Someone else explained how a carbon tax is different than carbon offsets, but I'll go a step further and say we should be using a cap & trade system. It would go something like this (at least in my egalitarian version):
Require "carbon credits" to be spent to legally generate carbon and other greenhouse gasses (GHGs).
Have a GHG treaty where signatories collectively decide on a GHG budget, i.e. an acceptable total level of GHG emissions.
Issue an equal amount of credits to each individual such that the total amount issued equals the total GHG budget.
Let people buy and sell carbon credits in a market system.