This article was originally published on The conversation. The publication contributed the article to Space.com’s Expert Voices: Op-Ed & Insights.
Matthew Warke, Research Fellow, School of Earth and Environmental Sciences, University of St Andrews
The earth will not be able to support and sustain life forever. Our oxygen-rich atmosphere may last just another billion years, according to a new study in Nature Geoscience.
As our sun ages, it becomes brighter, which means that the earth will receive more solar energy in the future. This increased energy will affect the surface of the planet and accelerate the weathering of silicate rocks such as basalt and granite. When these rocks weather, the greenhouse gas carbon dioxide is extracted from the atmosphere and trapped in carbonate minerals by chemical reactions. In theory, the earth should begin to cool down as carbon dioxide levels drop, but in about 2 billion years, this effect will be denied by the still bright sunshine.
Carbon dioxide, along with water, is one of the most important ingredients that plants need to perform photosynthesis. With declining carbon dioxide levels, less photosynthesis will occur and some species of plants may become completely extinct. Less photosynthesis means less oxygen production, and gradually the oxygen concentration in the Earth’s atmosphere will drop, creating a crisis for other forms of future life.
So, when will this happen? To find out, researchers from Japan and the United States used computer simulations to model the future evolution of the Earth’s carbon, oxygen, phosphorus, and sulfur cycles. They also considered climate change and how the earth’s surface (the crust, oceans and atmosphere) interact with the planet’s inside (the mantle).
They modeled two theoretical scenarios: an Earth-like planet with an active biosphere and a planet without an active biosphere. Interestingly, both scenarios yielded virtually the same results: oxygen levels began to drop drastically in the future for about 1 billion years. This finding suggests that the declining levels of carbon dioxide and plant photosynthesis do affect oxygen levels, but the effect of this process is secondary to long-term interactions between the mantle and surface environments. In short, it is the balance between the geochemistry of which rocks enter the mantle during subduction (see diagram below) and which gases are released from the mantle via volcanoes, which mostly influences how long the Earth’s atmosphere will remain oxygen rich.
The authors of the study conclude that our oxygen-rich atmosphere can only last about 1.08 billion years. To put this in context, oxygen only began to accumulate in the Earth’s atmosphere 2.5 billion years ago – during the Great Oxidation Event – and it is likely that oxygen levels have remained relatively low for most of the planet’s history, and only to almost modern levels did evolution of land plants rise about 400 million years ago.
Read more: Millions of years ago, the rise of oxygen in the Earth’s atmosphere caused deep freezing worldwide.
The end of oxygen would almost certainly be the end of the fact that the earth can support complex, aerobic forms of life. Although the details are discussed, and other environmental factors are discussed, scientists have long noted that the evolution and radiation of complex life on Earth are linked to periods of relative oxygen flux.
The authors of this study estimate that the total habitable life of the earth – before it loses its surface water – is about 7.2 billion years, but they also calculate that an oxygen-rich atmosphere may only occur for about 20% –30% of it. time.
Why does it matter? Imagine that we were aliens in another world looking for signs of life in heaven by searching for oxygen and ozone in the atmosphere of exoplanets. If our instruments move across Earth over 2 billion years from now, or 2 billion years ago, we can interpret a false negative – that such planets do not have a reliable ‘biosignature’, and we can continue our search .
The same problem is facing astronomers and planetary scientists today: what kind of exoplanets should we focus on, and what is a reliable biosignature of alien life? Liveability is not only a place around a star, but also a time in the evolution of a planet, and we must remain aware that we are limited to what we can currently see.
The future of our atmosphere has a strong resemblance to its distant past: low oxygen, rich in methane (if not carbon dioxide) with the possibility of organic haze. As the authors of the new study suggest, we will need to rethink the earth as an analogue about which gases to look for in exoplanet atmospheres and that we may need to rethink our interpretations of what these gases may indicate.
We need to better understand the history of the evolution of our own atmosphere over time and how the surface and inside of our planet evolved together. Only then will we be better able to determine if there is life in the radiance of other suns.
This article was published from The Conversation under a Creative Commons license. Read the original article.
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