This paper presents a self-consistent model for the evolution of gas produced in the debris disc of \beta Pictoris .
Our model proposes that atomic carbon and oxygen are created from the photodissociation of CO , which is itself released from volatile-rich bodies in the debris disc due to grain-grain collisions or photodesorption .
While the CO lasts less than one orbit , the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside .
The temperature , ionisation fraction and population levels of carbon and oxygen are followed with the photodissociation region model Cloudy , which is coupled to a dynamical viscous \alpha model .
We present new gas observations of \beta Pic , of C I observed with APEX and O I observed with Herschel , and show that these along with published C II and CO observations can all be explained with this new model .
Our model requires a viscosity \alpha > 0.1 , similar to that found in sufficiently ionised discs of other astronomical objects ; we propose that the magnetorotational instability is at play in this highly ionised and dilute medium .
This new model can be tested from its predictions for high resolution ALMA observations of C I .
We also constrain the water content of the planetesimals in \beta Pic .
The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C , O or CO detections .