The Earth is entering the final 10% of its lifespan, according to a US geoscientist.

Sun facts Mass - 1.99e30 kg Mass consumption/year - 10e17 kg Current age - 4.5e9 years Hydrogen-fuelled Life left - 10e10 years

Professor James Kasting, at Pennsylvania State University, calculates that the Earth's oceans will disappear in about one billion years' time, due to increased temperatures from a brightening Sun.However, well before the planet is left as an arid desert, the level of carbon dioxide in the atmosphere will be too low to support plant life, destroying the foundation of the food chains.

"The Sun, like all main sequence stars, is getting brighter with time and eventually temperatures will become high enough so that the oceans evaporate," said Professor Kasting.

At 60 degrees Celsius (140 degrees Fahrenheit), water becomes a major constituent of the atmosphere. Much of this water drifts up to the stratosphere and is lost into space. Eventually, all the oceans will leak out of the Earth's grasp.

Burnt-out planet

"Astronomers always knew that the oceans would evaporate, but they typically thought it would occur only when the Sun left the main sequence - that will be in five billion years."

Stars leave the main sequence when they stop burning hydrogen. The Sun will then become a red giant, swamping and obliterating Mercury. Venus will lose its atmosphere and become a burnt-out planet.

"However, my calculations show the oceans may evaporate much earlier," said Professor Kasting. "They are somewhat pessimistic and present a worst-case scenario, but they say a billion years."

The earlier loss of carbon dioxide will occur because as the climate gets hotter and wetter, more rock is weathered by rain. This dissolves carbon dioxide and hides it away on the ocean floor as calcium carbonate.

"Obviously, a billion, even a half billion years, is a long way off in the future," said Professor Kasting. "But these models can help us refine our understanding of the time that a planet remains in an orbit where life can exist."

"If we calculated correctly, Earth has been habitable for 4.5 billion years and only has a half billion years left."

Professor Kasting's comments were made at the annual meeting of the American Association for the Advancement of Science.