[Blueflag]

Simply put, the hydrogen ions come from water, H2O. Carbonic acid is the result as shown by a prior post. When the vapor pressure ( of dissolved CO2 ) changes, more CO2 enters or leaves the water, and the amount of carbonic acid correspondingly changes.

This is NOT an open system, it is a closed loop system. CO2 enters and leaves the oceans all the time. The oceans are not just a carbon sink.

[SAJ]

Ok, thanks. I'll take your word for it, certainly, but I've some trouble imagining the oceans as merely an overlarge fish tank, in terms of characterising the system as open or closed.

;^)