Asked by: Craig Morley. Norwich
If you sucked away the entire atmosphere, most higher vertebrates (including humans) would asphyxiate in seconds. Other land animals would follow within a few minutes, plants within a few hours. Meanwhile, the sudden drop in pressure would have a curious effect on the seas. The absence of pressure would let the fastest-moving molecules escape as gas (boil) at normal atmospheric temperature; however, as this process continued, only the slowest-moving molecules would be left, so that the rest of the water would very quickly freeze. Sea levels would drop substantially, but there would still be oxygen dissolved in the water that was left, so sea life, deep-water organisms in particular, might survive longer.
With no new oxygen dissolving in from the atmosphere, however, the amount in the water would steadily decrease as dead organisms sank to the bottom. It would take 500 years to fully deplete the deepest ocean. Anaerobic bacteria and the chemosynthetic bacteria living around sulphur vents on the seabed would survive, though. And volcanoes would still pump CO2 into the atmosphere. Given about a billion and a half years, the action of bacteria and algae in shallow seas might eventually replace the atmosphere. But don’t hold your breath.