A carbon tax would solve this.
It wouldn’t be. The massive cost already isn’t stopping it.
If (hypothetically) co2 tax would lead to reduction in other industries but keep emissions unchanged in planes, this is fine.
I personally feel the problem is more related to a bogus slot allocation/deallocation policy that forces an airline to fly an empty plane around to hold a slot. Could they not just remove that rather pointless requirement, and have some sort of market based approach to auction slots for a set period (say 1 year at a time)?!
The whole point of the current system is to combat old carriers that had local monopolies and refused to allow competition ( e.g. British Airways at Heathrow). Such a carrier could afford to spend money to prevent competitors from flying from their hubs, making themselves the only ( and thus more expensive) option.
The regulation is good, it just needs more fine tuning ( e.g. they should add a minimal passenger and/or cargo load, to make sure the flights aren't there just to keep the slots and are actually used).
I guess during the last pandemic of this scale, there wasn't really much of an aviation industry.
If you make fuel more expensive tickets will be more expensive and these empty flights will be a quite similar proportion of cost for airlines because the tax is the same with or without passengers.