Why does this make a difference? Unsaturated molecules are far less chemically stable and tend to break down easier. Since HFOs are less chemically stable, they do not survive long in the atmosphere – and so they do far less harm than the more stable saturated HFCs. The difference is dramatic. HFC 134a has a GWP of 1430 while HFO-1234yf has a GWP of 4.
However, HFOs have a design challenge to overcome: they are mildly flammable. The very instability that reduces their GWP increases their flammability. At present, building codes in the US generally do not recognize a difference between highly flammable refrigerants and mildly flammable ones. Most building codes do not allow the use of significant amounts of flammable refrigerant inside the building. ASHRAE is working on revising their Safety Standard for Refrigeration Systems, Standard 15. It is projected to be ready by January 2018. For more information on the work being done on flammable refrigerants check out this article in Contracting Business