So this sounds exactly like a system that is low, thanks for the info.
You probably have a leaking evaporator, which is common on these Volvos. It is very expensive to repair, like 8 hours of labor and several hundreds in parts, so recharge is your least expensive option. The best and longest lasting method is to evacuate it and recharge with the appropriate equipment, and this will be done at a repair shop.
The least expensive option is to go to a pars store and get a can of R134, plug it into the low side port near the RF fender and recharge the system. This will add enough to start cooling.