Researchers using computer simulations have calculated that Earth's oxygen-rich atmosphere will last for another one billion years before undergoing rapid deoxygenation, returning the planet to a state reminiscent of early Earth. The study highlights the temporary nature of planetary biosignatures. Meanwhile, scientists led by UC San Diego’s Scripps Institution of Oceanography are warning that oxygen is disappearing rapidly from oceans and freshwater systems today, potentially pushing the planet into an “unsafe space” with changes that could persist for centuries and may not be reversible within human lifetimes.
Modelling Earth's Atmospheric Future
For many years, discussions regarding the lifespan of Earth’s biosphere relied on scientific knowledge concerning the steady brightening of the Sun and the global carbonate-silicate geochemical cycle. To examine how the planet’s atmosphere will evolve, Kazumi Ozaki, Assistant Professor at Toho University and Christopher Reinhard, Associate Professor at Georgia Institute of Technology, created a computer model to simulate climate and biochemical processes. Their study was published in Nature Geoscience.
The theoretical framework driving such models typically points to a continuous decline in atmospheric carbon dioxide levels and long-term global warming on geological timescales. Researchers generally expect that the biosphere will terminate within two billion years due to a combination of overheating and carbon dioxide scarcity for photosynthesis. Consequently, atmospheric oxygen levels were anticipated to decline in the distant future, though the exact timeline remained unclear.
For many years, the lifespan of Earth's biosphere has been discussed based on scientific knowledge about the steady brightening of the Sun and global carbonate-silicate geochemical cycle,
says Ozaki.
Simulating Decades of Geological Time
Predicting planetary developments billions of years into the future involves substantial uncertainty. The results indicate that Earth’s oxygenated atmosphere will persist for approximately one billion years from the present.
If true, one can expect atmospheric O2 levels will also eventually decrease in the distant future. However, it remains unclear exactly when and how this will occur.
Following this window, rapid deoxygenation is projected to occur. The resulting atmosphere will closely mirror the conditions of early Earth prior to the Great Oxidation Event, which took place approximately 2.5 billion years ago and established breathable oxygen levels. Single-celled organisms are widely thought to be largely responsible for the Great Oxidation Event, a key epoch in the evolution of Earth.
Return to Anaerobic Life
When the projected atmospheric shift occurs, the planetary system will undergo drastic biochemical changes. The post-deoxygenation environment will feature elevated methane, low-levels of CO2, and the complete absence of an ozone layer.
This prospective state implies that Earth’s oxygen-rich atmosphere is not a permanent feature of the planet. According to the findings, oxygenated conditions may ultimately account for as little as 20 to 30 percent of Earth’s entire history.
Aquatic Deoxygenation and Planetary Boundaries
The new review examines aquatic deoxygenation, which refers to declining levels of dissolved oxygen in the ocean, coastal waters, rivers, lakes and streams. The researchers assessed how this growing problem interacts with the nine major Earth system processes included in the Planetary Boundaries framework. Introduced in 2009, the framework identifies environmental processes that are essential for maintaining a stable and resilient planet and tracks how human activity is pushing those systems beyond safe conditions.
The nine planetary boundaries are climate change, ocean acidification, biodiversity loss, atmospheric aerosol loading, stratospheric ozone depletion, freshwater change, land-use change, chemical pollution and biogeochemical flows including the nitrogen cycle. The researchers argue that dissolved oxygen levels should also be formally included. Human-caused warming, excessive nutrient pollution and changes in the movement and ventilation of deeper waters are the main forces driving aquatic deoxygenation. As oxygen levels fall, they can disrupt the biological and chemical processes that help regulate Earth’s climate.
The health and stability of our planet depends on the health and stability of aquatic ecosystems, which need oxygen to function normally,
said lead author Erica Ferrer, a Scripps Oceanography alumna and current postdoctoral scholar at UC Santa Barbara’s National Center for Ecological Analysis and Synthesis. This study is designed to elevate the profile of aquatic deoxygenation as a global threat and show that it does not operate in isolation.
The decline threatens organisms across aquatic food webs, from microscopic life to fish and sharks. Marine mammals can also suffer even though they breathe air at the surface, as oxygen loss can reduce or relocate their prey, damage habitats and alter the food webs they depend on.
Ferrer and Scripps biological oceanographer Lisa Levin, the study’s senior author, developed the idea for the review after attending COP25, the 2019 United Nations Climate Change Conference held in Madrid. They hope the findings will encourage researchers and policymakers to examine aquatic oxygen loss alongside climate change and pollution.
Cosmic Implications for Detecting Exoplanets
Beyond its local timeline, the research carries direct consequences for the search for life beyond the solar system. Astronomers searching for habitable worlds frequently look for biosignatures, which are atmospheric chemicals that can indicate biological activity.
Because Earth’s oxygenation is temporary rather than permanent, observations of distant exoplanets must account for the transient nature of atmospheric oxygen.