The end is nearer: Earth's demise moved up from 5 billion to 500 million
By Paul Recer
The Associated Press (Arlington Star Telegram, February 21, 2000)
WASHINGTON -- Earth is destined to dry up, burn up or freeze, scientists studying the planet's ultimate fate say. Don't worry about making final plans just yet: Those catastrophic scenarios are at least 500 million years in the future. In the very, very long term, Earth's future is grim, say experts who spoke at the national meeting of the American Association for the Advancement of Science. Astronomers once gave earthlings 5 billion years, but new computer studies suggest that life on Earth could die out much earlier.
Earth's fate depends on the sun, which, like any other star, will not last forever, said James F. Kasting, professor of geosciences at Pennsylvania State University. Kasting said the sun is gradually getting brighter and hotter. Eventually, it will raise Earth's temperature, and that's when the troubles will begin. "Bad things start to happen when the average temperature reaches 140 degrees Fahrenheit," Kasting said. "That's when the Earth will start losing its water." At that temperature, Earth's atmosphere will become 10 percent to 20 percent water, he said, and water vapor will rise into the stratosphere and break down chemically into oxygen and hydrogen. The hydrogen will escape into space, and the water will be lost. Kasting said astronomers have long known that would happen, perhaps in about 5 billion years. But the researcher said that the forecast has been refuted by new computer studies. "The most pessimistic calculation is that the oceans will disappear in about 1.2 billion years," he said, and Earth will become a waterless desert. But one new model suggests that Earth could get into trouble even sooner, Kasting said. Warm temperatures will cause the oceans to absorb carbon dioxide, thus removing from the atmosphere a gas essential for plant life, he said. In about 500 million years, the atmosphere will be so deficient in carbon dioxide that all plants will die, followed eventually by all life that depends on plants. "If we calculated correctly, Earth has been habitable for 4.5 billion years and only has a half-billion years left," Kasting said.
A second researcher, University of Michigan physics professor Fred Adams, predicts that Earth will eventually either freeze or fry. Adams said the sun, like stars of its type, will exhaust its fuel and balloon outward, becoming a withering sphere of gas, turning Mercury, Venus, Earth and Mars into burned out cinders -- if the planets survive at all. Probably no one will be alive to see that happen, however, because Adams believes that, in 3.5 billion years, the sun will be hot enough to sear Earth and wipe out all life forms. Adams said there is a long shot that Earth could escape a superheated fate. Fresh calculations show that the orbit of Jupiter, the solar system's biggest planet, could be disrupted by the gravity of a passing star, which could jar Earth from its orbit, possibly sending it into deep space. "Then, the Earth wouldn't fry," Adams said. "It would freeze." Some life, such as bacteria that live near thermal vents, warmed by radioactive heat from within the planet, might survive for billions of years in Earth's frigid spin through space. But Adams notes that the fate of the sun awaits every star. Eventually, all matter that can be converted to energy will have been exhausted. That will leave a cold, dark universe, containing only electrons, positrons and neutrinos, Adams said.