I haven't seen any papers either. One of the theories (or speculation) is as under :
There are a huge number of systems operating with R22 refrigerant worldwide. As more and more new systems use only alternate refrigerants (Europe is already doing that), the existing systems using R22 will slowly become obsolete or they get converted to R407C systems. The existing charge of R22 in these systems would hopefully be reclaimed thereby increasing the reserve of R22.
HVAC68