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JEFFREY KARGEL might as well be living on another planet. To him Earth is not homely and blue with swirling white clouds 鈥� it鈥檚 a hellish, lifeless planet glowing an angry red. In much of his world the sun never sets, cooking the ground to more than 2000 掳C and vaporising rock into a scorching sky. A giant ice cap dusted with nitrogen frost has grown in the never-ending midnight of the planet鈥檚 shadow. And in a strange twilight zone between these regions of constant day and constant night, sodium and potassium snow falls.

What Kargel is picturing is not some alien planet light years away. This is Earth in 7.5 billion years鈥� time, when the sun has swollen into a feisty red giant star. By creating models of the future Earth鈥檚 climate and geology, Kargel is hoping to jolt scientists into figuring out how Earth will end its days, something few have bothered to consider.

鈥溈烀ǘ淌悠祍 beat the next 100 years to death, which is understandable because that period is very relevant to us,鈥� says Kargel, a planetary scientist at the US Geological Survey in Flagstaff, Arizona. 鈥淏ut they鈥檝e ignored the distant future.鈥�

His portrait of Earth鈥檚 fate is a dramatic one. Today we may worry about man-made global warming, but in the long term, the sun will do the warming for us, big time. Just over a billion years from now, it will trigger runaway heating of the Earth鈥檚 atmosphere, boiling the oceans off into space. Eventually the sun will loom 250 times larger in the sky than it is today, and it will scorch the Earth beyond recognition. 鈥淥ne thing I know is that the Earth is going to be a very strange place,鈥� says Kargel.

While scientists have paid little attention to Earth鈥檚 distant future, some have at least thought about how the planet will change over the next couple of hundred million years. One of them is Christopher Scotese, a geologist at the University of Texas in Arlington. Over the past three decades, he and his colleagues have charted how the continents have drifted apart ever since the supercontinent Pangaea formed a single, giant land mass about 250 million years ago.

Scotese has also simulated how the continents will look in the future, assuming that they continue their current drifting pattern, with the Americas grinding away from Europe and Africa at just a few centimetres per year 鈥� about as fast as your fingernails grow. In 50 million years, he predicts, the Atlantic will be much wider than it is now, while Africa will have rammed into Europe, closing up the Mediterranean Sea and driving up a mountain range as grand as the Himalayas.

What happens next is less certain. But Scotese reckons that if past patterns are anything to go by, small subduction zones, where one plate dives beneath another, hold some clues. Existing zones on the western edge of the Atlantic ocean should seed a giant north-south rift that swallows heavy, old oceanic crust. The Atlantic will start to shrink, sending the Americas crashing back into the merged Euro-African continent. So roughly 250 million years from now, most of the world鈥檚 land mass will once again be joined together in a new supercontinent that Scotese and his colleagues have dubbed Pangaea Ultima. Here you would be able to walk from the Americas to Africa and Europe without getting your feet wet.

And what happens after Pangaea Ultima? It is impossible to say for sure, but Scotese suspects this supercontinent will itself break up. The process seems to be cyclic, so another giant land mass will probably form several hundred million years later. 鈥淲e should have another two or three Pangaeas,鈥� says Scotese.

After that, however, continental drift will wind down. The drifting is fuelled by heat from the Earth鈥檚 interior, which is gradually cooling. And the planet will eventually lose all its water, which keeps the continents on the move by softening the mantle. Today we worry that emissions of the greenhouse gas carbon dioxide are trapping the sun鈥檚 heat and raising global temperatures. But ironically, several hundred million years from now it will be a lack of CO₂ in the atmosphere that turns up the heat and causes catastrophe 鈥� the loss of the oceans and extinction of all life on Earth.

We have got CO₂ to thank for keeping the climate fairly constant. That鈥檚 because it acts as a natural thermostat. The Earth tucks away CO₂ in many different guises: as a gas in the atmosphere, as a weak solution of carbonic acid in the oceans, in an ice-like phase called clathrate hydrate near the poles, and in rock as carbonate minerals, oil and natural gas.

