Life is thriving on planet Earth thanks to oxygen. Oxygen is a highly reactive element; it can form compounds with nearly every other element on the periodic table, releasing energy in the process. In a process known as cellular respiration, organisms use oxygen to oxidize substrates (for example sugars and fats) and generate energy.
The Earth formed approximately 4.6 billion years ago. Intense volcanic activity released gases that formed a toxic atmosphere, likely a mixture of carbon dioxide, methane, ammonia, and water vapor. Traces found in ancient rocks suggest that about 2.7 billion to 2.8 billion ago for the first time oxygen was released into Earth's atmosphere, forming new minerals like iron-oxide. Scientists think that early photosynthetic microorganisms, able to use sunlight to chemically break carbon dioxide molecules into carbon and oxygen, caused the amount of carbon dioxide to decrease and oxygen to increase. But only around 2.45 billion to 1.5 billion years ago oxygen was becoming a significant component of Earth's atmosphere. Today Earth's atmosphere contains 78 percent of nitrogen and roughly 21 percent of oxygen.
Scientists all agree that life cannot go on forever on planet Earth. As the aging sun grows hotter, oceans on Earth will evaporate and the atmosphere will escape into space. Eventually, the sun will run out of energy and destroy itself together with the inner planets, including Earth.
A pair of researchers from Toho University and NASA Nexus for Exoplanet System Science has found evidence, via simulation, that Earth will lose its oxygen-rich atmosphere in approximately 1 billion years. In their paper published in the journal Nature Geoscience, Kazumi Ozaki and Christopher Reinhard describe the factors that went into their simulation and what it showed.
The researchers created a simulation of Earth that factored in variables that described the climate as well as geological and biological processes, and most importantly, the activity of the sun.
Climate, erosion, volcanism, plate tectonics all play a key role in regulating the oxygen level during various time periods, but by far the most important factor, the only process actively producing free oxygen, are oxygen-producing organisms like plants and algae.
The simulation shows that as the sun grew hotter, 1 billion years from now, releasing more energy, carbon dioxide levels in Earth's atmosphere will begin to drop due to the gas absorbing the heat and breaking down. In a warmer climate, increased chemical weathering rates will capture oxygen and carbon in rocks like limestone (or CaCO3). At the same rate, the uppermost layer of the atmosphere absorbs more energy from the sun. With more kinetic energy available, the light oxygen molecules will escape Earth's gravitational pull into space. Concentration levels of heavy molecules, like methane, will relatively increase as compared with today.
As carbon dioxide levels fall, photosynthetic organisms, like algae in the sea and higher plants on land, will begin to suffer, resulting in reduced production of oxygen. Over a period of just 10,000 years, carbon dioxide levels will drop so much that plant life would go extinct. Without plant life, oxygen levels will also drop, causing a mass extinction event among animals.
The result, according to the simulation, would be a planet with a nitrogen-methane-rich atmosphere and without higher life. Anaerobic creatures, microorganisms that instead of oxygen use other elements, like sulfur and methane, for cellular respiration, could still survive in underground habitats, like caves or fissures in rocks. As microbes were the first life-form on Earth, appearing some 4 billion years ago, they will also be the last.