Contacts:
A'ndrea Elyse Messer, (814) 865-9481, aem1@psu.edu
Vicki Fong, (814) 865-9481, vyf1@psu.edu

February 20, 2000

Earth's Oceans Destined to Leave in Billion Years

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.

*****

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