Washington, D.C. -- The Earth's oceans will disappear in about one billion years due to increased temperatures from a maturing sun, but Earth's problems may begin in half that time because of falling levels of carbon dioxide in the atmosphere, according to a Penn State researcher.
"The sun, like all main sequence stars, is getting brighter with time and that affects the Earth's climate," says Dr. James F. Kasting, professor of meteorology and geosciences. "Eventually temperatures will become high enough so that the oceans evaporate."
At 140 degrees Fahrenheit, water becomes a major constituent of the atmosphere. Much of this water migrates to the stratosphere where it is lost to the vacuum. Eventually, the oceans will evaporate into space.
"Astronomers always knew that the oceans would evaporate, but they typically thought it would occur only when the sun left the main sequence," Kasting told attendees today (Feb. 20) at the annual meeting of the American Association for the Advancement of Science. "That will be in 5 billion years."
Stars leave the main sequence when they stop burning hydrogen. The sun, a yellow, G-2 star, will then become a red giant encompassing the orbit of Mercury. Mercury will disappear and Venus will lose its atmosphere and become a burnt out planet. The Earth will suffer the same fate, even though it is outside the red giant's immediate reach.
"However, the oceans may evaporate much earlier," says Kasting, a faculty member with the College of Earth and Mineral Sciences. "My calculations are somewhat pessimistic and present a worst case scenario that does not include the effects of clouds, but they say a billion years."
This model was developed with Ken Caldeira, now at Lawrence Livermore Laboratory.
Things may go bad long before the Earth is a waterless desert. As the climate becomes warmer, the cycle of silicate rock weathering speeds up. This cycle removes carbon dioxide from the atmosphere and sequesters it in the oceans as calcium carbonate.
"The silicate weathering cycle stabilizes the Earth's climate for a time," says Kasting. "Eventually, atmospheric carbon dioxide levels will become so low that it will not be able to do so, but before then, there will not be sufficient carbon dioxide to sustain most plants."
Plants use carbon dioxide in photosynthesis to convert the sun's energy to sugars and other carbohydrates. Two main kinds of photosynthesis exist, C3 and C4. In a half billion years, the models predict that carbon dioxide will be at the compensation point for C3 plants which make up 95 percent of all plants. Below the compensation point, carbon dioxide is not concentrated enough for these plants to photosynthesize. C3 plants include trees and most crops.
C4 plants, which include corn, sugar cane and other tropical grasses, can still photosynthesize because they have an internal mechanism to concentrate carbon dioxide, but these plants cannot sustain the biosphere as we know it today.
"If carbon dioxide levels in the atmosphere continue to increase over the next few centuries, they could remain high for a very long time," says Kasting. "Then, after fossil fuels run out, it would take a million years or so for levels to return to present."
But even if there is a pulse of high carbon dioxide in the near future, by a half billion years, levels will be too low for productive plant life.
"Obviously, a billion, even a half billion years, are a long way off in the future," says Kasting. "However, these models can help us refine our understanding of the time that a planet remains in an orbit where life can exist."
Only a narrow spherical shell of space exists at a distance from a star that is neither too cold nor too warm for life. As a sun matures and brightens, that spherical shell moves outward. A planet must remain in the livable shell for long enough for life to evolve, even while that band moves outward. If planets lose their water supply, a mandatory requirement for life, earlier than previously thought, then that creates a shorter window for livable planets.
"If we calculated correctly, Earth has been habitable for 4.5 billion years and only has a half billion years left," says Kasting.
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