LECTURE 8: EARTH'S FUTURE
The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 billion years (1.1 Gyr), and by 40% over the next 3.5 Gyr.[129] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet's oceans.[130]
The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to the lethal levels for plants (10 ppm for C4 photosynthesis) in 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.[24] But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower volcanism).[131] After another billion years the surface water will have completely disappeared[132] and the mean global temperature will reach 70°C.[24] The Earth is expected to be effectively habitable for another 500 million years or so.[133]
The Sun, as part of its evolution, will expand to a red giant in about 5 Gyr. Models
predict that the Sun will expand out to about 250 times its present size,
roughly 1 AU
(150,000,000 km).[129][134] Earth's fate is less
clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without
tidal effects, the Earth will be in an orbit 1.7 AU (250,000,000 km) from the
Sun when the star reaches it maximum radius. Therefore, the planet is thought to
escape envelopment by the expanded Sun's sparse outer atmosphere, though most
(if not all) existing life would have been destroyed by the Sun's proximity to
Earth.[129] However, a more recent
simulation indicates that Earth's orbit will decay due to tidal effects and
drag, causing it to enter the red giant Sun's atmosphere and be destroyed.[134]
READING FOR THE NEXT LECTURE
