Refrigerants are really interesting - we phased out a bunch of them a few decades ago because they were destroying ozone, but what we replaced them with had high Global Warming Potential (GWP.) The new thing that all of the HVAC companies are working on are finding new refrigerants that have low GWP and work well in their equipment. One of the tricky things is a lot of the low GWP refrigerants are mildly flammable, so there's some thought about trying to revise the building codes to permit their use.