Some of my associates are opposed to GHG cap-and-trade. Here's my uneducated opinion. I'd be happy to be told where I've gone wrong.
We have been measuring CO2 in the atmosphere since 1980 at multiple locations accurately enough to make claims about global atmospheric content.
Since 1980 the averaged vector of the fraction of CO2 in the global atmosphere has been accelerating upwards.
We have since 2010 had sufficient techniques for measuring CO2 and methane production from industry to make reasonable claims about global human GHG emission, and this can be back-computed to 1980.
Therefore it is clear what fraction of the CO2 increase is caused by humans.
More than half of the cause of the acceleration in CO2 is human activity.
The increased CO2 and methane in the atmosphere causes climate change.
The climate change represents a direct economic cost to many humans in the same way that the use of arsenic by cotton produces represents a direct economic cost to rice and apple juice producers.
The 1990 levels of GHG in the atmosphere do not pose a major economic threat.
A fair solution is that each living human should be allocated a fraction of GHG production such that the atmosphere stabilizes at 1990 levels. This should be allocated to nation states on a per capita basis each year and regulated by a global cap-and-trade system.