The 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: These catastrophic scenarios are at least 500 million years into the future.
In the very, very long term, the Earth's future is grim, according to experts who spoke at the national meeting of the American Association for the Advancement of Science.
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.
The sun gradually is getting brighter and hotter. Eventually it will raise the Earth's temperature, and that's when the troubles begin.
"Bad things start to happen when the average temperature reaches 140 degrees Fahrenheit," said Kasting. "That's when the Earth will start losing its water."
At that temperature, the Earth's atmosphere becomes 10 to 20-percent water, he said, and water vapor rises into the stratosphere and breaks down chemically into oxygen and hydrogen. The hydrogen escapes to space, and the water is lost.
Kasting said astronomers long have known this would happen, perhaps in about 5 billion years. But the researcher said new computer studies suggest it could occur much earlier.
"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 Kasting said a new model suggests Earth could get into trouble even sooner.
Warm temperatures, he said, will cause the oceans to absorb carbon dioxide, thus removing from the atmosphere a gas essential for plant life. In about 500 million years, Kasting said, the atmosphere will be so short of 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," he said.
A second researcher, University of Michigan physics professor Fred Adams, predicts the Earth eventually will either freeze or fry.
Adams said the sun, like stars of its type, will exhaust its fuel and balloon outward with a torrid sphere of gas, turning Mercury, Venus, Earth and Mars into burned out cinders, if the planets survive at all.
There probably will not be anyone around to see that happen, however, because Adams thinks that in 3.5 billion years the sun will be hot enough to sear the Earth and wipe out all forms of life.
Adams said there is a long shot the Earth could escape this superheated fate. He said new calculations show that the orbit of Jupiter, the solar system's biggest planet, could be disrupted by the gravity of a passing star. This, in turn, could jar the 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 armed by radioactive heat from within the planet, might survive for billions of years in the Earth's frigid spin through a sunless space.
Adams notes that the fate that awaits the sun will visit all stars. Eventually, all matter that can be converted to energy will have been exhausted.
What will be left is a cold, utterly dark universe, containing only electrons, positrons and neutrinos.