Both cap-and-trade and emission taxing schemes are a way of establishing property rights over the right to emit green house gases (GHGs) into the atmosphere, but, at least in the US, cap-and-trade seems like the likely scheme to be established. If anthropogenic global warming is real, then the atmosphere as a waste sink and as an environmental good is a scarce resource because there are two competing benefits, so establishing property rights will help solve conflicts between those benefits. Establishing property rights brings up the question of initial distribution. Who should get the initial rights to emit GHGs?
To allocate initial property rights, libertarians often rely on the homestead principle, which is that a person can gain ownership over an un-owned scarce resource by being the first to use it. Unfortunately, the homestead principle is a little fuzzy here; it is difficult to tell whether the public or emitters were the first to use the atmosphere as a scarce resource. Clearly the public, as a collective and represented by the government, has some claim to the initial allocation since people use the atmosphere as an environmental good every day and have forever, but I think polluters also have some claim, though smaller than the public claim, because low CO2 emission levels probably have a negligible warming effect, and businesses have been emitting CO2 for a long time. It is for this reason that I am less concerned about handing out permits to current emitters as others are, for example, Coyote of Coyote Blog.
Now, if the current level of GHG emissions is higher than optimal (and that presumption is the basis for essentially all global warming legislation), allocating permits to emitters for the amount they currently emit is not consistent with the homestead principle, but neither is allocating all rights to the government. Since we don’t really know when the atmosphere became scarce, there has to be some relatively arbitrary judgment.