If for any reason the Earth starts to cool and an ice age looms, chemical reactions that suck CO₂ from the atmosphere into the Earth鈥檚 surface and oceans slow down. Meanwhile, volcanic activity still burps CO₂ into the atmosphere, so the greenhouse gas accumulates, turning the temperature back up. Conversely, if the atmosphere warms, this speeds up reactions that pull CO₂ out of the air, so the temperature falls again. 鈥淭his mechanism has worked for over 4 billion years, keeping conditions on the Earth not too extreme,鈥� says Kargel.

The trouble is that over the long term the sun is getting brighter 鈥� by about 1 per cent for every 100 million years (see 鈥淭he sun鈥檚 fiery future鈥�). It might not sound like much, but as the heat is turned up, CO₂ levels in the atmosphere will fall as a result of those heat-dependent chemical reactions. The 鈥渢hermostat鈥� will be overwhelmed. In 500 million years鈥� time, according to a climate model by James Kasting of Pennsylvania State University in University Park and his colleague Ken Caldeira, CO₂ levels will have fallen to just over 40 per cent of today鈥檚 level (Nature, vol 360, p 721).

Most plants will struggle to get enough of the gas for photosynthesis. 鈥淎bout 95 per cent of plant species will start to get into trouble,鈥� says Kasting. Pine, fir and tropical forests will give way to grasslands, shrubs and cacti that thrive on relatively low levels of CO₂. By about 900 million years from now, levels will be too low even for them. The lush green Earth will have turned a muddy brown.

One planet, two worlds

And it gets worse. In 1.2 billion years, the sun will be about 15 per cent brighter than it is today. The surface temperature on Earth will reach between 60 and 70 掳C and the chemical reactions that soak up atmospheric CO₂ will be so vigorous that almost all the CO₂ will have disappeared from the atmosphere. 鈥淭here will be no more carbon dioxide to compensate for the brightening sun,鈥� says Kargel.

The warming oceans will dramatically increase the humidity of the atmosphere, compounding the problem. Water is also a greenhouse gas, but it doesn鈥檛 act as a thermostat like CO₂. More water in the atmosphere means more heat, which evaporates more water. This vicious circle will trigger a runaway greenhouse effect. The oceans will all but disappear, leaving vast dry salt flats, and the cogs and gears of Earth鈥檚 shifting continents will grind to a halt. Complex animal life will almost certainly have died out.

No one has come up with detailed geological models to show how this drying Earth might look, but Kargel hazards a guess. With no mountain building in progress, he says, erosion by the remaining shallow, steamy rivers will be the dominant geological force for change. 鈥淚magine a steaming Mississippi river delta with 90 per cent of the water gone. There鈥檒l be lots of sluggish streams and the whole Earth will be flattening out. All the mountains will be eroded down to their roots.鈥� Huge swathes of the Earth might resemble today鈥檚 deserts in Nevada and southern Arizona, with low, rugged mountains almost buried in their own rubble.

At some point, ultraviolet radiation from the brightening sun will break down the water in the Earth鈥檚 steamy atmosphere into hydrogen and oxygen. Earth鈥檚 gravity would not be strong enough to retain the hydrogen, which would fizzle off into space. But the oxygen would remain, and at these temperatures could reach pressures of hundreds of atmospheres. 鈥淭he iron in rocks will absorb the oxygen, and Earth will become a rusty planet,鈥� says Kargel. It might start to look a bit like Mars.

Kargel also draws parallels with Venus, which is wrapped in thick, poisonous clouds of sulphuric acid. Eventually the greenhouse effect on Earth will have pushed temperatures up to 1000 掳C, hot enough to melt rock. Seas of scorching magma will form, and sulphate minerals like gypsum will break down. If a thin steamy atmosphere remains, it will form a lethal Venusian-style cocktail of sulphuric acid.

In September, at a meeting of the American Astronomical Society in Monterey, California, Kargel described new simulations of how the Earth will look even farther into its tortured future, after the sun has swollen into a red giant star in about 7 billion years鈥� time. No one can predict exactly how the Earth and moon will be orbiting the sun at that point, but one possibility is that the Earth will be 鈥渢idally locked鈥� to the sun. In other words, one side of the planet will be in permanent daylight while the other side will always be dark.

