PREDICTED FUTURE OF EARTH
Earth's long-term future is closely tied to that of the Sun. Over the next 1.1 Ga, solar luminosity will increase by 10%, and over the next 3.5 Ga by 40%. The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 500–900 Ma. The lack of vegetation will result in the loss of oxygen in the atmosphere, and animal life will become extinct. After another billion years all surface water will have disappeared and the mean global temperature will reach 70 °C (158 °F). From that point, the Earth is expected to be habitable for another 500 Ma, possibly up to 2.3 Ga if nitrogen is removed from the atmosphere. Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years, due to reduced steam venting from mid-ocean ridges.
The Sun will evolve to become a red giant in about 5 Ga. Models predict that the Sun will expand to roughly 1 AU (150,000,000 km), which is about 250 times its present radius. Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU from the Sun when the star reaches its maximum radius. Most, if not all, remaining life will be destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level). A 2008 simulation indicates that Earth's orbit will eventually decay due to tidal effects and drag, causing it to enter the Sun's atmosphere and be vaporized.
