Read more: “Strange skies: Seven wonders of the atmosphere“
Assuming that humans don’t mess it up completely, Earth’s atmosphere should remain broadly hospitable for many millions of years – but not in the longer term. The sun is slowly getting brighter as its core contracts and heats up. In a billion years it will be about 10 per cent brighter than today, heating the planet to an uncomfortable degree. Water evaporating from the oceans may set off a runaway greenhouse effect that turns Earth into a damp version of Venus, wrapped permanently in a thick, white blanket of cloud. Or the transformation may take some time and be more gentle, with an increasingly hot and cloudy atmosphere able to shelter microbial life for some time.
Either way, water will escape into the stratosphere and be broken down by UV light into oxygen and hydrogen. Oxygen will be left in the stratosphere – perhaps misleading aliens into thinking the planet is still inhabited – while the hydrogen is light enough to escape into space. So our water will gradually leak away.
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In 2 or 3 billion years, the oceans will have gone. If volcanoes continue to pump carbon dioxide into the air, Earth may come to resemble Venus even more closely, with a thick, super-hot CO2 atmosphere. Without ocean water to lubricate Earth’s plate tectonics, the planet could seize up, preventing buried carbon from returning to the air volcanically.
Then we may come to resemble Titan, the giant moon of Saturn, says Jonathan Lunine of Cornell University in Ithaca, New York. Most of Titan is dune-filled desert, just occasionally watered by violent storms of methane rain. Earth could become a warm version of this moon, with a few water-filled lakes at the poles breaking up the vast deserts. The skies will be clear most of the time, but once in a while they will suddenly billow with cloud and pour water onto the parched land. “For a while we’ve still got a lot of water in the atmosphere,” says Lunine. “With a brighter sun there is more energy going into the system, and the potential for very large storms.”
This environment might persist for a hundred million years while the last of the water remains. When it is finally gone, that will be the end of weather as we know it. So the long-range forecast is for cloud, followed by heavy showers, followed by… nothing very much.