On the day side of that future Earth, says Kargel, the red sun will appear 250 times wider than today, its globe looming across most of the sky. It would light up the skies quite far round the planet, with only a section about as wide as North America in true darkness at the back. Surrounding it would be an area in perpetual twilight.

Working with Bruce Fegley and Laura Schaefer of Washington University in St Louis, Missouri, Kargel used astronomical predictions of the sun鈥檚 increasing brightness to calculate temperatures on the Earth鈥檚 surface. They found that 7.57 billion years from now, the magma ocean directly in the glare of the sun will reach almost 2200 掳C. 鈥淎t that kind of temperature, the magma will start to evaporate,鈥� says Kargel.

The temperature on the night side is less easy to predict. 鈥淚f there was a thick atmosphere still blowing around, it could carry enough heat to the night side that even this dark side would be toasty warm,鈥� says Kargel. 鈥淏ut if you don鈥檛 have that atmosphere, then it could become extremely cold.鈥� The situation would be similar to that on Mercury, which only has a very thin atmosphere. Mercury鈥檚 midday temperatures of 350 掳C 鈥� hot enough to melt lead 鈥� fall to 鈭�170 掳C at night.

Kargel thinks the night side of the Earth could be even colder, at about 鈭�240 掳C. And this bizarre hot-and-cold Earth will set up some exotic weather patterns (see Graphic). On the hot side, metals like silicon, magnesium and iron, and their oxides, will evaporate out of the magma sea. In the warm twilight zones, they鈥檒l condense back down. 鈥淵ou鈥檒l see iron rain, maybe silicon monoxide snow,鈥� says Kargel. Meanwhile potassium and sodium snow will fall from colder dusky skies.

On the dark side, it could be cold enough for CO₂, sulphur dioxide and argon to freeze out into a giant ice cap, dusted with solid nitrogen icing. Underneath that will be plain old water ice 鈥� if any water remains on the planet, that is. And there is a small chance that in the twilight zone, this wild planet might retain a little memento of its distant past: a cosy liquid water ocean.

Which raises the question 鈥� could any life survive on this hostile future Earth? Probably not. But the last remaining habitable patches might excite the curiosity of an alien civilisation elsewhere in the galaxy, in the same way humans have launched spacecraft to seek out life in comfy pockets of the solar system. 鈥淚f some crazy aliens wanted to investigate this godforsaken world, the liquid water ocean might be where they鈥檇 set up camp,鈥� says Kargel. Who knows, maybe they would find rubble from today鈥檚 skyscrapers deep in sediments on the ocean floor.

鈥淎ll kinds of fanciful things are possible,鈥� says Kargel. He stresses that his model is not a reliable prophecy, more a broad-brush sketch of some potential future reality that might stir imaginations and kick-start some detailed studies. 鈥淚f we can get some students really pumped up about this so they go on to study the Earth鈥檚 fate, that would be fabulous.鈥�

Models like this could also help astronomers understand the environments on other planets orbiting nearby stars. More than a hundred planets have come to light over the past decade, but all the confirmed ones are gas giants like Jupiter, the easiest type to spot. However, future missions should be able to spot Earth-sized planets in the habitable zones around their stars.

For instance, NASA鈥檚 Terrestrial Planet Finder, aiming to launch around 2012, will search for small planets around roughly 150 stars and analyse their atmospheres. Models like Kargel鈥檚 will help scientists to predict the variety of Earth-sized worlds it might find. 鈥淲e鈥檙e going to see planets in all stages of their evolution, and we want to understand what we see,鈥� says Kasting, a member of NASA鈥檚 Terrestrial Planet Finder working group.

鈥淚t鈥檚 really important that we know what our brothers鈥� and sisters鈥� faces are going to look like,鈥� agrees Donald Brownlee, an astronomer at the University of Washington in Seattle. After all, he says, a distant Earth-sized planet might bear no resemblance to the serene blue world we know. 鈥淲e might see a pink planet with a melted surface and a hundred atmospheres of oxygen. Most people would say that鈥檚 a bizarre planet, but it鈥檚 not 鈥� it鈥檚 just our own future.鈥