to put it very simply, heat and humidity causes the sea evaporates to make clouds, the clouds drift over the land and rain, the water flows into the rivers and the rivers flow into the sea..
What goes round comes round. There's the same amount of water in the world that there always was. It's just that most of it is falling on my house at the present. maybe in 500 years there will be a terrific drought with all available water locked up in new ice caps.
Absolutley no comfort at all to those folk with their kitchens knee deep in sewage-tainted slush.
Hay ho.