There are two big reasons you can't fund a full UBI exclusively with a carbon tax.
The first is that the size of the carbon tax would be problematic. If you wanted to fund a UBI from e.g. VAT, or income tax, you might need a rate of something like 20%. If you wanted to fund the same sized UBI from a carbon tax, the rate would have to be thousands of percent, because burning carbon is not that huge a proportion of all economic activity. And then that would be very disruptive, because rather than some kind of gradual phase out of carbon, everyone would abandon it immediately to avoid the incredibly high tax.
Which is the second problem. Even if you used a less oppressive rate so that people would phase out carbon over some number of years rather than some number of weeks, that's eventually what would happen, and then your tax base dries up. It inherently can't be a long-term funding source because its purpose is to make carbon go away.
It is, however, a brilliant way to distribute the proceeds of a carbon tax. The "UBI" would then be a smaller amount (or require additional funding from general taxes), but it would effectively moot the entire economic impact of the tax (everyone is, on average, getting all the money back) while still creating a good incentive to be the first to stop burning carbon.
The first is that the size of the carbon tax would be problematic. If you wanted to fund a UBI from e.g. VAT, or income tax, you might need a rate of something like 20%. If you wanted to fund the same sized UBI from a carbon tax, the rate would have to be thousands of percent, because burning carbon is not that huge a proportion of all economic activity. And then that would be very disruptive, because rather than some kind of gradual phase out of carbon, everyone would abandon it immediately to avoid the incredibly high tax.
Which is the second problem. Even if you used a less oppressive rate so that people would phase out carbon over some number of years rather than some number of weeks, that's eventually what would happen, and then your tax base dries up. It inherently can't be a long-term funding source because its purpose is to make carbon go away.
It is, however, a brilliant way to distribute the proceeds of a carbon tax. The "UBI" would then be a smaller amount (or require additional funding from general taxes), but it would effectively moot the entire economic impact of the tax (everyone is, on average, getting all the money back) while still creating a good incentive to be the first to stop burning carbon.