How Long Will Earth's Oxygen-Rich Atmosphere Last? Scientists Say ~1 Billion Years

How Long Will Earth's Oxygen-Rich Atmosphere Last?

Summary: New simulations suggest Earth's oxygenated atmosphere could collapse in roughly 1 billion years if long-term processes reduce atmospheric CO2 below levels that sustain photosynthesis. This would not be an immediate threat to humanity, but it would transform surface ecosystems and favor oxygen-free microbes.

Oxygen is essential for most complex life on Earth, yet it is not the atmosphere's most abundant gas—nitrogen holds that title at about 78%, while oxygen makes up roughly 20%. Recent research using large-scale computer models indicates those oxygen levels may decline dramatically over geological time, potentially leaving the planet effectively oxygen-free in about 1 billion years.

The study and its conclusions

Scientists Kazumi Ozaki (Toho University) and Christopher Reinhard (Georgia Tech) developed a model of Earth’s long-term climate and biogeochemical cycles and ran it nearly 400,000 times to explore possible futures. Their results, published in Nature Geoscience in 2021, show plausible pathways in which atmospheric oxygen falls by orders of magnitude—up to a million-fold—over the next billion years under certain conditions.

Why this could happen

The main driver is the Sun’s gradual brightening. Astronomers estimate the Sun is roughly halfway through its life; in about a billion years it will be ≈10% brighter. Increased solar output will warm the planet and accelerate processes that reduce atmospheric carbon dioxide. Long-term geochemical feedbacks—especially rock weathering and carbon cycling—can draw down CO2 to concentrations too low for efficient photosynthesis.

Photosynthesis is the primary source of free oxygen in the atmosphere. If CO2 levels fall below the threshold needed for most plants and phytoplankton to carry out photosynthesis effectively, oxygen production would collapse while oxygen sinks (chemical reactions that remove O2) continue. That imbalance could drive atmospheric oxygen down to levels incompatible with air-breathing animals.

What would Earth look like?

To understand that future, it helps to look to the deep past. When Earth formed about 4.5 billion years ago, the atmosphere was dominated by methane, CO2 and water vapor and contained almost no free oxygen. Life began in oxygen-poor oceans; anaerobic microbes—organisms that do not require oxygen—were the planet’s earliest inhabitants. Cyanobacteria evolved photosynthesis roughly 2.7 billion years ago and gradually oxygenated the atmosphere, enabling complex aerobic life.

If the oxygen collapse predicted by the models occurs, surface ecosystems dominated by oxygen-breathing animals would likely disappear, and anaerobic microbes—many of which already persist in oxygen-free environments today—could become the dominant lifeforms once again. However, the timing and magnitude of these changes are model-dependent and involve significant uncertainties.

Note on uncertainty: These conclusions come from simulations that explore many scenarios. They illustrate plausible long-term futures driven by stellar evolution and Earth system feedbacks, not an immediate or inevitable timeline for human societies.