Earth’s Oceans Destined to Leave in Billion Years

Pennsylvania State University

University Park, Pennsylvania

Contacts:

A’ndrea Elyse Messer, (814) 865-9481, aem1@psu.edu

Vicki Fong, (814) 865-9481, vyf1@psu.edu

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.

**aem**

EDITORS: Kasting may be reached at 814-865-3207 or at kasting@essc.psu
by email.