Leaving a vehicle and air conditioning running.
Simple answer, don't do it! Your engine will very likely overheat if left idling, especially with things like the air running. An overheated engine will do itself major dirt. I don't have an alternative to suggest, but I KNOW I wouldn't do that. Don't ask me why I know this.
Locked-himself-out-of-his-vehicle-Bob
Sometimes a little help goes a long way
Gen,
Some cats, (& dogs, humans, birds, etc.) never make the transistion well to being on the road. If that happens to you, there a some pharmaceutical products that can make life much saner and more fun for all. I am not sure about the regulations of pet stores in Canada, but in the US some of these products are over the counter and some require a prescription. Just to keep in mind.
Sherpa Pet Carrier Backpack
Gen,
Sherpa is a widely-carried pet backline luggage line in the states. The Cat backpack model runs about $USD 70.00 and I have seen it at Petco, Petsmart and Target stores. But you may wish to order it on-line with a pick-up in one of those stores in Detroit or whatever.