In the future, in about seven billion years, the Sun will become hotter and turn into a red giant, which is likely to swallow the Earth. But the planet will cease to be suitable for living organisms much earlier. This will happen not only due to the evaporation of the oceans, but also due to major changes in the composition of the atmosphere. "Lenta.ru" tells in detail about the new scientific work of scientists from the United States and Japan, who believe that the maximum lifetime of complex life on Earth is about one billion years.
Thousands of destinies
Currently, the Earth's biosphere maintains a 20 percent oxygen content in the atmosphere through photosynthetic organisms. It is known that for most of the history of the Earth, the level of oxygen was lower than today, and its concentration in the atmosphere began to increase only after the appearance of terrestrial plants. The evolution of the biosphere has accelerated the geochemical cycles of such important chemical elements as phosphorus. However, photosynthesis alone is not enough to keep the planet high in oxygen.
Previous research on the future habitability of the Earth has focused on the relationship between the warming up of the Sun as it turns into a red giant, the carbonate-silicate geochemical cycle, and water loss. Over time, as the sun gets brighter, the concentration of carbon dioxide will fall, disrupting important geochemical cycles for the biosphere. A number of theoretical models suggest that the Earth's climate in the next two billion years will become humid due to a powerful greenhouse effect, as a result of which a large amount of water will begin to escape from the stratosphere into space.
The carbon cycle on earth
In the new study, scientists predicted future habitability for the Earth based on a detailed model that tracks the Sun's influence on geochemical cycles such as carbon, oxygen, phosphorus and sulfur. Experts added to this the methane cycle, which includes the metabolism of living organisms, as well as the redox exchange between the crust and the mantle, which makes it possible to track the processes that control the level of oxygen in the atmosphere on a geological time scale. Such a model is capable of covering billions of years of planetary history in the future.
The researchers used a stochastic approach, randomly fitting parameter values for the model, including changes in the rate of degassing of the Earth's mantle, as well as the acceleration of erosion. They set the initial conditions (initialization stage) for the Earth 600 million years ago, and then ran the model about 400 thousand times, covering the evolution of the planet to the present time. Of the entire sample of runs, only about five thousand reproduced conditions on Earth, close to modern ones. They were used to predict the future.
Everything is bad
Despite some uncertainty, in neither scenario would an oxygen-rich atmosphere last longer than 1.5 billion years. This is realized only in a deliberately impossible scenario, where the Sun does not increase its brightness. A decrease in oxygen concentration occurs due to an increase in the temperature of the Earth's surface, as well as due to a decrease in the amount of carbon dioxide in the atmosphere.
It is the decrease in the amount of carbon dioxide entering the atmosphere that will lead to photochemical destabilization of the atmosphere and a sharp drop in the oxygen level.This is due to both the geochemical carbon cycle affecting the oxygen cycle and a decrease in biospheric activity, that is, global photosynthesis. So, plants with C3 photosynthesis (most plants use this type of photosynthesis) will disappear after about 500 million years, which will affect atmospheric oxygenation.
Comparative sizes of the Sun at present and the red giant
The disappearance of plants suppresses chemical weathering and the associated phosphorus cycle, in which an important mineral is transferred from land to the ocean. The level of activity of marine ecosystems will also decrease over time.
The biosphere on Earth will be similar to the one that existed in the time of the Archean, before the Great Oxygen Event 2.45 billion years ago. In particular, the level of atmospheric oxygen at the new equilibrium state will be many orders of magnitude lower than at the present time, and the level of methane will increase sharply. At the same time, there will be one significant difference: a decrease in carbon dioxide levels, which increases the ratio of CH4 to CO2 and leads to the appearance of organic haze.
After the global temperature of the Earth's surface exceeds 300 Kelvin, further warming will begin to suppress residual land and marine biospheric activity. In any case, no one can live on the planet except microorganisms.
As the authors of the work write, organic haze can serve as a biosignature (a sign of the existence of life) on planets like the Earth, located in the main sequence star system. Such a potential planet is, for example, Kepler-452b, orbiting the star G2, whose age reaches about six billion years. This world currently receives 10 percent more heat from the parent star than the Earth does from the Sun. Organic haze can also provide long-term stability for a new climate in the future.
The artist's idea of the death of the Earth
The models used by scientists included the influence of the Earth's biosphere, but planets can also have completely different biospheres - for example, devoid of vegetation. To study how significant this influence is, scientists excluded the earth's biosphere from the model. As expected, the absence of terrestrial plants results in lower atmospheric O2 levels throughout planetary evolution. However, there will still be enough oxygen for a billion years to be detected by astronomical instruments. This result suggests that the presence or absence of the terrestrial biosphere (but not the biosphere in general) has only a secondary effect on the deoxygenation of the air envelope.
The work of the researchers will help the search for potentially habitable planets, since the time when an oxygen atmosphere exists is very limited, and only part of the Earth's history will be characterized by reliably detectable levels of oxygen. Direct detection of O2 in the visible wavelength range will be challenging for most of the lifetime of a planet like Earth, with the exception of 1.5-2 billion years. This roughly corresponds to 20-30 percent of the lifetime of the Earth as an inhabited world, including the age of microbes. At the same time, observation of traces of ozone in ultraviolet waves can widen this "window".