Whether or not nations will ever agree to mandatory reductions, your last sentence ignores the importance of the concept of negative externalities on the emission of CO2 gases. By definition, a negative externality occurs when two parties engaged in an exchange produce an undesired effect on a third party. In this case, fossil fuel producing and consuming parties are getting wealthy from the use of fossil fuels, with their consequent release of CO2. The CO2 is our negative externality, as it results in climate change that is disproportionately experienced by the poorest people in the poorest nations of the world. The Wikipedia article on negative externalities recognizes four means of addressing them, which I partially quote:

"

* Criminalization: As with many types of environmental and public health laws.
* Civil Tort law: Class action [by the third party], various product liability suits.
* Government provision: As with lighthouses, education, and national defense.
* Pigovian taxes or subsidies intended to redress economic injustices or imbalances.

."

Seen in this light, a cap-and-trade system seems to be a combination of the first and last: Criminalization and Pigovian taxes. Essentially, cap-and-trade is a combination of two processes: (1) creating and enforcing a legal limit to total production of CO2 - that is then distributed equitably on a per-capita basis - in order to keep aggregate releases below a level that would result in dangerous and potentially destabilizing changes to global climate (which is essentially a public good, non-rivalrous and non-excludable); and (2) "taxing" those who release more than their per-capita share of CO2 while redistributing this wealth to (i.e. subsidizing) those who consume less than their per-capita share and who, not coincidentally, are also currently the most common members of the third party sufferers.