and planets beyond the solar system,” Reinhard said.

The scientists are part of the NASA Nexus for Exoplanet System Science, a research network with the goal of studying the habitability of planets beyond the solar system, and their research was funded by the space agency's Exoplanet program.

Both researchers emphasized that their findings — which predict low carbon dioxide levels in a time of high temperatures  — do not contradict the science of human-caused climate change, in which high carbon dioxide levels are causing rising temperatures. The greenhouse gas is currently driving global warming by trapping heat within the atmosphere, but although it will trap much less heat in the far future as its concentration plummets, the resulting cooling effect will be countered by the sun's rising intensity, they said. Additionally, sunlight will become more intense on the scale of many millions of years, but that trend has been negligible across the last few centuries as climate change emerged.

The article, “The future lifespan of Earth’s oxygenated atmosphere,” was published March 1 in Nature Geoscience. The authors of the study were Kazumi Ozaki, Toho University and NASA Nexus for Exoplanet System Science; and Christopher Reinhard, Georgia Institute of Technology, NASA Nexus for Exoplanet System Science and NASA Astrobiology Institute. The lead author was Kazumi Okazi.