People's desire for heavily carbon consuming things vs lightly carbon consuming things varies massively. If you're worried about the poor don't ignore the externalities of their consumption but subsidise them directly via UBI or somesuch.
Rationing is a very worst of all possible worlds solution, losing you all the benefits of trade.
Just as a thought exercise, if anything was possible:
UBI seems to work towards the goal of making sure people don't die due to lack of resources (it is a basic income, after all). It's less clear how it works towards the goal of reducing carbon emissions.
Rationing, on the other hand, has the potential: Natural resources (publicly owned ones anyways), and perhaps natural limits like how much CO2 the skies can take, are collectively owned by the people. So it could make sense to distribute those amongst the people. The people could then sell them in a free® market. This means we can work towards both goals at once: Those seeking to pollute more, could simply buy the carbon credits from their fellow people, who can now better afford to live. And, at the same time, total pollution is capped-ish, depending on the scheme.
As a fun note, UBI is just rationing out the available funds for UBI, so it would suffer from any rationing-specific failings that carbon rationing would suffer from.
A tradeable carbon rationing is indeed equivalent to a UBI, but with side effects. In particular, if the market is efficient then the consumption of CO2 credits will exactly equal production, but the price will be unrelated to the actual externality cost or mitigation cost of the marginal CO2 release. So you either get more CO2 released than you would with an externality tax or you get less CO2 released than you should given that you can mitigate against that particular CO2 release.
Ideally you'd have credits being available for purchase at prices that correspond to the costs of mitigating their externalities (CO2 emission is not in and of itself evil, its the consequences that we don't want).
> A tradeable carbon rationing is indeed equivalent to a UBI
On the contrary -- I was saying it was not equivalent, because it also works towards the goal of reducing CO2 consumption, whereas I can't imagine how a UBI would do so.
> if the market is efficient then the consumption of CO2 credits will exactly equal production
The carbon credits I am imagining would not be "produced" per se -- they would, in total, represent the total amount of carbon we as a country want to emit, to reduce climate disaster, allocated equally to each individual, who all collectively "own" that natural limit. Those individuals can then sell their "contribution ration" to companies which wish to emit more than the CO2 allocated individually to their CEO, or whatever. So ideally credits will be available for purchase by the CEO, at whatever rate the CEO's fellow people are willing to charge the CEO. Mitigations will need to be done by humanity regardless, or else.
My point was, maybe it is a single number! It's whatever We The People decide, and it doesn't vary: a ton of CO2 released into the air does the same damage no matter what released it. You could spend it saving orphans, and it wouldn't make the CO2 heat the planet any less, and it wouldn't remove the need to find a new way to save orphans that doesn't emit CO2.
Indeed, examining the quote below...
> So you either get more CO2 released than you would with an externality tax or you get less CO2 released than you should given that you can mitigate against that particular CO2 release
...it's easy to miss the point that there's no such thing as "less CO2 released than you should" because the amount we should is negative: not only should we be emitting zero (not net zero, literally zero), and then we must do the mitigations on top of that.
In short, we've tried the existing carbon credit scheme without rationing the available credits, and it hasn't worked to achieve the goal. We need to try something different now.
No CO2 emissions is not the terminal value of the human race. It is a means, not an ends. The damage of that extra ton of CO2 is not infinite.
If releasing a marginal megaton of CO2 caused climate issues that cost a thousand lives over the next 200 years but we want to release that marginal megaton because it let's us save ten thousand orphans than we should release that CO2.
It might be that in the world we live in the correct CO2 per ton tax that takes into account externalities is literally greater than the price of capturing a ton of CO2, in which case all but niche cases (antique car drivers and space launchers probably) will stop producing CO2. But I don't know that and the estimates I've seen have a CO2 tax that pays for the climate externalities at less than a quarter of the price of atmospheric carbon capture (and thats the top end of CO2 tax estimates!).
> We want people to reduce their carbon consumption
Remember that this isn't the end goal. What we want is not to feel the effects of releasing CO2.
A CO2 tax that costs as much as it costs to mitigate the effects of that CO2 release reduces CO2 production to exactly the amount its worth emitting. Rationing means you either get more or less CO2 than this number.
People's desire for heavily carbon consuming things vs lightly carbon consuming things varies massively. If you're worried about the poor don't ignore the externalities of their consumption but subsidise them directly via UBI or somesuch.
Rationing is a very worst of all possible worlds solution, losing you all the benefits of trade.