Two and a half billion years hence, long after we are gone, Earth will lose its water, and the hardiest organisms will succumb to the sun.

Five billion years from now the sun will balloon into a red giant star and destroy Earth like a mote of dust in a blowtorch. By the time it does, though, our planet won’t be much to cry over. It will have spent its last 2 billion years not as a blue gem of life but as a dry, overheated ball of stone, like Venus.

Such are the grim forecasts coming out of the computer sitting on Ken Caldeira’s desk at Penn State. Caldeira and his colleague James Kasting have been simulating how the evolution of the sun will affect Earth’s atmosphere. On its way to red-gianthood, the sun is getting brighter and hotter; according to astronomers’ star models, it’s about 25 percent brighter now than when Earth was born 4.6 billion years ago. According to climate modelers like Caldeira and Kasting, the Earth has managed to maintain an equable temperature so far by turning down its greenhouse effect as the sun has turned up the heat.

The scheme works like this: More solar heat means more water evaporating from the ocean, more rain causing carbon dioxide to leave the atmosphere, and more carbon getting locked up in rocks. Although volcanoes return some heat-trapping CO2 to the atmosphere, there is still a steady decline in its concentration--which means Earth’s temperature stays stable and life can flourish.

But plant life needs atmospheric CO2 to photosynthesize, and animals like us need plants to eat. In coming eons, as the sun continues to warm, there will come a point at which so much CO2 has been pulled out of the atmosphere that plants and everything that eats them will die. In 1982 the British atmospheric chemist James Lovelock ran a simple computer simulation of this process. He concluded that the biosphere will expire from CO2 starvation in 100 million years. (It won’t be saved even by our pollution; the man-made greenhouse effect will be a mere blip on the geologic time scale.)

A decade after Lovelock’s forecast, Caldeira and Kasting decided to look into the crystal ball again. They changed a fundamental assumption of Lovelock’s: that all plants die when the atmospheric concentration of CO2 is less than 150 parts per million. In fact, some plants, such as grasses, use CO2 so efficiently that they can get by on less than 10 parts per million. They could take over the planet when other plants die off.

As a result, Caldeira and Kasting found, the biosphere’s demise can’t happen until at least a billion years from now. A few tough plants and animals may cling to existence even longer. Caldeira and Kasting kept their simulation running to see how the hardy survivors would meet their end. It turns out they will be baked and dried.

With no CO2 in its atmosphere, Earth will no longer be able to regulate its temperature, and it will begin to warm in tandem with the sun. A billion and a half years from now the temperature will hit 120 degrees, at which point everything but microbes will have died out. Two hundred million years later, the temperature will reach 212 degrees--boiling--and then all but the toughest microbes, of the kind that now live in seafloor hot springs, will hear time’s winged chariot hurrying near.

Even they will meet their end as Earth dehydrates. The atmosphere will be full of steam, some of which will rise to the stratosphere. There high-energy radiation from the sun will split the molecules into hydrogen and oxygen, and the lightweight hydrogen will fly away into space. By 2.5 billion years from now, all of Earth’s water will have been destroyed, leaving the planet sterilized.

Without water, the carbon cycle will screech to a halt. Volcanoes will continue belching up CO2, and now, in the rainless atmosphere, it will once again begin to build up, creating a massive greenhouse effect on an already scorched planet. Temperatures will race to 500 degrees and above. In 3 billion years Earth will resemble dead-hot Venus, with another 2 billion years left to sit around thinking about its salad days before the sun actually swallows it.

By then we’ll be long gone. If a species lives 10 million years, it’s long-lived, says Caldeira. So our own little blip of a greenhouse effect is much more worth worrying about than Caldeira’s apocalypse. Still, as he points out, every culture has a vision of creation and ending. Perhaps this is the ending for us.