When I bought a portable air conditioner unit, I was happy to see some genuine progress made in the refrigerants.
Long time ago, I bought my older portable air conditioner unit and it used R410a as the refrigerant. It consists of artificial hydrocarbons that have some hydrogen atoms substituted with fluorine. Apparently it doesn't cause ozone depletion which was why it was allowed as a refrigerant, but its global warning potential is horrible. Its global warming potential is over 2000, meaning that about half kilogram charge would cause as much damage as a ton of carbon dioxide if released to the atmosphere. To put that into context, in its lifetime the unit will probably consume around 5000 kWh of electricity, and at 100 g / kWh emissions from generating electricity in the relatively clean Finnish grid, the emissions caused by the electricity consumption would be 500 kg, half of the damage that would be caused if the unit releases all its refrigerant charge. Also the emissions from electricity generation are rapidly decreasing whereas the damage caused by R410a leak stays the same.
However, good refrigerants have been available for long. We have however been too scared of using them because they are flammable (but there's so little of these refrigerants in one AC unit so something like a barbecue grill would be a far greater flammability hazard). My new portable air conditioner unit uses R-290, or in other words propane. It has a global warming potential of 3. Only 0.24 kg of it is used, so this means if all of it are released, it corresponds to only 0.72 kg of CO2 emissions.
I wonder why the government doesn't get rid of special permits needed for installing split air conditioner units. With propane as refrigerant, it could be perfectly well vented to atmosphere with no damage.
So, look for propane (R-290) as the refrigerant.
I wonder why cars use today R-1234yf. Probably some chemical company was worried that their revenue stream from specialty AC chemicals would go away if propane was allowed and lobbied for everyone to switch to a more expensive patented chemical.