Water under pressure can hold a lot of carbon dioxide, but when the pressure is released, its capacity drops - and degassing occurs.
The pop of a champagne cork and the bubbles that pour out are the carbon dioxide the champagne can no longer hold.
But degassing isn't usually so dramatic - for instance, when it occurs between the atmosphere and the surface ocean.
In the dark waters of the deep ocean, the inhabitants live under conditions of enormous pressure and very low temperatures. The water down here can hold plenty of dissolved carbon. But when water from the deep ocean comes up to the surface through upwelling, the temperature is higher and the pressure is lower - so the surface waters release some of this dissolved carbon to the atmosphere as carbon dioxide.
This page forms part of the hints for Element On The Move.