16th March 2021 Earth's oxygen will last another billion years A new study calculates how long Earth's oxygen-rich atmosphere will persist, with a most likely timeframe of 1.08 billion years until the planet is rendered uninhabitable for most life.
Today, Earth is a highly oxygenated planet – from the atmosphere, down to the lowest depths of the oceans, it demonstrates all the hallmarks of an active photosynthetic biosphere. However, the long-term timeline for Earth's atmosphere remains uncertain, particularly for the very distant future. Solving this question has great ramifications not only for the future of Earth's biosphere but for the search for life on Earth-like exoplanets beyond our Solar System. Most scientists agree that our Sun will expand into a red giant about five billion years from now. Long before it reaches that stage, however, its ballooning size will create hellish conditions on Earth. Among the many changes to our home planet will be a gradual decline in oxygen levels. Exactly when and how this occurs has been the subject of much debate. A new study, published this month in Nature Geoscience, tackles this problem using a numerical model of biogeochemistry and climate to reveal that the future lifespan of Earth's oxygen-rich atmosphere is just over a billion years. "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 Kazumi Ozaki, Assistant Professor at Toho University, Japan. "One of the corollaries of such a theoretical framework is a continuous decline in atmospheric CO2 levels and global warming on geological timescales. Indeed, it is generally thought that Earth's biosphere will come to an end in the next two billion years due to the combination of overheating and CO2 scarcity for photosynthesis. If true, one can expect that atmospheric O2 levels will also eventually decrease in the distant future. However, it remains unclear exactly when and how this will occur." To examine how the Earth's atmosphere will evolve in the future, Ozaki and Christopher Reinhard, Associate Professor at Georgia Institute of Technology, constructed an Earth system model which simulates climate and biogeochemical processes. Because modelling future Earth evolution intrinsically has uncertainties in geological and biological evolutions, they adopted a stochastic approach, enabling the researchers to obtain a probabilistic assessment of an oxygenated atmosphere's lifespan.
Oxygen (O2), methane (CH4) and carbon dioxide (CO2) concentrations on Earth, from 500 million years ago, through to 2 billion years from now. Credit: K Ozaki/C Reinhard.
Ozaki ran the simulation more than 400,000 times – varying the model parameters – and found that Earth's oxygen-rich atmosphere will most likely persist for another 1.08 billion years. After that point, rapid deoxygenation will make the atmosphere reminiscent of the early Earth before its Great Oxidation Event around 2.5 billion years ago. "The atmosphere after the great deoxygenation is characterised by an elevated methane, low levels of CO2, and no ozone layer. The Earth system will probably be a world of anaerobic life forms," explains Ozaki. During its long history, our planet has seen varying levels of oxygen. Following the Great Oxidation Event of 2.4–2.0 Ga (billion years ago), it remained at a low and stable concentration before rising sharply during the Ediacaran Period (635–541 Mya) and then slowing in the Cambrian (541–485 Mya). As a percentage of the atmosphere, oxygen reached its highest levels in the Carboniferous, peaking at 35%. This enabled many insects, which breathe through their skins, to grow to gigantic sizes. It also fuelled wildfire activity. Since then, oxygen levels have fallen and rebounded again, and today stand at roughly 20%.
Earth's oxygen-rich atmosphere today represents an important sign of life that can be remotely detectable. However, this study concludes that it will enter a relatively rapid and terminal decline at some point, and suggests that rich concentrations of the gas might only be possible for 20–30% of Earth's entire history as an inhabited planet. Oxygen (and the photochemical by-product, ozone) is the most compelling biosignature in the search for life on exoplanets. If we can generalise these insights to Earth-like planets, then scientists need to consider additional biosignatures applicable to weakly-oxygenated and anoxic worlds, according to study authors Reinhard and Ozaki. The image of Earth as a dying planet, with only a handful of microscopic anaerobes clinging to survival, is a depressing contrast to the rich biodiversity we see around us today. This assumes, of course, that humanity or its descendants will be unable to save Earth via technological means, such as incremental shifting of the planet's orbit to match the retreating Goldilocks zone. The expanding Sun might also make Mars more habitable. Then again, the sheer length of time involved in this process would provide ample opportunity for Earth's remaining inhabitants to develop interstellar travel and leave the Solar System entirely to settle new systems. Our species by then might even have evolved into beings of pure energy, able to traverse dimensions and into other universes. Perhaps the Earth by then could be left as a kind of galactic relic – a monument to our ancient past.
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