In a billion years, a lack of oxygen will wipe out life on Earth

Matthew Warke, University of St Andrews

Earth will not be able to sustain and sustain life forever. Our oxygen-rich atmosphere may only last a billion years, according to a new study from Nature Geoscience.

As our Sun ages, it becomes brighter, which means that in the future the Earth will receive more solar energy. This increased energy will affect the surface of the planet, accelerating the weathering of silicate rocks such as basalt and granite. When these rocks survive, the greenhouse gas, carbon dioxide, is extracted from the atmosphere and through chemical reactions locked up in carbonate minerals. In theory, the Earth should start to cool as carbon dioxide levels drop, but in about 2 billion years this effect will be canceled out by the ever-increasing glare from the Sun.

Carbon dioxide, along with water, is one of the key ingredients that plants need for photosynthesis. With the drop in carbon dioxide levels, less photosynthesis will occur and some types of plants may disappear altogether. Less photosynthesis means less oxygen production, and gradually oxygen concentrations in Earth’s atmosphere will drop, creating a crisis for other future life forms.

So when will this happen? To find out, Japanese and American researchers used computer simulations to model the future evolution of carbon, oxygen, phosphorus and sulfur cycles on the Earth’s surface. They also looked at climate change and how the Earth’s surface (crust, oceans, and atmosphere) interacts with the planet’s interior (mantle).

They modeled two theoretical scenarios: an Earth-like planet with an active biosphere and a planet without an active biosphere. Interestingly, the two scenarios produced broadly similar results: oxygen levels began to drop drastically about 1 billion years into the future. This finding suggests that while declining carbon dioxide levels and plant photosynthesis affect oxygen levels, the effect of this process is secondary to long-term interactions between mantle and surface environments. In short, it is the balance between the geochemistry of the rocks that enter the mantle during subduction (see diagram below) and the gases emitted by the mantle via volcanoes, which appears to primarily affect the length of time that the Earth’s atmosphere will remain rich in oxygen.

Scheme of subduction.
Subduction is the process of rocks sinking into the Inner Earth. But these rocks can carry some gases with them. stihii / shutterstock

The study’s authors conclude that our oxygen-rich atmosphere can only last about 1.08 billion years longer. To put this in context, oxygen only began to accumulate in Earth’s atmosphere 2.5 billion years ago – during the Great Oxidation Event – and it is likely that levels oxygen levels have remained fairly low for most of the planet’s history, only increasing near modern levels after the evolution of land plants about 400 million years ago.

The end of oxygen would almost certainly mark the end of Earth’s ability to support complex, breathing life forms. Although details are debated and other environmental factors are at play, scientists have long noted that the evolution and radiation of complex life on Earth appears to be linked to periods of relative oxygen abundance.

The authors of this study estimate that the total habitable lifespan of the Earth – before it loses its surface waters – is around 7.2 billion years, but they also calculate that an atmosphere rich in oxygen can only be present for about 20-30% of it. time.

Why is this important? Imagine we are aliens in another world scanning the skies for signs of life looking for oxygen and ozone in the atmosphere of exoplanets. If our instruments passed over Earth in 2 billion years, or 2 billion years ago, we might interpret a false negative – that such planets did not have a reliable “biosignature” – and continue our research.

Planets in space.
Some exoplanets may look like Earth now, but they won’t in the future. Jurik Peter / shutterstock

The same problem arises for astronomers and planetologists today: what kind of exoplanets should we target and what is a reliable biosignature of extraterrestrial life? Habitability is not just a place around a star, but a time in the evolution of a planet, and we must be aware that we are limited to what we can see at this time.

The future of our atmosphere strongly resembles its distant past: poor in oxygen, rich in methane (if not in carbon dioxide) with the possibility of organic mists. As the authors of the new study suggest, by using Earth as an analogue, we may need to think more broadly about what gases to look for in exoplanet atmospheres and we may need to rethink our interpretations of what these gases are. can indicate.

We need to better understand the history of how our own atmosphere has evolved over time and how the surface and interior of our planet have evolved together. Only then will we be in a better position to determine if there is life under the reflections of other suns.

Matthew Warke, Research Fellow, School of Earth and Environmental Sciences, University of St Andrews

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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